Tritylodontidae was a successful advanced cynodont clade with a close relationship to mammals, but falling outside the clade Mammaliaformes. Stereognathus ooliticus was the first tritylodontid to be named and described in 1854, but since then no comprehensive description for this species has been produced. A second species, S. hebridicus, was named in 1972 and diagnosed based solely on size difference, being larger than the S. ooliticus holotype. We reexamined all postcanine tooth material attributed to the genus Stereognathus to test the species diagnosis and identify diagnostic morphological characters for this genus. We find no statistical difference in size distribution between S. ooliticus and S. hebridicus postcanine specimens. Specimens previously attributed to the different species fall along an ontogenetic spectrum of size, with no clear clustering. Morphologically, we affirm many previously described features for Stereognathus and identify new morphological features in upper and lower postcanines. We find no morphological features to distinguish these two species, and therefore we synonymize S. hebridicus under S. ooliticus. We reevaluate the scoring of S. ooliticus in previous phylogenetic analyses, generating a new tree using rescored Stereognathus characters. Finally, we suggest that similar reevaluations and redescriptions of other poorly described tritylodontid material are necessary to further clarify relationships among Tritylodontidae and the evolution of characters in derived genera such as Stereognathus.

Tooth attachment and implantation are two classical descriptors of dental anatomy. Tooth attachment distinguishes between teeth that are either fused to the jaw by bone, or suspended within a socket by a periodontal ligament. Tooth implantation describes the geometry of this attachment and has been broadly divided into acrodonty, pleurodonty, and thecodonty. Among extant amniotes, only mammals and crocodilians are considered truly thecodont, because they possess a complex attachment system that includes a periodontal ligament and true tooth sockets. These two amniote groups diverged from a common ancestor over 300 million years ago and are thought to have evolved thecodonty independently. This view has recently come under a great deal of scrutiny with the discovery of complex tooth attachment systems, including the presence of a ligamentous tooth attachment in numerous non-mammalian, non-crocodilian amniotes. This has spurred debate and inconsistencies over the conventional usage of tooth attachment and implantation categories and their evolutionary significance. We provide a comparative histological approach for describing tooth attachment and implantation in an exemplary, traditionally thecodont taxonomic group: the non-avian dinosaurs. The comparisons between theropod, hadrosaurid, and ceratopsid teeth show that all dinosaurs have ligamentous tooth attachments composed of identical dental tissues to those in mammals and crocodilians, but they show a diverse array of tooth attachment geometries, replacement modes, and bone architectures supporting the dentition. The methodology we follow allows researchers to tease apart phylogenetically and functionally significant features of tooth attachment and implantation that could be used in future studies.

Hystricognathous rodents are among the most common members of African mammal faunas of the mid-Paleogene, but their record has so far remained limited to North and northeast Africa. Here we report the first Paleogene record of hystricognaths from the Atlantic margin of North Africa. The fossils come from the westernmost part of the Sahara, east of the Dakhla peninsula, Morocco, from estuarine deposits dating to the earliest Oligocene (Dakhla level C₂ [DAK C₂]). Several tens of isolated teeth plus three jaw fragments document seven species of hystricognaths (Gaudeamus cf. aslius, G. cf. hylaeus, Phenacophiomys occidentalis, gen. et sp. nov., Birkamys aff. korai, Mubhammys atlanticus, sp. nov., Neophiomys minutus, sp. nov., and ?Phiocricetomys sp.). Despite the extensive east-west geographic distance, the majority of hystricognath taxa recorded in DAK C₂ document primarily close relatives of taxa that are known from a latest Eocene Egyptian locality (L-41) and from early Oligocene localities of both Egypt and Libya. This highlights the widespread eastwest distribution of hystricognaths across North Africa, a distribution that reflects the existence of roughly similar tropical environmental conditions in northern latitudes of Africa at that time. The presence of seven hystricognath species plus five anomaluroid species in sympatry during the earliest Oligocene demonstrates that rodents were particularly diverse near the global cooling recorded at the Eocene-Oligocene transition. We describe and compare the species and new species of hystricognaths with their sub-coeval counterparts from northern and northeastern Africa and then discuss the paleobiogeographic and paleoenvironmental implications of that discovery.

