PENNARAPTORAN SYSTEMATICS TODAY

This chapter will cover clade definitions, the relationships within clades as well as the occasional controversial relationships between different clades. Phylogenies arising from a recent iteration of the Theropod Working Group (TWiG) matrix by Pei et al. (2020) and the recent Mesozoic avialan matrix of Wang et al. (2018a) are used as the main comparative backbones in this section (see Methods), with every effort made to incorporate key findings from all previous work. We opt to consider a range of crown bird phylogenies here given substantial disagreements between them (see Crown Birds section below).

Pennaraptora Foth et al., 2014: A node-based clade defined as the last common ancestor of Oviraptor philoceratops Osborn, 1924, Deinonychus antirrhopus Ostrom, 1969, and Passer domesticus Linnaeus, 1758, and all its descendants (Foth et al., 2014). Pennaraptora was proposed by Foth et al. (2014) on the basis of a phylogenetic analysis using a modified TWiG data matrix with the Latin penna meaning “bird feather” in reference to the pennaceous feathering shared by these animals. While it is possible that such feathers may yet have a wider distribution among theropods (Foth et al., 2014), as hinted by evidence from earlier-diverging coelurosaurians such as ornithomimosaurians (Zelenitsky et al., 2012; but see Xu, 2020), a close phylogenetic relationship between Paraves and Oviraptorosauria has been recovered by a range of studies including with both clades as sister taxa (Senter et al., 2012; Turner et al., 2012; Xu et al., 2015; Lefèvre et al., 2017), with Paraves in a sister relationship with a (Therizinosauria + Oviraptorosauria) clade (Makovicky et al., 2005), and with Paraves, Therizinosauria, Oviraptorosauria and Alvarezsauroidea in a polytomy (Choiniere et al., 2010). Agnolín and Novas (2013) recovered Alvarezsauroidea as the most closely related clade to Paraves with Oviraptorosauria sister to this grouping. Therizinosaurians and alvarezsauroids have been consistently recovered as the closest relatives of pennaraptorans. Each clade has been recovered as the most closely related to Pennaraptora (Therizinosauria: Senter et al., 2012; Turner et al., 2012; Xu et al., 2015; Alvarezsauroidea: Senter, 2007; Zanno, 2010) as well as in a trichotomy with Pennaraptora (Brusatte et al., 2014).

Oviraptorosauria Barsbold, 1976: A stem-based taxon containing all maniraptorans closer to Oviraptor philoceratops Osborn, 1924, than to Passer domesticus Linnaeus, 1758 (sensu Sereno, 1998; Maryańska et al., 2002). Oviraptorosaurian fossils had been referred to ornithomimosaurians (e.g., Gauthier, 1986) and birds (e.g., Maryańska et al., 2002) largely due to their edentulous jaws, even though Oviraptorosauria had already been proposed as a new taxon (Barsbold, 1976). Despite being known from more than 40 reported genera, fully resolved interrelationships remain out of reach for a large portion of known oviraptorosaurian taxa, partly because some of them are missing significant amounts of data owing to their preservation as flattened and/or highly fragmentary specimens (Ji et al., 1998; Lü and Zhang, 2005; Zanno and Sampson, 2005; Sullivan et al., 2011; Longrich et al., 2013). Another obstacle to reconstructing fully resolved interrelationships is the lack of overlap between the elements preserved in specimens of different species. For example, several North American caenagnathid species are based solely on lower jaw remains whereas others are based on postcranial holotypes (Longrich et al., 2013), complicating the testing of taxonomic assignments and reconstruction of quantitative phylogenetic hypotheses (but see Funston and Currie, 2016, for an alternative view). Incisivosaurus has been recovered as the earliest-diverging oviraptorosaurian by a number of analyses that excluded oviraptorosaurian taxa known from poorly preserved specimens, e.g., Protarchaeopteryx, Ningyuansaurus, and Luoyanggia (Longrich et al., 2013; Brusatte et al., 2014; Lamanna et al., 2014; Lü et al., 2016, 2017; Pei et al., 2020) (figs. 1, 2A, C). However, if these poorly known taxa are included, early-diverging oviraptorosaurians collapse into a polytomy (Funston and Currie, 2016; Yu et al., 2018; fig. 2B, D). Oviraptorosauria has two main lineages, the Caenagnathidae and the Oviraptoridae, that are consistently recovered as monophyletic sister taxa (Longrich et al., 2013; Lamanna et al., 2014; Lü et al., 2015; Funston and Currie, 2016; Lü et al., 2016; Lü et al., 2017; Yu et al., 2018; fig. 2A–D). These two lineages together form the Caenagnathoidea, which is sister to Avimimus (Lamanna et al., 2014; Pei et al., 2020) (fig. 1). Within Caenagnathidae, Microvenator and the giant Gigantoraptor are consistently recovered as early-diverging members, whereas the interrelationships between later-diverging members remain unresolved largely due to the issue of nonoverlapping elements between specimens of different species (Lamanna et al., 2014; Lü et al., 2015; Lü et al., 2016; Lü et al., 2017; fig. 2C). Taxonomic revision of problematic taxa like Caenagnathus sternbergi, Macrophalangia canadensis, and “Alberta dentary morph 3” (fig. 2A) and the exclusion of poorly known taxa from the matrix like Leptorhynchos gaddisi and Ojoraptorsaurus has improved topological resolution among later-diverging caenagnathids (Funston and Currie, 2016; Yu et al., 2018; fig. 2B, D). Elmisaurus, Apatoraptor, and Leptorhynchos elegans are consistently recovered as the three latest-diverging caenagnathids, although the position of Apatoraptor and Leptorhynchos gaddisi interchanged in two recent analyses (Funston and Currie, 2016; Yu et al., 2018; fig. 2B, D). Previously, two sister clades have been recovered within Oviraptoridae, the Oviraptorinae and Ingeniinae (Longrich et al., 2010). Over the past decade, successive discoveries of new Chinese oviraptorids preserving interesting character combinations have unsurprisingly collapsed many parts of the phylogeny into different polytomies (Longrich et al., 2013; Lamanna et al., 2014; Lü et al., 2015, 2016, 2017). Funston and Currie (2016) and Yu et al. (2018) provide the most highly resolved interrelationships among oviraptorids to date: Nankangia is recovered as the earliest-diverging oviraptorid and all polytomies are resolved (fig. 2B, D). The oviraptorid interrelationships of these two phylogenies are largely consistent with those of other recent analyses (Lamanna et al., 2014; Lü et al., 2015, 2016, 2017), except the placement of Ganzhousaurus at an earlier-diverging position (fig. 2B–D). However, this topological resolution may be short-lived as many newly reported Chinese specimens have yet to be included, e.g., Huanansaurus, Tongtianlong, Beibeilong, and Corythoraptor (Lü et al., 2015, 2016, 2017; Pu et al., 2017). Thus, while a phenomenal amount of oviraptorosaurian material is known, including specimens subject to detailed monographic work, future work combining and updating existing phylogenetic datasets is a much-needed priority (Funston and Currie, 2016; Lü et al., 2017).

Oviraptorosauria is generally seen as the sister clade of Paraves, together forming the more inclusive clade Pennaraptora (Brusatte et al., 2014; Foth et al., 2014; Xu et al., 2017; Pei et al., 2020) (see Pennaraptora and Paraves sections below). However, the phylogenetic placement of Scansoriopterygidae within Pennaraptora has been contentious. Scansoriopterygids have been placed within Avialae (Xu et al., 2011; Senter et al., 2012; O'Connor and Sullivan, 2014), as an early-diverging lineage within Oviraptorosauria (Agnolín and Novas, 2013; Brusatte et al., 2014; Agnolín et al. 2019; Pei et al., 2020), as an early-diverging paravian clade outside the traditional (Deinonychosauria + Avialae) clade (Turner et al., 2012; Xu et al., 2015; Lefèvre et al., 2017), and in a polytomy with Deinonychosauria and Avialae (Xu et al., 2017). Herein we consider Scansoriopterygidae to be early-branching oviraptorosaurians (fig. 1). More exhaustive descriptions of scansoriopterygid specimens as well as application of developmental insights to the study of nonadult scansoriopterygids may help to give this group a more stable phylogenetic position. This is sorely needed to deepen our understanding of oviraptorosaurians and early-diverging pennaraptorans more broadly.

Paraves Sereno 1997: A stem-based taxon containing Passer domesticus Linnaeus, 1758, and all coelurosaurians closer to it than to Oviraptor philoceratops Osborn, 1924 (sensu Holtz and Osmólska, 2004; after Turner et al., 2012). The last main iteration of the TWiG matrix (Pei et al., 2020) produced a pruned, reduced consensus tree that reaffirmed that Paraves comprises exclusively of Deinonychosauria and Avialae (Gauthier, 1986; Sereno, 1997; Norell et al., 2001; Senter et al., 2004, 2012; Turner et al., 2007a, 2012), bringing recent uncertainty associated with the Brusatte et al. (2014) matrix iteration to a close. Disregarding the most unstable taxa, reasonable group supports (frequency differences between 76 and 100) were calculated by means of no-zero-weight symmetric resampling (Goloboff et al., 2003) for Paraves and the key internal nodes Troodontidae, Dromaeosauridae, Unenlagiinae, and Eudromaeosauria in a recent TWiG study (Pei et al., 2020). The no-zero-weight resampling increases or decreases character weights, but never eliminates characters. With no-zero-weight resampling, support is lowered only because of conflict among characters and not just because of absence of information; it is thus an appropriate resampling scheme for palaeontological datasets. The identification of rogue taxa was carried out in two steps, first identifying a list of possible rogues with a heuristic procedure (the chkmoves command of TNT), then selecting taxon subsets from that list with an optimality-based method (the prupdn command of TNT). This combined routine was implemented in a script called bothprunes (see Pei et al., 2020, for additional details). Other datasets generally have low nodal supports for paravian nodes (Senter et al., 2012; Turner et al., 2012; Brusatte et al., 2014; Foth et al., 2014; Xu et al., 2015), with Bremer supports typically between 1 and 2 for most of them (but see Xu et al., 2015). Despite the differences in pennaraptoran interrelationships recovered in previous studies (Zheng et al., 2009; Xu et al., 2010, 2011, 2015; Senter et al., 2012; Turner et al., 2012; Evans et al., 2013; Godefroit et al., 2013a, 2013b; Brusatte et al., 2014; Foth et al., 2014; Cau et al., 2015; DePalma et al., 2015) there are some seemingly robust paravian synapomorphies: a laterally facing glenoid fossa was recovered by Pei et al. (2020) and at the equivalent node by Turner et al. (2012) and Brusatte et al. (2014) and is related to the extension of the glenoid floor onto the external surface of the scapula, a paravian synapomorphy of Agnolín and Novas (2013), Senter et al. (2012), and Xu et al. (2015), and a synapomorphy of Foth et al. (2014) for the node (Jinfengopteryx + Paraves). Agnolín and Novas (2013), Brusatte et al. (2014), and Pei et al. (2020) recover paravians as sharing a proximal surface of ulna that is divided into two distinct fossae. At the paravian node in Pei et al. (2020) and at the node (Deinonychosauria + Avialae) in Turner et al. (2012) and Brusatte et al. (2014), an acromion margin of the scapula with a laterally everted anterior edge is also shared. Three equivalent paravian synapomorphies were recovered by Pei et al. (2020), Brusatte et al. (2014), and Senter et al. (2012), one of which was also recovered by Xu et al. (2015) and Agnolín and Novas (2013): basipterygoid process absent or very subtle; carotid process present on posterior cervical vertebrae; posterior trochanter is developed as a slightly projected tubercle or flange. Additionally, short and stout proximal chevrons were recovered as a common synapomorphy in this study and in Brusatte et al. (2014) and Xu et al. (2015). Some paravian synapomorphies have been shown to have a more inclusive distribution. For example, large maxillary and dentary teeth and an astragalus fused to the calcaneum but not to the tibia were paravian synapomorphies in the analysis of Turner et al. (2012), but Pei et al. (2020) found these to be avialan and deinonychosaurian synapomorphies respectively. However, some reconstructed tree topologies have been so seemingly different that their synapomorphies have no overlap with other studies. For example, Foth et al. (2014) found that avialans share closer affinities to troodontids than to dromaeosaurids and recovered anchiornithines as early-diverging birds.

FIG. 1.

Pennaraptoran reduced strict consensus tree using extended implied character weighting in analysis of updated TWiG dataset (modified from Pei et al., 2020). Skeletal reconstructions used with the permission of Scott A. Hartman. Scale bars = 10 cm.

Continued

FIG. 2.

Existing oviraptorosaurian phylogenies. A. Adams consensus tree of Longrich et al. (2013). B. Strict consensus tree of Funston and Currie (2016). C. Strict consensus tree of Lü et al. (2017). D. Strict consensus of Yu et al. (2018).