The Pliocene fossil record of Australia has revealed the oldest known occurrences of modern peramelemorphian (bandicoot and bilbies) genera such as Perameles, cf. Peroryctes, and Chaeropus. Recent phylogenetic analyses based on morphology have questioned the previously accepted understanding about generic relationships of some of these Pliocene taxa. These doubts limit our ability to develop independent divergence models based on molecular data because they depend on fossil records in order to calibrate minimum rates of change. Hence, there is a need to critically review the Pliocene fossil record. To this end, we have examined Pliocene specimens of peramelemorphians in museum collections across Australia and performed a comprehensive phylogenetic analysis based on dental and cranial morphology to review and accordingly revise subfamilial taxonomy. As part of this revision, we describe here two new species, one from the Chinchilla Local Fauna (LF) in Queensland (Perameles wilkinsonorum, sp. nov.) and one from both the Big Sink LF in the Wellington Caves area and the Bow LF in New South Wales (Silvicultor karae, gen. et sp. nov.). We reassign the two ‘Peroryctes’ species from Hamilton LF, Victoria, to a new genus (Silvicultor). We summarize the distribution of peramelemorphians during the Pliocene and show how climate change appears to have shaped their subsequent Quaternary distribution.

Fragmentary cervical vertebral elements of a gigantic pterosaur are described from the upper Campanian—Maastrichtian Nemegt Formation in the Gobi Desert. With an estimated width of a posterior centrum across the postexapophyses of 198 mm, this taxon represents one of the largest pterosaurs currently known. This is the first discovery of a pterosaur from the Nemegt Formation, adding further evidence that gigantic pterosaurs were widely distributed in Eurasia and North America during the latest Cretaceous.

The moment arm of a muscle defines its leverage around a given joint. In a clinical setting, the quantification of muscle moment arms is an important means of establishing the ‘healthy’ functioning of a muscle and in identifying and treating musculoskeletal abnormalities. Elsewhere in modern animal taxa, moment arm studies aim to illuminate adaptions of the musculoskeletal system towards particular locomotor or feeding behaviors. In the absence of kinematic data, paleontologists have likewise relied upon estimated muscle moment arms as a means of reconstructing musculoskeletal function and biomechanical performance in fossil species. With the application of ‘virtual paleontological’ techniques, it is possible to generate increasingly detailed musculoskeletal models of extinct taxa. However, the steps taken to derive such models of complex systems are seldom reported in detail. Here we present a case study for calculating three-dimensional muscle moment arms using Stegosaurus stenops Marsh, 1887, to highlight both the potential and the limitations of this approach in vertebrate paleontology. We find the technique to be mostly insensitive to choices in muscle modeling parameters (particularly relative to other sources of uncertainty in paleontological studies), although exceptions do exist. Of more concern is the current lack of consensus on what functional signals, if any, are contained within moment arm data derived from extant species. Until a correlation between muscle moment arm and function can be broadly identified across a range of modern taxa, the interpretation of moment arms calculated for extinct taxa should be approached with caution.

The validity of the parvipelvian ichthyosaur genus Protoichthyosaurus was evaluated by examining the type material and recently recognized specimens. Protoichthyosaurus has a wide forefin with at least five primary digits, an anterior digital bifurcation in the forefin, a humerus that is nearly equal in width distally and proximally, and a coracoid with welldeveloped anterior and posterior notches, features it shares with the more common genus Ichthyosaurus. Previously, Protoichthyosaurus was synonymized with Ichthyosaurus because of an incomplete understanding of forefin variation in the latter taxon. However, Protoichthyosaurus has several features in the forefin that distinguishes it from Ichthyosaurus, including the presence of three elements in the distal carpal row, a contact between distal carpal 3 and the ulnare, a bifurcation of distal carpal 2 to form two elements in the metacarpal row, the proximal element of the bifurcation nearly separating distal carpal 2 from distal carpal 3, distal carpal 3 separating the intermedium from contact with distal carpal 4, and the absence of digit V. A phylogenetic analysis confirms that Protoichthyosaurus is the sister taxon of Ichthyosaurus and is nested in Ichthyosauridae. We recognize two species, P. prostaxalis and P. applebyi, sp. nov., but not P. prosostealis. Protoichthyosaurus is unequivocally from the Hettangian but might be present in the Rhaetian to lower Sinemurian. The genus is geographically widespread across the U.K.