Continued

Deinonychosauria Colbert and Russell, 1969: A node-based taxon containing the last common ancestor of Troodon formosus Leidy, 1856, and Velociraptor mongoliensis Osborn, 1924, and all its descendants (sensu Sereno, 1998; after Turner et al., 2012). Dromaeosaurids and troodontids have been traditionally united as the Deinonychosauria by the “sickle claw” on their second toe among other characteristics and considered the sister group of birds (Gauthier, 1986; Sereno, 1997; Norell et al., 2001, 2009; Makovicky et al., 2005; Novas and Pol, 2005; Senter, 2007; Senter et al., 2012; Turner et al., 2012). A deluge of discoveries over the last decade has called this status quo into question (Lü et al., 2007; Senter et al., 2012; Evans et al., 2013; Godefroit et al., 2013a; Godefroit et al., 2013b; Foth et al., 2014; DePalma et al., 2015; Lü and Brusatte, 2015; Pittman et al., 2015), seemingly supporting every combination of taxa and demonstrating that historically diagnostic features of the clade have more inclusive distributions than once thought or have parallel origins (Zheng et al., 2009; Xu et al., 2010, 2015; Xu et al., 2011; Senter et al., 2012; Turner et al., 2012; Evans et al., 2013; Godefroit et al., 2013a, 2013b; Brusatte et al., 2014; Foth et al., 2014; Cau et al., 2015; DePalma et al., 2015). Deinonychosaurian monophyly was reaffirmed in a recent TWiG study under both extended implied-weighting and equal-weighting searches and in the strict consensus trees (Pei et al., 2020; fig. 1), as well as independently (Lefèvre et al., 2017). Seemingly robust synapomorphies of this clade include: a splenial exposed as a broad triangle in the lateral surface of the dentary between the dentary and angular (Senter et al., 2012; Turner et al., 2012; Pei et al., 2020); dorsal vertebrae with interspinous ligament scars that terminate below the apex of the neural spine (Turner et al., 2012; Pei et al., 2020); an astragalus fused to the calcaneum but not to the tibia (Turner et al., 2012; Brusatte et al., 2014; Pei et al., 2020); penultimate phalanx of digit II highly modified for extreme hyperextension, and its ungual is larger and more curved than that of digit III (Turner et al., 2012; Pei et al., 2020), but a dromaeosaurid synapomorphy in Brusatte et al. (2014) and equivalent to synapomorphies of Senter et al. (2012) and Xu et al. (2015); slightly raised bicipital scar of the ulna (Turner et al., 2012; Pei et al., 2020).

Dromaeosauridae Matthew and Brown, 1922: A stem-based taxon containing Dromaeosaurus albertensis Matthew and Brown, 1922, and all deinonychosaurians closer to it than to Troodon formosus Leidy, 1856, or Passer domesticus Linnaeus, 1758 (sensu Sereno, 1998; after Turner et al., 2012). This clade is consistently supported by robust synapomorphies, particularly in relation to the paroccipital process of the skull. The dromaeosaurid paroccipital process is: elongate and slender with nearly parallel dorsal and ventral edges (Turner et al., 2012; Brusatte et al., 2014; Foth et al., 2014; Pei et al., 2020); has a dorsal edge twisted anterolaterally at its distal end (Turner et al., 2012; Brusatte et al., 2014; Foth et al., 2014; Pei et al., 2020); possesses a ventral flange at its distal end (Brusatte et al., 2014; Pei et al., 2020); and has a ventral edge at its base that is situated at the midheight of the occipital condyle or further ventrally (Brusatte et al., 2014; Pei et al., 2020). Other cranium-based synapomorphies include dentary and maxillary teeth with confluent tooth crowns and roots (Turner et al., 2012; Brusatte et al., 2014; Foth et al., 2014; Pei et al., 2020) as well as an anterior location of the anterior tympanic recess and anterior tympanic crista, with little or no development of the recess posterior to the basipterygoid processes (Turner et al., 2012; Brusatte et al., 2014; Foth et al., 2014; Pei et al., 2020). Past studies are contradictory on the absence/presence of the accessory tympanic recess dorsal to the crista interfenestralis: absent according to Turner et al. (2012) and Pei et al. (2020) but present according to Brusatte et al. (2014) and Foth et al. (2014). Two of the few robust postcranial synapomorphies are: posterior trunk vertebrae with parapophyses projected distinctly on pedicels (Turner et al., 2012; Brusatte et al., 2014; Foth et al., 2014; Pei et al., 2020) and a well-developed ginglymus on the distal end of metatarsal II (Turner et al., 2012; Agnolín and Novas, 2013; Brusatte et al., 2014; Foth et al., 2014; Pei et al., 2020). Rogue taxa can significantly impact support for previously recovered dromaeosaurid synapomorphies, e.g., many synapomorphies are shared with the analysis of Foth et al. (2014) only if Pyroraptor is pruned. The basic dromaeosaurid topology places unenlagiines at an earlier-diverging position and microraptorines and Eudromaeosauria at later-diverging positions (Senter, 2007; Senter et al., 2012; Turner et al., 2012; Evans et al., 2013; Brusatte et al., 2014; Foth et al., 2014; Han et al., 2014; DePalma et al., 2015; Lü and Brusatte, 2015; Pei et al., 2020; fig. 1). However, some analyses have suggested that unenlagiines and microraptorines are early-branching members of Averaptora, a paravian clade that includes avialans and is most closely related to dromaeosaurids (Agnolín and Novas, 2011; Agnolín and Novas, 2013). Mahakala is typically recovered as the earliest-diverging dromaeosaurid (Turner et al., 2007a, 2012; Brusatte et al., 2014; Pei et al., 2020) potentially forming a clade with an additional dromaeosaurid from the Djadokhta Formation, Halszkaraptor escuilliei (Cau et al., 2017). This early-diverging clade is then followed by the Unenlagiinae. Unenlagiinae Bonaparte, 1999, is a stem-based taxon containing Unenlagia comahuensis Novas and Puerta, 1997, and all coelurosaurs closer to it than to Velociraptor mongoliensis Osborn, 1924, Dromaeosaurus albertensis Matthew and Brown, 1922, Microraptor zhaoianus Xu et al., 2000, and Passer domesticus Linnaeus, 1758 (sensu Sereno et al., 2005; after Turner et al., 2012). Within the group, Austroraptor is typically recovered as a sister taxon of Unenlagia, which are both in turn sister to Buitreraptor (Turner et al., 2012; Agnolín and Novas, 2013; Pei et al., 2020). Rahonavis, originally described as a long, bony-tailed bird (Forster et al., 1996), has been identified as a member of the Unenlagiinae (Makovicky et al., 2005) with additional referred material and anatomical work continuing to support that relationship (Forster et al., 2020), but some authors still find it to be an early-diverging avialan (Agnolín and Novas, 2013; Agnolín et al. 2019). Unenlagiine synapomorphies shared by some recent studies include a pubic shaft that is vertically oriented when articulated and an ilium with a reduced supraacetabular crest that is still separate from the anti-trochanter (Foth et al., 2014; Pei et al., 2020) and has a postacetabular process with a concave dorsal edge (Agnolín and Novas, 2013; Pei et al., 2020). (Microraptorinae + Eudromaeosauria) is usually recovered as the latest-diverging dromaeosaurid clade (Senter et al., 2012; Turner et al., 2012; Brusatte et al., 2014; Xu et al., 2015; Pei et al., 2020; fig. 1). Pei et al. (2020) found this clade to lack a deep and sharp groove on the lateral side of the dentary and to have a relatively reduced maxillary fenestra.

Microraptorinae Senter et al., 2004, is a stem-based taxon containing Microraptor zhaoianus Xu et al., 2000, and all coelurosaurians closer to it than to Dromaeosaurus albertensis Matthew and Brown, 1922, Velociraptor mongoliensis Osborn, 1924, Unenlagia comahuensis Novas and Puerta, 1997, and Passer domesticus Linnaeus, 1758 (sensu Sereno et al., 2005; after Turner et al., 2012). Microraptorine interrelationships have been elusive in main TWiG dataset studies (Turner et al., 2012; Brusatte et al., 2014) as well as related ones (Lü and Brusatte, 2015), but some datasets relying on TWiG data as its backbone have recovered resolved microraptorine topologies (Senter et al., 2012; Xu et al., 2015). Both Pei et al. (2020) and Gianechini et al. (2018) extended this improved resolution to a main TWiG study by recovering the two largest microraptorines, Tianyuraptor and Zhenyuanlong, as the earliest-diverging microraptorines (fig. 1), as suggested by Senter et al. (2012) for Tianyuraptor, but contrary to the eudromaeosaurian affinity recovered by DePalma et al. (2015). This is supported by the lack of prominent median posterior process along the posterior edge of the ischium in Tianyuraptor and Zhenyuanlong, unlike other microraptorines and some early-diverging paravians (Pei et al., 2020). Clarifying the character state distributions of several potentially important features will help to test this phylogenetic hypothesis. For example, a posteriorly curved pubis and a radial shaft less than half of the ulnar width were recovered as synapomorphies of smaller-bodied microraptorines excluding Tianyuraptor (Pei et al., 2020), but as microraptorine synapomorphies by Xu et al. (2015) and Agnolín and Novas (2013). Pei et al. (2020) found a proximally pinched metatarsal III was shared by a more inclusive group of microraptorines excluding Tianyuraptor and Zhenyuanlong, whereas Senter et al. (2012) recovered this as a microraptorine synapomorphy. Small-bodied microraptorines have a sub-arctometatarsalian foot that is absent in Zhenyuanlong and has an uncertain condition in Tianyuraptor. The plesiomorphic unspecialized pedal phalanx II-2 appears to be present in small-bodied microraptorines whereas the specialized condition exists in Zhenyuanlong but is unknown in Tianyuraptor. Pruning incomplete specimens like IVPP V22530 substantially improves the resolution of later-diverging microraptorine interrelationships (Pei et al., 2020). Using automated phylogenetic analysis pipelines (Pei et al., 2020) to further explore the impact of rogue taxa on microraptorine phylogeny would therefore be worthwhile. It is important to bear in mind that some studies have recovered microraptorines as the closest avialan relatives as part of a paravian clade called Averaptora, rather than as dromaeosaurids (Agnolín and Novas, 2011, 2013). Thus, more work is clearly needed to bridge this large discrepancy.

Eudromaeosauria Longrich and Currie, 2009, includes the vast majority of dromaeosaurids. It is a node-based taxon containing the last common ancestor of Saurornitholestes langstoni Sues, 1978, Deinonychus antirrhopus Ostrom, 1969, Dromaeosaurus albertensis Matthew and Brown, 1922, and Velociraptor mongoliensis Osborn, 1924, and all its descendants (Longrich and Currie, 2009; sensu Turner et al., 2012). Long-standing eudromaeosaurian synapomorphies include: a sharp demarcation between the postorbital process and the frontal orbital margin in dorsal view (Turner et al., 2012; Pei et al., 2020); a pedal phalanx II-2 with a long and lobate flexor heel with the midline ridge extending onto its dorsal surface (Turner et al., 2012; Pei et al., 2020); thoracic centra approximately equal in anteroposterior length and midpoint mediolateral width (Foth et al., 2014, after Pyroraptor is pruned; Pei et al., 2020); lateral surfaces of dorsal vertebrae possess pneumatic foramina (Pei et al., 2020; equivalent node in Xu et al., 2015; clade with equivalent taxa in Agnolín and Novas, 2013).

A relatively early-diverging position was recovered for the diminutive Bambiraptor within Eudromaeosauria (Senter et al., 2012; Agnolín and Novas, 2013; DePalma et al., 2015; Pei et al., 2020; fig. 1), but Bambiraptor and Saurornitholestes were nested within Velociraptorinae in recent main TWiG dataset studies (Turner et al., 2012; Brusatte et al., 2014), and indeed their taxonomic distinction is based on a few traits that are known to change ontogenetically. The long-standing monophyletic subclades Dromaeosaurinae and Velociraptorinae are still maintained, but not without pruning several species known from fragmentary remains (Yurgovuchia, Acheroraptor, V. osmolskae, and Utahraptor) (Pei et al., 2020; fig. 1). Dromaeosaurinae Matthew and Brown, 1922, is a stem-based taxon containing Dromaeosaurus albertensis Matthew and Brown, 1922, and all coelurosaurians closer to it than to Velociraptor mongoliensis Osborn, 1924, Microraptor zhaoianus Xu, 2000, Unenlagia comahuensis Novas and Puerta, 1997, and Passer domesticus Linnaeus, 1758 (sensu Sereno et al., 2005; after Turner et al., 2012). Dromaeosaurines appear to share serrations on all premaxillary and maxillary teeth and have premaxillary teeth that all have a D-shaped cross section (Turner et al., 2012; Pei et al., 2020). However, Pei et al. (2020) recovered these synapomorphies despite pruning Utahraptor. That study found Dromaeosaurus in a sister relationship with (Achillobator + Atrociraptor) as in Turner et al. (2012), but unlike Xu et al. (2015), where Dromaeosaurus is later diverging than Achillobator and Atrociraptor and also unlike Brusatte et al. (2014) where these taxa are paraphyletic. Velociraptorinae Barsbold, 1983, is a stem-based taxon containing Velociraptor mongoliensis Osborn, 1924, and all coelurosaurians closer to it than to Dromaeosaurus albertensis Matthew and Brown, 1922, Microraptor zhaoianus Xu, 2000, Unenlagia comahuensis Novas and Puerta, 1997, and Passer domesticus Linnaeus, 1758 (sensu Sereno et al., 2005; after Turner et al., 2012). Pei et al. (2020) recovered (Linheraptor + Tsaagan) as the earliest-diverging velociraptorine clade, but remaining velociraptorine interrelationships were controversial as they differed between the two weighting schemes adopted. Later-diverging velociraptorine interrelationships were better resolved at the expense of Luanchuanraptor and V. osmolskae: Deinonychus was recovered as a later-diverging velociraptorine than (Linheraptor + Tsaagan), which has a sister relationship with Adasaurus and the clade (Balaur + Dakotaraptor). Balaur, a strange double-sickle-clawed velociraptorine (Csiki et al., 2010; Turner et al., 2012; Brusatte et al., 2013; Lü and Brusatte, 2015), has also been recovered as an avialan (Cau et al., 2015). Dakotaraptor is a eudromaeosaurian that has been recovered as both a velociraptorine (Pei et al., 2020) and as a dromaeosaurine (DePalma et al., 2015). Recent analyses have not converged on the same velociraptorine synapomorphies (Turner et al., 2012; Brusatte et al., 2014; Pei et al., 2020), but some potential ones are worth bearing in mind: cranial nerve X–XII openings located in a shallow bowl-like depression; basal tubera dorsoventrally deeper than the occipital condyle. Pei et al. (2020) recovered Yurgovuchia, Acheroraptor, V. osmolskae, and Utahraptor as rogue taxa in their analysis. This is not unexpected as they are all based on fragmentary material that is missing the anatomy relating to dromaeosaurine and velociraptorine synapomorphies. Utahraptor is the best preserved among these four taxa but still has a poorly resolved phylogenetic position despite possessing at least one dromaeosaurine synapomorphy (Pei et al., 2020). Ongoing descriptive work on a range of different-sized Utahraptor individuals (Jim Kirkland, personal commun.) has particular promise in further elucidating eudromaeosaurian phylogeny. Dromaeosaurids are known for their “rodlike” tails that deviate from the typical coelurosaurian tail condition (Senter et al., 2012; Pittman et al., 2013), but unlike Senter et al. (2012), Pei et al. (2020) did not recover tail-based synapomorphies at any internal dromaeosaurid nodes except as a trait of Luanchuanraptor. This hints that their tail evolution may be more complex than previously appreciated or that a more sequential evolutionary pattern may yet emerge as more anatomical data becomes available in the future.

Troodontidae Gilmore, 1924: This group is defined as a stem-based taxon containing Troodon formosus Leidy, 1856, and all coelurosaurians closer to it than to Velociraptor mongoliensis Osborn, 1924, or Passer domesticus Linnaeus, 1758 (sensu Sereno, 1998; after Turner et al., 2012). Troodontid phylogeny is probably the least resolved of the paravian groups and does not have subclades established to the same degree as in dromaeosaurid and avialan phylogeny. Seemingly persistent troodontid synapomorphies include a laterally expanded supraorbital crest of the lacrimal anterior and dorsal to the orbit (Turner et al., 2012; Foth et al., 2014; Pei et al., 2020) (excluding anchiornithines); an asymmetrical foot with a slender metatarsal II and a very robust metatarsal IV (Turner et al., 2012; Foth et al., 2014; Pei et al., 2020) (excluding anchiornithines); metatarsal IV wider than either II or III (Turner et al., 2012; Pei et al., 2020) (excluding anchiornithines); smaller, more numerous and more closely appressed anterior dentary teeth compared to those in the middle of the tooth row (Senter et al., 2012; Brusatte et al., 2014; Pei et al., 2020) (in Brusatte et al., 2014, the anchiornithines are included); metatarsal II shorter than IV, but reaching distally further than the base of the metatarsal IV trochlea (Foth et al., 2014; Pei et al., 2020); absence of the basisphenoid recess between the basisphenoid and basioccipital (Xu et al., 2015; Pei et al., 2020); oval foramen magnum taller than wide (Xu et al., 2015; Pei et al., 2020); dentary teeth set in an open groove (Xu et al., 2015; Pei et al., 2020); postorbital process of the laterosphenoid with pneumatic depression on ventral surface (Xu et al., 2015; Pei et al., 2020); and conjoined basal tubera narrower than the occipital condyle (Xu et al., 2015; Pei et al., 2020). In Brusatte et al. (2014) the concave step in the anterior margin of the maxilla is a troodontid synapomorphy, but this same anatomical feature was recovered as a paravian synapomorphy by Pei et al. (2020). In general, some Jehol troodontids such as Sinovenator and Jingfengopteryx are often recovered as the earliest-diverging members of the group, sometimes including a pair of small-bodied Mongolian troodontids as well in a Jingfengopterygid clade (Turner et al., 2012). Other taxa like Sinusonasus and Jianianhualong, which have transitional features, are later diverging in their systematic positions, and most Late Cretaceous troodontids are the latest-diverging within the group. In some analyses the transitional taxa collapse nodes into polytomies and produce a single nested troodontid clade with no distinction between Jehol or Mongolian groups (Xu et al., 2017). Main TWiG datasets recover at least two troodontid clades of varying composition (reduced strict consensus tree: Brusatte et al., 2014; Pei et al., 2020; fig. 1; strict consensus tree: Turner et al., 2012) near the base of the troodontid phylogeny, but have yet to include Jianianhualong and other new taxa that seem to be blurring traditional clade lines. Anchiornithines have been recovered as the earliest-diverging troodontids in some analyses of past TWiG dataset iterations (e.g., Turner et al., 2012; Brusatte et al., 2014; Gianechini et al., 2018) as well as modified versions of them (Agnolín et al. 2019) whilst other studies have resolved them with early-diverging avialans (e.g., Agnolín and Novas, 2013; Pei et al., 2020; fig. 1) or as early-diverging paravians (e.g., Lefèvre et al., 2017). Anchiornis is known from hundreds of fossils preserved in different postures (Hu et al., 2009; Pei et al., 2017; Wang et al., 2017a), offering the most complete picture of anchiornithine anatomy currently available. This taxon has a key role to play in further investigating the taxonomic status of anchiornithines. In this study Xiaotingia maintains its close affinities with Anchiornis as part of the Anchiornithinae (fig. 1). Its phylogenetic position has also been contentious, since it was originally recovered as part of a deinonychosaurian subclade with Anchiornis and Archaeopteryx (Xu et al., 2011) and later recovered as a troodontid (Turner et al., 2012).

Mesozoic Avialae: Avialae Gauthier, 1986, is a stem-based taxon containing Passer domesticus Linnaeus, 1758, and all coelurosaurians closer to it than to Dromaeosaurus albertensis Matthew and Brown, 1922, or Troodon formosus Leidy, 1856 (sensu Maryańska et al., 2002; after Turner et al., 2012). Although our understanding of the phylogenetic relationships of Mesozoic birds has improved enormously over the past three decades, there remain major challenges as systematists scramble to keep up with the rapidly expanding species diversity primarily coming out of the Jehol deposits in northeastern China. Current analyses targeted at Mesozoic bird relationships have experienced a decline in overall support as new taxa with unusual character combinations blur the distinction between major clades (Zhou et al., 2012). This is primarily because character matrices have failed to keep abreast of new morphological diversity through revision and expansion of the character set. This is not to say that character matrices have not been expanded, but with new species and even clades identified every year, it is unsurprising that most analyses fail even to keep up with new operational taxonomic units (OTUs).

Most, but not all, 21st-century descriptions of new taxa have included a phylogenetic analysis to support taxonomic inferences based on morphological observations. All major analyses stem from either the Norell and Clarke (2001) data set, which consists of 201 characters, or the Chiappe (2002) data set, consisting of 169 characters. Generally, Norell and Clarke's (2001) matrix includes more characters targeted at resolving later-diverging avialans derived from their work on Apsaravis (Clarke and Norell, 2002) and Clarke's (2004) revision of Ichthyornis, whereas the Chiappe (2002) character list focuses more heavily on early-diverging birds based on his work on the El Brete enantiornithines and the early-diverging ornithuromorph Patagopteryx (Chiappe, 1995). Chiappe (2002) included Allosauroidea, Velociraptorinae, and the Troodontidae as outgroups but did not illustrate their relative relationships. The Alvarezsauroidea, at one time considered to be a clade of flightless birds (Suzuki et al., 2002), were also initially included (Chiappe, 2002). In contrast, Norell and Clarke (2001) used the OTU Dromaeosauridae to root their analysis. Following their usage, all subsequent analyses born from the Chiappe (2002) list have also used the Dromaeosauridae as the outgroup (O'Connor et al., 2011; Zhou et al., 2014; Wang et al., 2018a) (fig. 3).

FIG. 3.

Wang et al. (2018a) analysis of 70 taxa scored across 280 morphological characters. Published results using A. new technologies; B. traditional search using implied weighting, k = 16.

Continued

The Chiappe (2002) character list was expanded by Gao et al. (2008), whose new matrix included 15 new taxa and 73 additional characters (total 242 characters), some of which were new and others taken from Norell and Clarke (2001). This list was further expanded to include three new characters and a large number of additional taxa (O'Connor et al., 2009, 2011). The O'Connor et al. (2011) character list has since been expanded independently by Wang et al. (2014, 2018a) and Atterholt et al. (2018). Wang et al. (2014) added 17 new characters (262 characters) and Atterholt et al. (2018) added seven (252 characters). In both cases the added characters were targeted towards increasing resolution in the Enantiornithes. The Wang et al. (2014) matrix was modified by Chiappe et al. (2019), in which some characters were added while others were deleted and some scorings were modified (212 characters). Wang and colleagues expanded their matrix again in 2018, and it has 70 taxa scored over 280 (18 new) characters (Wang et al., 2018a; fig. 3). In 2014, Li's group added 19 characters to the Norell and Clarke (2001) matrix but included only 36 taxa (Li et al., 2014).

All these aforementioned analyses stem from careful firsthand observation of almost all the analyzed OTUs. However, they all fail at one critical level: in only using the OTU Dromaeosauridae as the outgroup, these analyses included only one potential avialan ancestor. This made it impossible to test hypotheses regarding the closest dinosaurian relatives of Avialae. A notable recent difference between the analyses of these various workers is the parameters utilized for the analysis. For example, although Wang et al. (2017b) and O'Connor et al. (2016) both use the TNT software (Goloboff et al., 2008a), Wang et al. (2018a) have recently shifted to using “new technology” search algorithms (fig. 3A). O'Connor et al. (2014; Atterholt et al., 2018) continue to use traditional search terms and have more recently started applying implied weighting (Goloboff et al., 2008b) to account for the extreme homoplasy observed during early avialan evolution because this method more strongly weights characters that have more homology (less homoplasy) (Wang and O'Connor, 2017; Xu, 2018). Here, the Wang et al. (2018a) analysis is rerun with a traditional search using implied weighting (Goloboff et al., 2008b) with a gentle concavity (Goloboff et al., 2018), k = 16 (fig. 3B) (see Methods). The results are presented together with the results published by Wang et al. (2018a) based on “new technology” search algorithms in TNT (fig. 3A).

There are a few other matrices that have been utilized in attempts to resolve the phylogeny of Mesozoic birds (Cau and Arduini, 2008; Lefèvre et al., 2014). However, these appear to rely heavily on published information, which is often incomplete if taxa are not monographed and can at times be inaccurate (e.g., “Dalingheornis”: Zhang et al., 2006; Lockley et al., 2007). Controversial taxa that have received only preliminary study can produce erroneous results in analyses when their purported morphologies are taken at face value (e.g., Jixiangornis (Ji et al., 2002; Lefèvre et al., 2014)).