Upper and lower tritylodontid postcanine teeth from the Middle Jurassic (Bathonian) Itat Formation of the Berezovsk coal mine, Krasnoyarsk Territory, Russia, are referred to Stereognathus sp. The genus Stereognathus Charlesworth, 1855 (Polistodon He and Cai, 1984, Xenocretosuchus Tatarinov and Mashenko, 1999, and Montirictus Matsuoka et al., 2016, are new synonyms), is similar to Bocatherium and differs from all other tritylodontids in having an upper postcanine cusp formula of 2:2:2. It differs from Bocatherium in the quadrangular shape of the upper postcanine teeth. Stereognathus also differs from all other tritylodontids in having a more elaborate interlocking system in the upper postcanine teeth, each of which bears a mesial cingulum connecting cups B0, M0, and L0. Stereognathus shares with Bienotheroides single-rooted lower postcanine teeth, and these two genera are closely related. Discovery of Stereognathus in the Itat Formation underlines the similarity of the vertebrate assemblage from this stratigraphic unit to the British Bathonian vertebrate assemblages and the Early Cretaceous vertebrate fauna from the Ilek Formation in Siberia. The presence of Stereognathus during an ∼40 Ma time interval in Siberia suggests a long stasis in the evolution of the vertebrate faunas of this region.

The ankle bone assembly of pterosaurs has received little attention, even though it is critical for understanding the functional morphology of the leg and the foot and has far-reaching implications for interpretations of stance and gait in ornithodirans in general, as well as for any role the leg may have had in the flight of pterosaurs. Of particular importance are the distal tarsal bones, which are seldom preserved clearly. Their concave proximal facets articulate with the medial and lateral condyles (comprising the astragalus and, at least basally, the calcaneum) of the tibiotarsus. Distally, they articulate with metatarsals II–IV, and the relatively large metatarsal V articulates on the distolateral side of the lateral distal tarsal. The homology of these bones in pterosaurs can be established with reference to other early-branching ornithodirans, and the morphology of the bones implies similar functional roles and ranges of motion. The medial distal tarsal is likely the fusion of distal tarsals 2 3, and the lateral distal tarsal is distal tarsal 4, a pattern reflected in ontogeny. The pterosaur ankle was capable of plantarflexion, but adduction and abduction of the feet were greatly limited. A synoptic survey of available tarsal bones of pterosaurs shows that the morphology of these bones remained relatively unchanged from the most basal pterosaurs to the pteranodontids and the azhdarchoids. Comparisons among a variety of ornithodirans show that the basic functional pattern did not vary importantly, although some ornithodiran subgroups evolved unique schemes of development and sequential ossification.

Lystrosaurus is iconic for surviving the Permo-Triassic Mass Extinction and becoming the most abundant terrestrial vertebrate during the Early Triassic. Previous reports of skeletal remains of the Triassic species being found in fossilized burrows hint at a possible reason for its success, but unequivocal evidence showing that Lystrosaurus individuals were the burrow makers was lacking. I present here the first articulated skeleton of Lystrosaurus in a fossilized burrow from the Lower Triassic of the South African Karoo Basin, along with taphonomic evidence indicating that this individual was the burrow maker. The species is identified as L. curvatus, the only Lystrosaurus species recovered from above and below the inferred Permo-Triassic extinction horizon. It provides the first evidence of burrowing in a Permian species of Lystrosaurus, suggesting that this behavior was more prevalent than previously thought. Based on its size, the specimen is inferred to be a juvenile, showing that Lystrosaurus was capable of excavating burrows at young ontogenetic stages. The abundance of Lystrosaurus body fossils and similar-sized burrows from Lower Triassic strata suggests that burrowing played a pivotal role in the success of this genus in harsh, unpredictable postextinction conditions. Given the abundance of these burrows throughout the Lower Triassic Lystrosaurus Assemblage Zone, Lystrosaurus may have acted as an ecosystem engineer and refuge provider for other species, which may help to explain the high species diversity in the lowermost Triassic in the Karoo Basin.

Odontocetes first appear in the fossil record during the early Oligocene and soon after show a near-worldwide distribution. However, little is still known of their early diversity, especially in the North Pacific Region. A new taxon of stem odontocete with heterodont dentition, Olympicetus avitus, gen. et sp. nov., is described herein from the upper Oligocene Pysht Formation in Washington State, U.S.A. The material consists of two partial skulls belonging to a juvenile and a neonate and includes part of the dentition and a tympanic bulla. Olympicetus shares many similarities with the Oligocene odontocetes Simocetus rayi and Ashleycetus planicapitis but displays a combination of characters that distinguish it from these and other coeval taxa. A phylogenetic analysis shows Olympicetus in a polytomy with Xenorophidae, a clade of early high-frequency-hearing odontocetes mainly known from the western Atlantic. Furthermore, Olympicetus displays characters that can be considered as structural proxies for echolocation, such as having premaxillary sac fossae and the maxilla expanded over the frontal. In addition to Olympicetus, other marine tetrapods from the Pysht Formation include marine birds, desmostylians, pinnipeds, and early mysticetes, giving us a unique insight into late Oligocene marine tetrapod faunas of the North Pacific.