There are several relationships in Mesozoic avialan phylogeny that have stabilized in recent analyses, and other regions of the tree where recent analyses still fail to find consensus. Archaeopteryx is the earliest-diverging avialan according to the definition above followed by most Mesozoic ornithologists (possible troodontids such as Anchiornis are not included in these analyses) and Jeholornis, another long bony-tailed bird, is typically resolved as later diverging than Archaeopteryx but earlier diverging than the Pygostylia, all birds whose abbreviated tail ends in a fused, compound element, the pygostyle (Li et al., 2014; Atterholt et al., 2018; Wang et al., 2018a). The Enantiornithes and the Ornithuromorpha (node-based definition that we follow in this volume because of the preference of our authors; Euornithes Sereno et al., 1998, is the stem-based definition) are consistently resolved as sister taxa, forming the Ornithothoraces (Chiappe, 2002; Clarke, 2004; Gao et al., 2008; Wang et al., 2018a). The relative placement of the two major recognized early-diverging pygostylian clades, the Sapeornithiformes and Confuciusornithiformes, is unstable; the two groups have both been resolved as the sister taxon to the Ornithothoraces (Li et al., 2014; Wang et al., 2014). The “mosaic” distribution of derived avialan features within Paraves obscures attempts at resolving aspects of early avialan phylogeny. For example, while the absence of an ossified sternum suggests Sapeornis is earlier diverging, it has a more reduced (and thus derived) manus with only two manual unguals, whereas in Confuciusornis the manus retains the plesiomorphic condition of three manual unguals, and these are hypertrophied relative to most other paravians (Chiappe et al., 1999; Zhou and Zhang, 2002; Zheng et al., 2014) (fig. 4). The Wang et al. (2018a) analysis suggests that the Confuciusornithiformes are the earliest-diverging pygostylians known, even more primitive than the recently identified Jinguofortisidae (fig. 3).

Enantiornithine phylogeny is poorly resolved (O'Connor et al., 2011; Wang et al., 2018a), but several groups have become apparent in recent analyses. Protopteryx, Iberomesornis, and the Pengornithidae are fairly consistently resolved as very early-diverging taxa (Wang et al., 2017b; Atterholt et al., 2018; Wang et al., 2018a). Jehol “longirostrine” taxa are commonly resolved into two clades, one formed by Longirostravis, Shanweiniao, and Rapaxavis, and the other by Longipteryx and Boluochia (O'Connor et al., 2009, 2016; Wang et al., 2014; Zhou et al., 2014). These two clades are often but not always resolved forming a larger clade (the Longipterygidae; fig. 3) (Wang et al., 2014, 2018a). The most diverse and reasonably resolved clade of enantiornithines is the Bohaiornithidae, although its recognition through cladistic analysis required additional characters to be added and scored in relevant phylogenetic data matrices (Wang et al., 2014, 2018a) and a recent analysis included in the description of a new bohaiornithid-like species failed to resolve this clade (Chiappe et al., 2019). The only recognized Late Cretaceous family is the Avisauridae (Chiappe, 1992). Through the support of seven additional characters targeted at avisaurid relationships, this clade has recently been demonstrated to consist of North American and South American subclades, each consisting of three taxa (Atterholt et al., 2018).

The Ornithuromorpha is slightly more resolved than the Enantiornithes, probably due to its lesser diversity (fig. 3). Archaeorhynchus and Patagopteryx are consistently resolved as among the earliest-diverging taxa (You et al., 2006; Li et al., 2014; O'Connor et al., 2016; Wang et al., 2017b), although in the enlarged analysis of Wang et al. (2018a) Patagopteryx has a later-diverging position. The secondary loss of flight in the Late Cretaceous Patagopteryx may be creating a false signal interpreted by some analyses as indicative of an earlier-diverging position (fig. 1B). Vorona, Chaoyangia, Jianchangornis, and Zhongjianornis are also consistently resolved as early-diverging taxa (Wang et al., 2014; Zhou et al., 2014; O'Connor et al., 2016). The most diverse family is the Hongshanornithidae (Wang et al., 2015; 2017b; 2018a). Yanornis, Yixianornis, and Songlingornis are often grouped together based on sternal morphology forming a clade referred to as the Songlingornithidae (Clarke et al., 2006; Li et al., 2014; Wang et al., 2018a). Gansus from the slightly younger Xiagou Formation is typically resolved as later diverging than Jehol ornithuromorphs (Zhou et al., 2014; Wang et al., 2018b).

FIG. 4.

Major evolutionary transformations in the avian furcula, coracoid and sternum, and manus in context of a simplified phylogeny, as documented in A. Archaeopteryx, B. Jeholornis, C. Confuciusornis, D. Sapeornis, E. Early Cretaceous Enantiornithes (Parabohaiornis), and F. Early Cretaceous Ornithuromorpha (Yanornis). Generally during early avian evolution, the furcula, coracoid, and sternum become more craniocaudally elongate, while the manual digits become reduced and fusion between the metacarpals increases. Illustrations not to scale.

The Ornithurae generally consists of Ichthyornis, Hesperornis, and Aves, although whether the Ichthyornis lineage or the Hesperornithiformes is more closely related to modern birds is unresolved (Clarke, 2004; O'Connor et al., 2011; Li et al., 2014; Wang et al., 2018a). Since the most common definition of Ornithurae is node based with Hesperornis and living birds as reference taxa (Gauthier and de Queiroz, 2001), this means that Ichthyornis may or may not be a member of this clade. It is for this reason that stem-based definitions may better serve until early avialan relationships are better understood. Although Ichthyornis was traditionally resolved as later diverging (Chiappe, 2002; Clarke, 2004; You et al., 2006), increased character sampling in the Hesperornithiformes resulted in this clade moving crownward (O'Connor et al., 2011). New cranial information on Ichthyornis supports the earlier-diverging position of this taxon (Atterholt et al., 2018; Field et al., 2018). However, the position resolved for this taxon using the Wang et al. (2018a) matrix is quite different depending on how the data is analyzed, further highlighting the uncertainty that surrounds the relative placement of these later-diverging lineages (fig. 3).

In recent analyses, Bremer support is typically very low (O'Connor et al., 2017; Wang et al., 2018a). This likely reflects similarities between early-diverging ornithuromorphs and enantiornithines that have blurred the distinction between these two major clades and not missing data (O'Connor and Zhou, 2013). Take, for example, the taxon Schizooura lii: although clearly a member of the Ornithuromorpha, this species possesses a Y-shaped enantiornithine-like furcula; the coracoid lacks a lateral process (at one time a synapomorphy of the Ornithuromorpha); and the cranial surface of the humerus is flat (Zhou et al., 2012). Chaoyangia preserves two features observed clearly only in ornithuromorphs, but sometimes is resolved in the Enantiornithes, and thus a well-resolved tree can be found only if it is excluded (O'Connor et al., 2014). In many recent analyses, Ornithothoraces forms a single clade in trees only one or two steps longer than the most parsimonious solution (O'Connor and Zhou, 2013; Wang et al., 2018a). This mosaic distribution of morphologies among taxa and the dwindling number of clear morphological differences between the Enantiornithes and the Ornithuromorpha as early-diverging taxa are uncovered has diminished the number of synapomorphies in support of each node, resulting in weak support for most traditional clades (O'Connor et al., 2014; Zelenkov, 2017). The only way to increase support is to identify new synapomorphies for each clade and increase the amount of morphological data incorporated into the character matrix, no easy task given the varying preservational limitations set by the known diversity and the rapid rate of discovery principally in China (O'Connor and Zhou, 2013), but also elsewhere (de Souza Carvalho et al., 2015; Atterholt et al., 2018). Currently, all synapomorphies in support of Ornithothoraces, Enantiornithes, and Ornithuromorpha are ambiguous because of the large number of taxa and substantial amount of preservational variability between specimens. Although the rate of discovery has outpaced the rate at which character sets have been expanded, the Wang et al. (2018a) dataset exemplifies the continual efforts that are being made to encapsulate greater morphological variation. This dataset has roughly 100 more characters than datasets from 15 years ago and double the number of OTUs.

It is often difficult to compare results of various analyses due to differences in the included taxa. Nearly half of all Mesozoic avialans are not even included in phylogenetic analyses because of their highly fragmentary nature (O'Connor, 2009; O'Connor et al., 2014). With the exception of specimens from a few key deposits, most of the Mesozoic fossil record consists of incomplete taxa based on a small number of incomplete elements (e.g., Flexomornis, Limenavis), a single bone (e.g., Avisaurus, Yungavolucris), or sometimes less (e.g., Lectavis, Almatiornis). Such incomplete OTUs in already weakly supported phylogenies can result in the collapse of the Ornithothoraces (e.g., Chaoyangia), and some (e.g., Mystiornis, Flexomornis) even cause the Pygostylia to collapse (O'Connor and Zhou, 2013; O'Connor et al., 2014; Atterholt et al., 2018).

Zhongornis is known only from a single young juvenile specimen and, given the extreme ontogenetic changes likely experienced by most paravians, its phylogenetic affinities cannot be determined with any certainty at this time. Originally considered a member of a distinct lineage of birds characterized by a tail morphology intermediate between that of Archaeopteryx and that of pygostylians (Gao et al., 2008), the specimen was later suggested to be a juvenile scansoriopterygid (O'Connor and Sullivan, 2014). New data on pygostyle formation suggests that Zhongornis may in fact be a juvenile pygostylian (Rashid et al., 2018).

Anchiornithine taxa—Anchiornis, Aurornis, Eosinopteryx, and Xiaotingia—have been gathered into a distinct clade of early branching avialans (Anchiornithinae: we use this stem-based taxon containing Anchiornis instead of Anchiornithidae because the former was proposed earlier [Xu et al. 2016]) either all together (Agnolín and Novas, 2011; Agnolín and Novas, 2013; Foth et al., 2014; Pei et al., 2017, 2020; Rashid et al., 2018) or as a subset of the four taxa (Xu et al., 2011). This contrasted with past analyses involving the main TWiG data matrix or modified versions, which recovered anchiornithines or a subset of them as part of the Troodontidae (Turner et al., 2012; Brusatte et al., 2014; Agnolín et al., 2019), as well as previous studies that found them to be early-diverging deinonychosaurians (Senter et al., 2012; Xu et al., 2015) or early-diverging paravians (Lefèvre et al., 2017). Despite such major differences in the phylogenetic position of this clade these avialan synapomorphies stand out (Turner et al., 2012; Brusatte et al., 2014; Foth et al., 2014): transition point proximal to caudal 7; in lateral view, the dorsal border of the antorbital fossa is formed by the lacrimal and nasal.

Eight individual Archaeopteryx specimens (of 11 published) were recently recovered in a polytomous clade that is sister to the nonanchiornithine avialans (Pei et al., 2020; fig. 1). With the Haarlem specimen pruned (recently proposed as an anchiornithine: Foth and Rauhut, 2017), six Archaeopteryx specimens (the London, Berlin, Munich, Thermopolis, Eichstätt, and 11th specimens) form a group sister to a large avialan clade comprising of the Solnhofen specimen and remaining later-diverging avialans. However, the former group was supported by synapomorphies that cannot be scored in the Solnhofen specimen, so additional data may yet show a monophyletic Archaeopteryx group (although some fragmentary specimens are probably only referable to more inclusive Mesozoic avialan clades: Rauhut et al., 2018).

Rahonavis was originally described as an early-diverging avialan (Forster et al., 1996) and has been found in a later-diverging position than Archaeopteryx (Cau, 2018; Novas et al., 2018). However, it has also been recovered as a dromaeosaurid by a range of other studies (Makovicky et al., 2005; Turner et al., 2012), which is the position favored in this volume.

Crown birds (Aves): The last decade has seen a great deal of progress with regard to resolving the interrelationships of the major groups of living birds. A series of large-scale “phylogenomic” studies has iteratively brought us closer than ever to a complete picture of crown bird interordinal relationships (e.g., Ericson et al., 2006; Hackett et al., 2008; McCormack et al., 2013; Jarvis et al., 2014; Claramunt and Cracraft, 2015; Prum et al., 2015; Kimball et al., 2019). Ongoing work seeks to continue progress on resolving recalcitrant portions of the bird tree of life, as well as achieving the goal of sequencing and analyzing the whole genomes of all known extant avialan species.

Crown birds (Aves) comprise nearly 11,000 described extant species, and are divided into two major subclades: Palaeognathae and Neognathae (Gill and Donsker, 2017). Palaeognathae comprises the paraphyletic, flightless ratites (i.e., the extant ostriches, rheas, kiwis, emus, and cassowaries), and the volant tinamous of the New World. Over the last decade, the surprising hypothesis that ratites are paraphyletic with respect to tinamous has received accumulating support from molecular phylogenetic data sets (Hackett et al., 2008; Harshman et al., 2008; Phillips et al., 2010; Haddrath and Baker, 2012; Smith et al., 2013; Baker et al., 2014; Jarvis et al., 2014; Mitchell et al., 2014; Prum et al., 2015; Yonezawa et al., 2016; Grealy et al., 2017; Reddy et al., 2017) and is corroborated by embryological data (Faux and Field, 2017), suggesting that flightlessness and large body size have arisen repeatedly throughout palaeognath evolutionary history. Neognathae are divided into Galloanserae (chickenlike and ducklike birds) and the major clade Neoaves, which represents over 95% of living bird diversity. The earliest fossil crown bird yet identified, Asteriornis from the Maastrichtian of Belgium, appears to represent an early galloanseran (Field et al., 2020). This implies that at least the stem lineages of Palaeognathae, Galloanserae, and Neoaves were present in the Late Cretaceous.

The phylogenetic interrelationships of the major subclades of Neoaves are hotly contested, and according to one recent high-profile phylogenomic study, represent “the greatest unresolved challenge in dinosaur systematics” (Prum et al., 2015). While several recent studies have sought to clarify the deep branching pattern among the major neoavian clades (e.g., Hackett et al., 2008; Jarvis et al., 2014; Prum et al., 2015; Reddy et al., 2017; Kimball et al., 2019), these studies have recovered incompatible neoavian topologies, raising questions about the influence of data type, taxon sampling, and analytical approach in driving topological results. Indeed, recent work (Suh, 2016) hypothesises that a consistent, bifurcating branching pattern along much of the backbone of Neoaves may never be obtained, since the early evolutionary history of Neoaves may have been characterized by extremely rapid cladogenesis and high rates of incomplete lineage sorting. Although the subject is still controversial (see Field et al., chapter 5, on molecular rate variation), the hypothesis of a rapid radiation of Neoaves in the aftermath of the end-Cretaceous mass extinction has gained mounting support in recent years (Feduccia, 1995; Ericson et al., 2006; Longrich et al., 2011; Feduccia, 2014; Jarvis et al., 2014; Prum et al., 2015; Berv and Field, 2018). Importantly, uncertainty regarding the precise origination times of the deepest clades of crown birds, as well as Aves itself, has clouded understanding of the influence of the end-Cretaceous mass extinction on avialan evolution. However, the known neoavian fossil record is entirely post-Cretaceous at present (Longrich et al., 2011; Field, 2017; Ksepka et al., 2017), consistent with the idea that lingering topological uncertainty at the base of Neoaves may indeed be related to rapid early Cenozoic cladogenesis (see Field et al., chapter 5, on molecular rate variation).

Despite ongoing controversy related to the branching pattern at the base of Neoaves, recent phylogenetic studies are consistent in recognizing about 10 major constituent neoavian subclades (Reddy et al., 2017; Field et al., chapter 5). Figure 5 depicts competing topological hypotheses presented by two recent avialan phylogenomic studies (Jarvis et al., 2014; Prum et al., 2015), illustrating that despite some topological uncertainty, the composition of most major clades is consistent across studies. Major outstanding topological controversies influencing our understanding of avialan macroevolution include:

FIG. 5.

Competing phylogenetic topologies for crown birds from recent phylogenomic studies of Jarvis et al. (2014) and Prum et al. (2015). Both topologies support Palaeognathae (black) and Galloanserae (red) as successive sister taxa to the rest of extant birds (Neoaves). Interrelationships within Neoaves differ, but the same major constituent clades are largely supported (see legend). The monotypic hoatzin (Opisthocomus hoazin) is inferred to be the sister taxon of Telluraves in Prum et al. (2015), and sister to a Charadriiformes + Gruiformes clade in Jarvis et al. (2014). Prum et al. (2015) supports a monophyletic Columbaves (fuschia), uniting Otidimorphae (bustards, cuckoos, turacos) and Columbimorphae (doves, mesites, sandgrouse), whereas Jarvis et al. (2014) find Columbimorphae as the sister taxon to Mirandornithes, and Otidimorphae as sister to Strisores.

While ongoing large-scale phylogenetic studies are likely to shed light on these and other questions related to neoavian phylogeny in the relatively short-term, targeted phylogenomic studies of major avialan subclades are continuously illuminating finer-scale phylogenetic relationships across the bird tree of life. For example, a recent phylogenomic study of passerines (a massive avian subclade comprising over 6000 living species) represents the first effort to sequence representatives of all extant passerine families, revealing important insight into this charismatic superradiation (Oliveros et al., 2019).

DISCUSSION

Pennaraptoran systematics has dramatically improved over the past few decades through immense efforts from all corners of the globe, but there remain areas to improve in all clades, particularly the enigmatic Scansoriopterygidae and Anchiornithinae.

Contentious Groups and Groups with ‘Blurry’ Boundaries: For groups with contentious phylogenetic positions like the Anchiornithinae and Scansoriopterygidae and for groups with “blurry” boundaries, an automated pipeline of phylogenetic analysis, like the one described in Methods, will be helpful in further exploring phylogenetic placement through easier and more efficient running of analyses using alternative character codings and considering all phylogenetically informative but highly fragmentary specimens.

Powered-flight Origins: The topology in figure 1 provided the context to determine that only paravians meet the minimum thresholds of modern powered flight (in terms of wing loading and specific lift estimates seen in today's flying birds: see Pei et al. (2020) for details). This is despite the reduced body size, pennaceous feathers, and capable respiratory system among nonparavian pennaraptorans (Turner et al., 2007a; Dececchi and Larsson, 2013; Benson et al., 2014; Foth et al., 2014; Lee et al., 2014; Xu et al., 2014; Brusatte et al., 2015). Powered flight potential among microraptorines and unenlagiines underscores the need for more intensive systematic study of these groups, particularly the more enigmatic unenlagiines (Pei et al., 2020). Recovered early-diverging positions of the larger microraptorines Tianyuraptor and Zhenyuanlong imply a decrease in body size and an increase in relative forelimb length across Microraptorinae (Pei et al., 2020). Discovering microraptorines that cover temporal and geographical ranges outside the currently known Early Cretaceous Asian forms and Late Cretaceous North American ones may yet reveal the drivers and development of this flight potential.

METHODS

BACKGROUND

Pennaraptorans are a clade of vaned feathered coelurosaurian dinosaurs that are comprised of the Oviraptorosauria, Scansoriopterygidae, Dromaeosauridae, Troodontidae, and Avialae (see Pittman et al., in the previous chapter for additional information). They include the only dinosaurs to have evolved flight and the only ones to have persisted to the present day.

Oviraptorosauria

Oviraptorosaurian fossils were first discovered in the 1920s and are now represented by more than 40 genera spanning a size range across three orders of magnitude (table 1). The 1920s to 1940s, the 1970s, 1980s, 1990s, and the past 20 years have been key periods in our documentation of the oviraptorosaurian fossil record, which is limited to Laurasian continents and dominated by discoveries from Asia and North America (fig. 1). The last 30 years have seen the discovery of most known oviraptorosaurian taxa, particularly from the Cretaceous of China and the Late Cretaceous of North America and Mongolia. These discoveries have greatly broadened our understanding of this group, including in regard to the evolution of their beaked and strangely pneumatized skulls, as well as the origin of brooding in theropod dinosaurs.

Asia: This continent is the home of the first described oviraptorosaurian species (Osborn, 1924), the eponymous species Oviraptor. Asia is also home to more than 75% of valid oviraptorosaurian genera. The most important sources of Asian oviraptorosaurians are the Early Cretaceous (Hauterivian-Aptian) Jehol Lagerstätte of northeastern China, the Campanian-Maastrichtian Ganzhou oviraptorid fauna of southern China (Nanxiong Formation) as well as the southern Mongolian Campanian Djadokhta Formation (and the similar Wulansuhai (Bayan Mandahu) Formation in China), Campanian-Maastrichtian Barun Goyot Formation, and the Maastrichtian Nemegt Formation.

FIG. 1.

Geographic distribution of pennaraptoran theropods illustrated on palaeogeographic globes of the Late Jurassic (Oxfordian-Tithonian), Early Cretaceous (Berriasian-Albian), and Late Cretaceous (Cenomanian-Maastrichtian). Palaeomaps modified from GPlates ( www.gplates.org) (Müller et al., 2018).

TABLE 1

Oviraptorosaurian fossil record

continued

Jehol oviraptorosaurians represent the oldest unequivocal oviraptorosaurian records, and the six described taxa include some articulated specimens preserving feathers, gastroliths, and stomach contents. The early-diverging oviraptorosaurians Incisivosaurus, Protarchaeopteryx, Similicaudipteryx, Caudipteryx, Xingtianosaurus, and Ningyuansaurus are the only known toothed forms and have less specialized skulls compared to later oviraptorosaurians (Ji et al., 1998; Zhou et al., 2000; Xu et al., 2002a; Balanoff et al., 2009; Qiu et al., 2019). Caudipteryx is known for its pennaceous feathered arms, gastroliths, and a tail plume probably used for display purposes (Ji et al., 1998; Zhou et al., 2000; Pittman et al., 2013; Persons et al., 2014). It is known from two species (Ji et al., 1998; Zhou et al., 2000), although this is contested. Both specimens were recovered from the ∼125 Ma Yixian Formation in Liaoning province, with both 2-D and 3-D preservations. Two specimens of Similicaudipteryx, another Yixian Formation genus, show radical changes to feather morphology during ontogeny (Xu et al., 2010a). However, these two specimens might be specimens of Incisivosaurus (Xu, 2020). Ningyuansaurus possibly preserves seeds within its body cavity (Ji et al., 2012). Xingtianosaurus is the most recently named Jehol genus, which is known from an articulated postcranial skeleton (Qiu et al., 2019). Luoyanggia is an Aptian- to Albian-aged oviraptorid from the Haoling Formation of Henan, central China, which was previously thought to be Late Cretaceous in age (Lü et al., 2009; Xu et al., 2012a).

The Late Cretaceous Ganzhou fauna of Jiangxi, southern China, has the greatest known diversity of oviraptorid oviraptorosaurians with seven reported genera in the Campanian-Maastrichtian Nanxiong Formation: Banji, Huanansaurus, Jiangxisaurus, Tongtianlong, Ganzhousaurus, Nankangia, and Corythoraptor (Xu and Han, 2010; Lü et al., 2013a, 2015, 2016, 2017; Wang et al., 2013a; Wei et al., 2013). Embryos of an oviraptorid have also been recovered from this formation (Wang et al., 2016a). Heyuannia is an oviraptorid genus described from a partial skeleton from the Maastrichtian Dalangshan Formation of Guangdong, southern China (Lü, 2003). “Ingenia,” or Ajancingenia yanshini, from the Campanian-Maastrichtian Barun Goyot Formation of southern Mongolia (Barsbold, 1981; Easter, 2013) has been referred to this genus as a second species, H. yanshini, but this involves a very large geographical and temporal separation between species (Funston et al., 2017). Shixinggia is another described Guangdong oviraptorid from the Maastrichtian Pingling Formation (Lü and Zhang, 2005). Yulong is a chicken-sized oviraptorid represented by excellent fossil material from the Upper Cretaceous Qiupa Formation of Henan, central China (Lü et al., 2013b), while Beibeilong is a caenagnathid known from a perinate skeleton and some eggs from the Cenomanian-Turonian Gaogou Formation of the same province (Pu et al., 2017). Anomalipes is a recently reported caenagnathid from the Campanian Wangshi Group of Shandong Province, known only from hind-limb elements (Yu et al., 2018). The largest known oviraptorosaurian—the caenagnathid Gigantoraptor—was recovered in the northernmost frontier of China from the Campanian-Maastrichtian Erlian (Iren Dabasu) Formation of Nei Mongol (Inner Mongolia) (Xu et al., 2007). This is also the locality for one of the smallest oviraptorosaurians, Avimimus, which was first reported from similarly aged rocks in Mongolia (Kurzanov, 1981), although these assignments would benefit from review, as they may represent different taxa. The Campanian Wulansuhai (Bayan Mandahu) Formation, also in the Gobi Desert region, is the home to the oviraptorids Machairasaurus and Wulatelong and some other indeterminate oviraptorid material (Longrich et al., 2010; Xu et al., 2013b).

Mongolian oviraptorosaurians are dominated by oviraptorids, with three genera from the Campanian Djadokhta Formation (Oviraptor, Citipati, and Khaan) (Osborn, 1924; Clark et al., 2001, 2002; Balanoff and Norell, 2012), four genera from the Maastrichtian Nemegt Formation (Gobiraptor, Nomingia, Rinchenia, and Nemegtomaia) (Barsbold, 1986; Barsbold et al., 2000; Lü et al., 2004, 2005; Fanti et al., 2012; Funston et al., 2017; Lee et al., 2019) and three from the Campanian-Maastrichtian Barun Goyot Formation (“Ingenia”/Ajancingenia/Heyuannia yanshini, Conchoraptor, and Nemegtomaia [also from the Nemegt]; see Fanti et al., 2012, for details of Maastrichtian portion) (Barsbold, 1981; 1986; Longrich et al., 2010; Funston et al., 2017). Several skeletons are known for Khaan and Citipati from the rich fossil beds of Ukhaa Tolgod, including brooding specimens, single species group associations and embryos (Norell et al., 1995, 2001; Clark et al., 2001).

Avimimus is a small, early-diverging oviraptorosaurian closer to Caenagnathidae and Oviraptoridae that is known from multiple formations in Mongolia, including the Djadokhta, Nemegt, and Barun Goyot (Kurzanov, 1981; Longrich et al., 2010). Elmisaurus is a caenagnathid from the Nemegt Formation (Osmólska, 1981; Currie et al., 2016). The holotype of the caenagnathid Caenagnathasia is a pair of dentaries from a single individual recovered from the Turonian Bissekty Formation of Uzbekistan (Currie et al., 1993). A partial dentary referred to Caenagnathasia is known from the Erlian (Iren Dabasu) Formation of Nei Mongol, China (Yao et al., 2015). Few caenagnathid skull elements have been reported in Asia; these are from the perinate Beibeilong and the mandible of Gigantoraptor and a similarly sized specimen from the Gobi Desert (Xu et al., 2007; Tsuihiji et al., 2015; Ma et al., 2017; Pu et al., 2017).

North America: The early-diverging caenagnathid Microvenator was recovered from the Aptian-Albian Cloverly Formation and is a historically important specimen and likely that of a juvenile. It is the continent's oldest oviraptorosaurian (Makovicky and Sues, 1998). Late Cretaceous caenagnathids dominate the North American oviraptorosaurian fossil record. Chirostenotes, currently known from the species C. pergracilis, was the first discovered caenagnathid as well as the first described North American oviraptorosaur (Gilmore, 1924). The Campanian Dinosaur Park Formation of Canada is the most important source of North American caenagnathids including Chirostenotes (also referred to possible material in the Campanian Two Medicine and Maastrichtian Hell Creek formations of the northern United States [Osmólska et al., 2004]), Leptorhynchos (Longrich et al., 2013) (also the Campanian-Maastrichtian Aguja Formation of the southern United States) as well as Caenagnathus, the caenagnathid that lends its name to the clade (Currie et al., 1993). Hagryphus is a caenagnathid from the Campanian Kaiparowits Formation of Utah, known from forelimb material (Zanno and Sampson, 2005). Moving into the latest Cretaceous, Epichirostenotes and Apatoraptor are caenagnathids from the Campanian-Maastrichtian Horseshoe Canyon Formation of Canada. Both have preserved skull elements, and the holotype of Apatoraptor is a largely articulated partial skeleton. Ojoraptorsaurus is a caenagnathid known from pubic bones recovered from the Maastrichtian Ojo Alamo Formation of the southwestern United States (Sullivan et al., 2011; Funston and Currie, 2016). Anzu is the largest described caenagnathid from North America and is one of the best-preserved North American oviraptorosaurians (Lamanna et al., 2014). It is known from the Maastrichtian Hell Creek Formation of North and South Dakota (Lamanna et al., 2014). Fossil eggshell material and undescribed skeletal material from the top of the Cedar Mountain Formation (Cenomanian-Turonian) of Utah represents an even larger taxon that was similarly sized to Gigantoraptor (Makovicky et al., 2015; Tucker et al., 2020).

Europe: Oviraptorosaurians are poorly known from Europe with representation from only isolated postcranial material (Naish et al., 2001; Csiki and Grigorescu, 2005) whose referrals have been subsequently challenged (Csiki et al., 2010; Allain et al., 2014).

Isolated elements from Cretaceous strata of Gondwana have been interpreted as deriving from oviraptorosaurians, but these records have not withstood subsequent reevaluation. An isolated cervical from the Maastrichtian El Brete Formation of Argentina was described as an oviraptorosaurian (Frankfurt and Chiappe, 1999), but has since been reinterpreted as a noasaurid theropod (Agnolín and Martinelli, 2007). Elements from the Lower Cretaceous Otway Group of Australia described as an oviraptorosaurian lower jaw fragment and dorsal vertebra (Currie et al., 1996), have since been attributed to Unenlagiinae or other theropod clades (Agnolín et al., 2010). To date, no unambiguous records of oviraptorosaurians from Gondwanan continents exist.

Scansoriopterygidae

Scansoriopterygids are a bizarre group of early-diverging Laurasian oviraptorosaurians or paravians, known only from the Middle and Late Jurassic Haifanggou Formation and Late Jurassic Tiaojishan Formation of north China so far (∼168–155 Ma) (Czerkas and Yuan, 2002; Zhang et al., 2002; Zhang et al., 2008a; Turner et al., 2012; Brusatte et al., 2014; Xu et al., 2015a; Wang et al. 2019a; Pei et al., in press) (fig. 1; table 2). Known from five (or six: O'Connor and Sullivan, 2014) feathered Chinese specimens, only one definitive and possibly two somatically mature individuals exist. Two of these specimens (Yi qi and Ambopteryx longibrachium) possess feathered, membranous wings (Xu et al., 2015a; Wang et al., 2019a) and one possesses a pygostyle (Wang et al., 2019a). Epidendrosaurus and Epidexipteryx are two well-accepted genera, but Scansoriopteryx may be the same genus as Epidendrosaurus. The Early Cretaceous Zhongornis, originally described as a bird (Gao et al., 2008), may be a scansoriopterygid instead (O'Connor and Sullivan, 2014), but this has been contested (Rashid et al., 2018). The notion that scansoriopterygids are early-branching avialans (Xu et al., 2011a; Czerkas and Feduccia, 2014) has been replaced by anatomical evidence grouping some or all scansoriopterygids with oviraptorosaurians (Turner et al., 2012; Agnolín and Novas, 2013; Brusatte et al., 2014; Pei et al., in press) or as early-branching paravians (Turner et al., 2012; Godefroit et al., 2013a, 2013b; Xu et al., 2015a; Wang et al., 2019a).

TABLE 2

Scansoriopterygid fossil record

TABLE 3

Dromaeosaurid fossil record

continued

continued

Dromaeosauridae

Dromaeosaurid fossils have been found on almost all modern continental landmasses including members that appear to have had volant capabilities (Turner et al., 2012; Pei et al., in press; fig. 1; table 3).

North America: In 1922, Dromaeosaurus, from the Campanian Dinosaur Park Formation of Alberta, Canada, was the first dromaeosaurid to be described. It lends its name to the clade and is known from partial cranial and very fragmentary postcranial material (Matthew and Brown, 1922; Currie, 1995). The Dinosaur Park Formation has also yielded Saurornitholestes, a relatively completely known taxon thought to be from only one species, Saurornitholestes langstoni (Sues, 1978; Turner et al., 2012). This taxon lacked a proper diagnosis until recently and is likely represented by multiple partial skeletons. Recently, an exquisite skull and skeleton from Dinosaur Provincial Park were recovered allowing for a revised diagnosis (Currie and Evans, 2020). Evidence of tooth-marked bones and a broken tip of a tooth still embedded in a bone suggest that this taxon ate azhdarchid pterosaurs on occasion (Currie and Jacobsen, 1995). Hesperonychus elizabethae is known from a single incomplete pelvis and referred pedal bones recovered from the Dinosaur Park Formation (Longrich and Currie, 2009. This taxon is North America's only named microraptorine and the youngest one worldwide by almost 45 million years (Longrich and Currie, 2009). Atrociraptor marshalli is a fragmentary taxon recovered from the similarly aged Horseshoe Canyon Formation from the same part of Canada (Currie and Varricchio, 2004). It consists of a partial rostrum, including both premaxillae, a right maxilla, and both dentaries. The snout of this dromaeosaurid appears to be quite short and deep, given the abbreviated nature of the facial process of the maxilla. Across the border in neighboring Montana, the Campanian Two Medicine Formation is home to the relatively well-preserved dromaeosaurid Bambiraptor (Burnham et al., 2000). The holotype of Bambiraptor feinbergi is quite small and typically considered a juvenile to subadult (Currie and Varricchio, 2004; Norell and Makovicky, 2004). However, attempts at histologically sampling the single known skeleton of this taxon have been unsuccessful. It is possible that Bambiraptor is a juvenile specimen of Saurornitholestes (Burnham et al., 2000; Norell and Makovicky, 2004): both taxa lack detailed and adequate diagnoses and differ only in the length of the suborbital process of the frontal, a feature that is undoubtedly influenced by ontogeny. Furthermore, the Bambiraptor feinbergi type specimen is known to be a chimera, as there are elements of three different similarly sized lower legs included in the holotype. The youngest North American dromaeosaurids are from the Maastrichtian Hell Creek and Ojo Alamo formations of the United States. From the Hell Creek Formation: the velociraptorine Acheroraptor temertyorum, which is known from a complete right maxilla and a referred right dentary (Evans et al., 2013) as well as the significantly larger Dakotaraptor steini, which was originally described as a dromaeosaurine (DePalma et al., 2015), but recently recovered as a velociraptorine (Pei et al., in press). From the Ojo Alamo Formation: the velociraptorine Dineobellator notohesperus, which is known from fragmentary cranial and postcranial material (Jasinski et al., 2020). Deinonychus, Utahraptor, and Yurgovuchia are the oldest widely accepted dromaeosaurids from North America with an Aptian/Albian age for the former (Ostrom, 1969) and a Barremian age for the latter two taxa (Kirkland et al., 1993; Senter et al., 2012). Deinonychus and Utahraptor are known from a large amount of material, much of it undescribed (personal commun., J. Kirkland), and Utahraptor ostrommaysorum remains the largest dromaeosaurid known. Yurgovuchia doellingi is represented by associated postcranial remains. The oldest record of Dromaeosauridae in North America relates to controversial fragmentary material from the Late Jurassic Morrison Formation (Heckert and Foster, 2011). Deinonychus antirrhopus remains the best-represented dromaeosaurid from North America. It is known from at least eight partially articulated and disarticulated skeletons from the Cloverly and Antlers formations. A partial egg associated with an adult has also been recovered (Grellet-Tinner and Makovicky, 2006). The osteology of this taxon was described in detail in Ostrom's monograph (Ostrom, 1969) and has been revisited by subsequent studies (Norell and Makovicky, 1997, 1999; Norell et al., 2006).

Asia: Velociraptor, arguably the most famous dromaeosaurid, was the second dromaeosaurid to be described, in 1924 (Osborn, 1924). It is one of the best-known genera, with several complete or near-complete skeletons, and lends its name to the subfamily Velociraptorinae. Velociraptor was recovered from the Campanian Djadokhta Formation of southern Mongolia, which is among the most productive strata for dromaeosaurids anywhere on Earth. Several specimens of Velociraptor tell us much about its palaeobiology. The famous “fighting dinosaurs” appears to preserve Velociraptor attacking a large Protoceratops (Kielan-Jaworowska and Barsbold, 1972). Another specimen shows the presence of quill knobs on the ulna (Turner et al., 2007a), and yet another preserves stomach contents that include the remains of a pterosaur (Currie and Jacobsen, 1995). A second species, V. osmolskae, is known from paired maxillae and a left lacrimal described from similar rocks across the border in Nei Mongol, China (Wulansuhai (Bayan Mandahu) Formation) (Godefroit et al., 2008). This appears to be a valid taxon despite the paucity of its preserved fossil material (Turner et al., 2012). Djadokhta Formation outcrops at Ukhaa Tolgod have yielded Tsaagan mangas, a velociraptorine larger than Velociraptor (Norell et al., 2006) that is closely related to Linheraptor exquisitus, with a nearly complete holotype skeleton from the Wulansuhai Formation (Xu et al., 2010b, 2015b). The Djadokhta Formation has also yielded the earliest diverging noneudromaeosaurian dromaeosaurid Mahakala omnogovae, which is known from a partial skeleton including the back of the skull (Turner et al., 2007b; Turner et al., 2011). It was recovered from the Tögrögiin Shiree locality in Mongolia. Recent work described an additional dromaeosaurid from the Djadokhta Formation, Halszkaraptor escuilliei, and recovered it as the sister taxon to Mahakala, although parts of the sole specimen have been forged (Cau et al., 2017). Hulsanpes is another enigmatic specimen purported to be a dromaeosaurid (Osmólska, 1982). It is from the Campanian -Maastrichtian Barun Goyot Formation at the Khulsan locality in Mongolia. It consists only of a partial right metatarsus and pes (and possibly an associated braincase). Although considered a dromaeosaurid by recent analyses (Cau et al., 2017; Cau and Madzia, 2018), because of the extremely fragmentary nature of the material this identification has been repeatedly challenged (Turner et al., 2012).

The Gobi Desert has yielded a number of taxa occupying other parts of the Late Cretaceous: Achillobator from the Cenomanian-Santonian Bayan Shireh Formation (Perle et al., 1999) and Adasaurus from the Maastrichtian Nemegt Formation (Bayankhongor) of southwestern Mongolia (Barsbold, 1983). Adasaurus was only recently well figured and described (Turner et al., 2012). IGM 100/20 is the only specimen considered to be Adasaurus and is known from a partial skull and postcranial skeleton. Additional cranial and postcranial remains (IGM 100/22 and 100/23) likely pertain to a different taxon from the older Baynshiree Formation. Shanag is the only Early Cretaceous Mongolian dromaeosaurid, belonging to the Berriasian-Barremian Öösh Formation (Turner et al., 2007c).

In contrast, China has a large number of Early Cretaceous forms, but fewer Late Cretaceous ones. The Barremian-Aptian Yixian Formation and Aptian Jiufotang Formation of northeastern China, which yield part of the Jehol Biota, are home to many microraptorines, a non-eudromaeosaurian subclade that is known only from one fragmentary specimen outside Asia (Longrich and Currie, 2009). Despite their name, microraptorines were not all small and appear to be reasonably large ancestrally (Pei et al., in press). Their well-known arm and leg feathers are exemplified in the group's namesake Microraptor, where they are extremely long and are thought to have enabled volant capabilities, although this remains an area of intense study (Dyke et al., 2013; Dececchi et al., 2016; Pei et al., in press). Microraptor is from the Aptian Jiufotang Formation and is known from three species M. zhaoianus, M. gui, and M. hanqingi (Xu et al., 2000, 2003; Gong et al., 2012); however, the status of M. gui (Senter et al. 2004) and M. hanqingi have been questioned (Turner et al., 2012; Pei et al., 2014). The other known Jiufotang microraptorine is Wulong (Poust et al., 2020). The Yixian Formation has the microraptorines Changyuraptor (Han et al., 2014), Graciliraptor (Xu and Wang, 2004a), Sinornithosaurus (Xu et al., 1999), Zhongjianosaurus (Xu and Qin, 2017) and the larger seemingly early-diverging forms Tianyuraptor and Zhenyuanlong (Zheng et al., 2009; Lü and Brusatte, 2015; Pei et al., in press). Microraptorines are otherwise rare in Asia: IVPP V22530 is from the younger Aptian-Albian Bayan Gobi Formation of Nei Mongol, northern China (Pittman et al., 2015) and suspected microraptorine tracks have been discovered in the Aptian Jinju Formation of Gyeongsangnamdo, South Korea (Kim et al., 2018). Shanag is possibly a microraptorine as well, as found in some phylogenetic analyses (Gianechini et al., 2018). Luanchuanraptor, known from a partial skeleton, was discovered from the Campanian-Maastrichtian Qiupa Formation of Henan, central China (Lü et al., 2007), and a recent analysis found it closely related to its Late Cretaceous Mongolian relative Velociraptor (Pei et al., in press). Tracks of two differently sized coeval deinonychosaurs have been found in the Barremian-Aptian Tianjialoue Formation of Shandong, eastern China, but the identity of their makers remains elusive (Li et al., 2008a).

A small partial braincase forms the type of Itemirus medullaris from the Turonian Bissekty Formation of Uzbekistan, which was originally described as an earlier-diverging theropod (Kurzanov, 1976). More recently, two phylogenetic analyses have recovered it as a velociraptorine (Longrich and Currie, 2009) and dromaeosaurine (Sues and Averianov, 2014).

Europe: Variraptor was named as a dromaeosaurid from the Late Campanian–Early Maastrichtian Grès à Reptiles Formation of France (LeLoeuff and Buffetaut, 1998). However, it was shown to lack dromaeosaurid synapomorphies and was superseded by Pyroraptor (Late Campanian–Early Maastrichtian of La Boucharde, France) as the only known Late Cretaceous European dromaeosaurid taxon (Allain and Taquet, 2000; Turner et al., 2012). Prior to the discovery of Pyroraptor, only indeterminate Late Cretaceous dromaeosaurid material had been known in Europe (Allain and Taquet, 2000) from elsewhere in France (Buffetaut et al., 1986; LeLoeuff et al., 1992; LeLoeuff and Buffetaut, 1998) and from Portugal (Antunes and Sigogneau, 1992) and Romania (Weishampel and Jianu, 1996). Despite being represented by only extremely fragmentary remains, the unique biogeography of Pyroraptor and its near contemporaneousness with Late Cretaceous taxa from neighboring continents (Campanian and Maastrichtian of Provence, France) made it an important taxon (Allain and Taquet, 2000). Understanding of Late Cretaceous European dromaeosaurids dramatically increased with the discovery of Balaur, a more complete partial skeleton of an island-dwelling velociraptorine from the Maastrichtian Sebeş Formation of Alba county, Romania (Csiki et al., 2010; Brusatte et al., 2013). The animal is perhaps most distinctive for its double sickle claw on the foot, due to the unusual hypertrophy of the first pedal ungual in addition to the typically enlarged and trenchant second pedal ungual of dromaeosaurids and other deinonychosaurs. Although recently argued to be an avialan (Cau et al., 2015), its status as a velociraptorine was recently reaffirmed (Pei et al., in press).

Knowledge of Early Cretaceous European dromaeosaurids is sparse and superficial. Reexamination of historic reptilian tooth and fragmentary jaw material from the Berriasian Lulworth Formation of the U.K. led to Nuthetes being reassigned as a dromaeosaurid taxon (Milner, 2002), and then being narrowed to the subfamily Velociraptorinae (Sweetman, 2004). However, this assignment was later contested by one of the original authors as possible tyrannosaurid material instead (Rauhut et al., 2010). Six fused sacral vertebrae from the Berriasian-Barremian Wessex Formation of the U.K. form the type of Ornithodesmus cluniculus (Seeley, 1887), which probably belongs to a dromaeosaurid (Norell and Makovicky, 1997; Naish and Martill, 2007). However, this specimen has a complex taxonomic history including past identifications as a bird, pterosaur, troodontid, and earlier-diverging theropod (Anonymous, 1887; Seeley, 1887; Howse and Milner, 1993; Naish et al., 2001). Dromaeosauroides bornholmensis is a taxon known from a tooth from the Early Cretaceous of Denmark (Bonde and Christiansen, 2003).

TABLE 4

Troodontid fossil record

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Africa: Rahonavis ostromi of the Maastrichtian Maevarano Formation (Rogers et al., 2013) of Madagascar's Mahajanga Basin was first described as an avialan (Forster et al., 1998) as supported by others (Agnolín and Novas, 2013; Cau, 2018; Novas et al., 2018). However, it has also been recognized as one of the first discovered Gondwanan dromaeosaurids (Makovicky et al., 2005; Turner et al., 2012; Pei et al., in press), which we follow in this volume. A dromaeosaurid from the Albian-Cenomanian Wadi Milk Formation of Sudan (Dromaeosauridae incertae sedis (Turner et al., 2012)) is the first and only African record reaching into the Early Cretaceous (Rauhut and Werner, 1995).

South America: The discovery of Unenlagia from the Turonian-Coniacian Portezuelo Formation of Patagonia, Argentina (Calvo et al., 2007) provides strong support that dromaeosaurids were not exclusively Laurasian, but occupied Gondwana as well (Novas and Puerta, 1997). This landmark discovery was followed by recognition of a second species, U. paynemili, in addition to the original U. comahuensis from the same formation (Calvo et al., 2004) as well as the new genus Neuquenraptor (Novas and Pol, 2005). However, the latter might be a junior synonym of Unenlagia (Makovicky et al., 2005), but this remains unclear (Brissón Egli et al., 2017). Buitreraptor from the Cenomanian-Turonian Candeleros Formation of Patagonia extended the South American record of dromaeosaurids into the earliest Late Cretaceous (Makovicky et al., 2005) and also provided evidence for a monophyletic Unenlagiinae in Gondwana, while Austroraptor demonstrated that their record extended to the end of the Cretaceous (Campanian-Maastrichtian Allen Formation) (Novas et al., 2009) and solidified Patagonia, Argentina, as a hotspot for dromaeosaurid fossils. Pamparaptor is based on a deinonychosaurian foot from the Portezuelo Formation that is distinct from specimens of Unenlagia (Porfiri et al., 2011). This material has possible unenlagiine affinities, but does not nest exclusively with that clade in phylogenetic analyses (Gianechini et al., 2018). Overoraptor of the Cenomanian-Turonian Hiuncul Formation of Patagonia is known from fragmentary postcranial material (Motta et al., 2020). Described as a paravian, it was recovered as a stem avialan in a phylogenetic analysis (Motta et al., 2020). However, the closeness of its phylogenetic position to contemporaneous Patagonian unenlagiine dromaeosaurids as well as its highly modified deinonychosaurian digit II-2, suggests that Overoraptor might instead be an unenlagiine. Unquillosaurus is based on a left pubis from the Maastrichtian Los Blanquitos Formation of Patagonia (Powell, 1979). It may be a dromaeosaurid (Martínez and Novas, 2006) and was previously proposed as an indeterminate maniraptoran theropod (Novas and Agnolín, 2004) and as an earlier-diverging theropod (Powell, 1979). South American records outside Argentina are rare, but possible unenlagiine elements have been reported from the Late Cretaceous Bauru group of Brazil (Candeiro et al., 2012; Delcourt and Grillo, 2017).

Antarctica: Two isolated teeth associated with a partial left foot and fragments from the right foot from the Maastrichtian Snow Hill Island Formation of James Ross Island, Antarctica were referred to Dromaeosauridae (Case et al., 2007). These were subsequently reinterpreted as indeterminate deinonychosaurian material (Turner et al., 2012). Ely and Case (2019) have recently described this specimen as Imperobator antarcticus, and recovered it as a nondromaeosaurid paravian.

Troodontidae

Troodontids were first recognized in the late 19th century in North America and it is on that continent and in Asia where most fossils have been found (fig. 1; table 4). Troodontids are otherwise scarce and have been traditionally thought of as a Laurasian group, but a single tooth now suggests that troodontids were possibly present in Gondwana (fig. 1; table 4).

North America: The first troodontid genus Troodon was given to a tooth discovered in the Campanian Judith River Formation of Montana in the mid-19th century (Leidy, 1856). Originally thought to belong to a fossil lizard and then a pachycephalosaur, this is one of three historic North American troodontid genera, alongside Polyodontosaurus (Gilmore, 1932) and Stenonychosaurus (Sternberg, 1932). North American Campanian- and Maastrichtian-aged troodontids have experienced a prolonged period of taxonomic instability, including the role of Troodon as a wastebasket taxon (see Zanno et al., 2011, for further details) once it was recognized as a theropod (Sternberg, 1945). Campanian material referred to this genus comes from the Judith River Formation (Lawver and Jackson, 2017) as well as the Dinosaur Park (Brown et al., 2013) and Oldman formations of Alberta, Canada (Ryan and Russell, 2001), the Two Medicine Formation of Alberta, Canada, and Montana (Foreman et al., 2008), the Kaiparowits and Wahweap formations of Utah (Eaton et al., 1999), the El Gallo Formation of Baja California, Mexico (Weishampel et al., 2004) and the Campanian-Maastrichtian Wapiti, Horseshoe Canyon and St. Mary River formations of Alberta, Canada (Ryan et al., 1998; Ryan and Russell, 2001). The Two Medicine material includes eggs, some with embryos, and nests (Varricchio and Jackson, 2016) as well as skeletons. Troodon has been reported from Maastrichtian strata including the Ferris Formation of Wyoming (Lillegraven and Eberle, 1999), the Hell Creek Formation of Montana, Wyoming, North Dakota, and South Dakota (Fastovsky and Bercovici, 2016), the Prince Creek Formation of Alaska (Fiorillo et al., 2016), the Lance Formation of Wyoming (Carpenter, 1982), and the Scollard Formation of Alberta, Canada (Weishampel et al., 2004). Troodon has even been assigned to material from the Lower Cretaceous Dakota Formation of Utah (Eaton et al., 1999), although this rock unit, now known as the Naturita Formation, has been reassigned to the early Late Cretaceous (Tucker et al., 2020). Material from the Dinosaur Park Formation has been assigned a different species name, T. inequalis, from the original T. formosus (Currie, 2005). The discovery of Talos, a partial postcranial skeleton from the Campanian Kaiparowits Formation of Utah, provided a chance to reappraise North American troodontid material, which led to the suggestion that Troodon is a nomen dubium and support for the genus Pectinodon (Longrich, 2008; Zanno et al., 2011). The latter, known from teeth and juvenile skeletal material from the Maastrichtian Lance Formation of Wyoming, was originally described as an additional species of Troodon, T. bakkeri (Carpenter, 1982). Continued efforts to address the taxonomic confusion arising from North America's problematic, highly fragmentary historic holotypes led to the resurrection of the genus Stenonychosaurus for some troodontid skeletal material from the Dinosaur Park Formation (Evans et al., 2017). This analysis was supported by subsequent work that assigned some of this Stenonychosaurus material to the new genus Latenivenatrix (van der Reest and Currie, 2017). Albertavenator was named from a distinctive partial left frontal recovered from the Maastrichtian Horseshoe Canyon Formation of Alberta, Canada (Evans et al., 2017). “Saurornitholestes” robustus from the Campanian Kirtland Formation of San Juan Basin, New Mexico, is an indeterminate troodontid frontal (Evans et al., 2014), originally referred to a new species of the dromaeosaurid Saurornitholestes (Sullivan, 2006). Geminiraptor, an incomplete maxilla from the Cedar Mountain Formation of Utah is arguably one of the most important North American troodontid specimens because, as the only Early Cretaceous record, it provides a crucial point of comparison with better-known Chinese contemporaries (Senter et al., 2010). A tooth that is the holotype of Koparion (Chure, 1994), and the partial articulated skeleton that forms the type of Hesperornithoides miessleri (Hartman et al., 2019) are possible Jurassic troodontid records, both from the Morrison Formation of the western United States.

Asia: The Gobi Desert of Mongolia provided the first Asian record of troodontids: Saurornithoides from the Campanian Djadokhta Formation of southern Mongolia (Osborn, 1924). Its reasonably complete skull and partial postcranium was particularly important in the early days of troodontid research. This animal was known from one species, S. mongoliensis, that was later joined by a second species, S. junior, from the younger Maastrichtian Nemegt Formation (Barsbold, 1974), although S. junior is now ascribed to Zanabazar (Norell et al., 2009). Other Djadokhta taxa include Byronosaurus, which is known from a large amount of cranial material and some postcranial material (Norell et al., 2000; Makovicky et al., 2003) including, perhaps, two perinates (Bever and Norell, 2009; but see Pei et al., 2017a). Gobivenator and Almas are well-preserved, recently described specimens from this formation, with Gobivenator one of the best three-dimensionally preserved troodontids in existence (Tsuihiji et al., 2014; Pei et al., 2017a). Linhevenator tani, known from a partial, eroded skeleton, was discovered from the similar Campanian Wulansuhai (Bayan Mandahu) Formation across the border in Nei Mongol, northern China (Xu et al., 2011b, 2012b). A single leg from the same formation was originally identified as a juvenile Saurornithoides specimen (Currie and Peng, 1993) and was later assigned to the new taxon Philovenator (Xu et al., 2012b). Mongolia and Russia provide the latest Cretaceous records. Borogovia and Tochisaurus are known from fragmentary hind-limb elements (Osmólska, 1987; Kurzanov and Osmólska, 1991), and like Zanabazar, were recovered from the Maastrichtian Nemegt Formation of southern Mongolia. “Troodon” records from the Maastrichtian Kakanaut and Udurchukan formations of Russia are expected to belong to one or more new genera given the recent revisions to Troodon taxonomy in North America (Averianov and Sues, 2007; Zanno et al., 2011; Evans et al., 2017; van der Reest and Currie, 2017). A single tooth from the Maastrichtian Kallamedu Formation of India potentially represents the only troodontid record from Gondwana (Goswami et al., 2013), despite the group being known for over 150 years. Occurrences from China and Uzbekistan extend the Asian troodontid record back into the earliest Late Cretaceous as well as the Early Cretaceous, providing the only described taxa from these time intervals worldwide. Xixiasaurus is from the Coniacian-Santonian Majiacun Formation (Lü et al., 2010) of Henan, China, and Urbacodon is from the Cenomanian Dzharakuduk Formation of Navoi Viloyat, Uzbekistan (Averianov and Sues, 2007). The Early Cretaceous troodontid record of Asia is well represented in China by at least eight named genera. The oldest record is Jinfengopteryx from the Hauterivian-Barremian Huajiying (Qiaotou) Formation of Hebei, China, that was originally described as an avialan and whose stomach may contain preserved seeds (Ji et al., 2005; Pan et al., 2013). Sinovenator, Mei, Sinusonasus, Daliansaurus, Liaoningvenator, and Jianianhualong were all discovered from the Barremian-Aptian Yixian Formation of northern China (Xu et al., 2002b, 2017; Xu and Norell, 2004; Xu and Wang, 2004b; Pan et al., 2013; Chang et al., 2017; Shen et al., 2017a, 2017b). This formation and the Djadokhta Formation represent the most important sources of troodontid material globally. Sinovenator was the first troodontid reported from the Yixian Formation (Xu et al., 2002b). Initially represented by a partial skull and a few incomplete postcranial skeletons (Xu et al., 2002b), later material included a partial skull with a well-preserved braincase (Yin et al., 2018). Mei was first described on the basis of an exquisitely-preserved skeleton with a bird-like sleeping posture, which is arguably the most complete Early Cretaceous troodontid specimen known (Xu and Norell, 2004; Pan et al., 2013). Sinusonasus, Daliansaurus, and Liaoningvenator all have a similar size as Sinovenator, and each of them were reported from a single, near complete skeleton (Xu and Wang, 2004b; Shen et al., 2017a, 2017b). Jianianhualong is only known from a single flattened specimen from the Yixian Formation, which preserves the first record of asymmetrical feathers in troodontids (Xu et al., 2017). Sinornithoides is known from the Aptian-Albian Ejinhoro Formation of Nei Mongol, China, based on a near complete skeleton (Russell and Dong, 1993). Across the border in Mongolia, unnamed Early Cretaceous troodontids have been reported, including the well-known “Early Cretaceous troodontid” MPC-D 100/44 and MPC-D 100/140 (Barsbold et al., 1987; Tsuihiji et al., 2016).

TABLE 5

Mesozoic avialan fossil record

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In northeastern China, the Middle Jurassic Bathonian Haifanggou Formation yields Pedopenna (Xu and Zhang, 2005), while the Late Jurassic Oxfordian Tiaojishan Formation yields Anchiornis, Auronis, Caihong, Eosinopteryx, Serikornis, and Xiaotingia (Xu et al., 2008, 2011a; Godefroit et al., 2013a, 2013b; Lefèvre et al., 2017; Hu et al., 2018). These taxa have been proposed as members of the Anchiornithinae, a controversial clade of long-tailed, early-diverging paravians. Anchiornithines were first described as early avialans, but their phylogenetic placement lacks consensus (Hu et al., 2009; Lee and Worthy, 2011; Xu et al., 2011a; Agnolín and Novas, 2013; Godefroit et al., 2013a, 2013b; Foth and Rauhut, 2017; Lefèvre et al., 2017; Pei et al., 2017b, in press). If they are troodontids as recovered in some recent works, they would be the oldest fossils of these animals (Hu et al., 2009). Hesperornithoides, from the Morrison Formation of Wyoming, is also a potential Jurassic troodontid (Hartman et al., 2019). Two Early Cretaceous taxa from the Yixian Formation, Liaoningvenator and Yixianosaurus, have been recovered as anchiornithines and are the only anchiornithine taxa besides Ostromia (previously an archaeopterygid) that have been found outside the Tiaojishan Formation (Cau et al., 2017; Foth and Rauhut, 2017; Shen et al., 2017b). However, the phylogenetic placement of Yixianosaurus remains controversial (Dececchi et al., 2012; Xu et al., 2013a; Cau et al., 2017; Foth and Rauhut, 2017; Lefèvre et al., 2017).

DISCUSSION

The pennaraptoran fossil record has expanded phenomenally since they were first discovered in the mid 19th century, bringing about huge leaps in our understanding of the group. Archaeopterygiformes, anchiornithines, and scansoriopterygids tell us that the clade had originated by the Late Jurassic, with other pennaraptoran groups either unknown at that time as in oviraptorosaurians (Osmólska et al., 2004) or based on fragmentary specimens as in dromaeosaurids (Heckert and Foster, 2011). The anchiornithines are the best represented group of Jurassic paravians after Archaeopterygiformes, but their status as birds, troodontids, early-diverging deinonychosaurs, or sister to Paraves remains controversial, even though consensus for their near-avialan status is emerging (Pei et al., in press). The search for Jurassic-aged pennaraptorans should therefore remain a priority moving forward, both from lagerstätten that have recovered them already, like the Solnhofen Limestone of southern Germany and the Tiaojishan Formation of northern China, and from new localities. Although Konservat Lagerstätten are few and far between, it is heartening to note that new exposures containing Jehol and Yanliao biota fossils are cropping up across northern China with increasing collection efforts. They are also present, but undersampled in Mongolia, where only a few feathers have been excavated (Kurochkin, 2000). In general, the uneven nature of the pennaraptoran record, which is biased toward key formations like the Yixian and Djadokhta, needs to be better counterbalanced to ensure our understanding of this group is not being biased by potential local or regional factors. This is easier said than done, but a healthy awareness of this issue will at least help to minimize any chance of conflating separate evolutionary signals.

Oviraptorosauria: Tooth-bearing early-diverging oviraptorosaurians like Incisivosaurus and Caudipteryx remain rare and oviraptorosaurians with more ancestral theropod body plans are expected in the Late Jurassic, but have not been found. This is potentially the most important priority for future work because it should shed more light on the evolution of the beak and the changes involved in skull specialization. Future finds of later-diverging taxa that could better characterize the caenagnathid and oviraptorid split would also be very useful, especially as early-diverging caenagnathids are not known from complete cranial material and include giant, evidently specialized forms like Gigantoraptor (Ma et al., 2017). Oviraptorosaurians and Scansoriopterygidae are currently exclusively Laurasian groups, but experience in other pennaraptoran groups, including those with a longer collection history, suggests that future Gondwanan finds are possible. Thus, efforts to seek such material whether in the field or in existing collections could be fruitful.

Scansoriopterygidae: Scansoriopterygids are among the least known early-diverging pennaraptorans because of their representation by a small pool of specimens. More specimens, particularly from adult growth stages, will be critical in solidifying the taxonomic status of the group and uncovering key events in their evolutionary history as well as their correct phylogenetic position. Yi preserves feathered, membranous wings that appear to be an alternative dinosaurian volant strategy to feathered, muscular wings (Xu et al., 2015a; Wang et al., 2019a). This astonishing discovery warrants extensive further study that will require additional soft-tissue-preserving specimens. Further discoveries are also needed to determine whether scansoriopterygids were a short-lived experiment or they persisted to the terminal Cretaceous like oviraptorosaurians.

Dromaeosauridae: Dromaeosaurids are among the most widely distributed pennaraptorans after birds. This, coupled with their generally more ground-based lifestyle compared with early birds (early birds could probably cross barriers more easily), provides the best opportunity to understand the impact of Mesozoic biogeography on pennaraptoran evolution. This is examined in the next chapter on coelurosaurian biogeography. Encouraging potential for further finds in underrepresented parts of Gondwana, e.g., the Wadi Milk Formation of Sudan and James Ross Island, Antarctica, underscores the importance of Dromaeosauridae in understanding pennaraptoran biogeography more generally. The reconstruction of flight capabilities in Microraptor makes microraptorines an obvious subclade to focus more attention on (Pei et al., in press). However, the unenlagiine Rahonavis also has similar flight potential, and so this clade should also be studied more intensively, especially given that it represents the only detection of nonavialan flight potential in Gondwana (Pei et al., in press).

Troodontidae: Troodontids are thought to be a Laurasian clade, but the discovery of a possible troodontid tooth from the Kallamedu Formation of India (Goswami et al., 2013) justifies further search efforts to confirm this Gondwanan record and explore biogeographic differences among troodontids in more detail (see Ding et al., chapter 4). The taxonomic status of Anchiornithinae should be another study priority and will benefit from Jurassic nonavialan paravian finds, particularly from the Solnhofen and Tiaojishan as well as the sparse Early Cretaceous of North America. The discovery of more troodontid specimens with transitional anatomical features between longer-armed earlier-diverging forms and shorter-armed later-diverging forms (e.g. Sinusonasus and Jianianhualong (Xu et al., 2017)) would also shed more light on troodontid character evolution.

Avialae: Despite the incredible number of new specimens unearthed within the past four decades, there remain numerous major gaps in the fossil record of stem avialans. There is a 20 million year gap in the record between the 155–150 Ma Archaeopteryx and the beginning of the Jehol avifauna captured by the 131 Ma Huajiying Formation. Specimens from this 20 Ma gap are critical to understanding early skeletal transitions such as the evolution of the pygostyle and the evolution of the first avialan edentulous beak, let alone a host of other features like solidification of the pectoral girdle and plumage specialization. Notably, non-ornithothoracines are almost exclusively found in the Solnhofen limestones (Archaeopterygiformes) and in the Early Cretaceous Jehol lagerstätten, which may suggest early-diverging lineages went extinct fairly early, being unable to compete with ornithothoracines. No Mesozoic avialan has been collected from the African continent, despite its great potential (although remains are known from Madagascar). Globally, the Early Cretaceous record is far stronger than the Late Cretaceous record (mostly due to the Jehol Biota), but there are currently no Early Cretaceous avialan fossils known from North America. A major gap in the avialan fossil record consists of the conspicuous absence of fossils documenting the crownwardmost portion of the avian stem lineage, i.e., crownward of the Late Cretaceous ornithurine groups Hesperornithiformes, Ichthyornithiformes, and Iaceornis. Similarly, the earliest stages of crown-bird evolution are poorly known at present, and many of the greatest questions regarding the early evolutionary history of Aves will be resolved by new discoveries of crown birds from the Late Cretaceous and early Paleogene, including questions related to avialan survivorship, ecological selec- tivity, and recovery across the end-Cretaceous mass extinction. It is hoped that the coming years will yield avialan fossils filling the critical temporal and geographic gaps discussed above—and in the process, shed important new light on the Mesozoic and Cenozoic evolutionary history of avialan pennaraptorans.