Early chondrichthyans (cartilaginous fishes) possess a rich record of scale fossils that have impacted on taxonomic and phylogenetic studies of early jawed vertebrates, but the significance of these scales remains elusive due to the lack of well-defined characters. Gualepis, a scale-based genus from the Lower Devonian of Yunnan, China, was initially recognized as an early chondrichthyan of uncertain affinity. We investigate new material of Gualepis elegans from the type locality. By combining microtomographic data with morphological and histological examination of specimens at successive growth stages, we have reconstructed the 3D growth pattern of the scales. During growth, Gualepis scales added new odontodes appositionally, which has previously been documented in the dermal skeletons of Seretolepis, Kathemacanthus, Parexus, and other crownward members of the stem chondrichthyans. Phylogenetic analysis resolves Gualepis as a non-acanthodid stem chondrichthyan near the crown chondrichthyan node. Our 3D approach is potentially useful to render more reliable scale-based characters, especially regarding scale growth patterns. This study provides the first 3D virtual growth model of stem chondrichthyan scales with a Seretolepis-type morphogenesis, offering new insights into the early evolution of chondrichthyan dermal skeletons.

The Carboniferous is characterized by drastic climatic and environmental fluctuations, which include multiple phases of glaciation resulting in an icehouse climate. Additionally, dynamic continental reconfigurations forced the contraction of the Rheic Ocean resulting in the closure of the Rheic–Tethyan Gateway, which precluded further faunal exchanges between the North American and Eurasian marine realms. Interestingly, cartilaginous fishes seem to be relatively immune to these drastic climatic and environmental changes. The Eurasian fossil record of Paleozoic sharks is strongly biased towards intensively sampled localities from England, Ireland, Scotland, and the Russian Platform. Here we present rare dental material from the Serpukhovian (early Carboniferous) of Austria, adding new information to the paleogeographic distribution of ctenacanthiform sharks. The new material revealed the first record of the genus Saivodus in Central Europe and allowed us to recognize a new species, Cladodus gailensis sp. nov., and a remnant of fossilized cartilage. In an attempt to identify possible linkages between climatic or environmental fluctuations on shark diversity throughout the Carboniferous, we provide a synopsis of the distribution and diversity of elasmobranchs based on primary literature. This preliminary assessment at genus level indicates two pronounced events of extinction, with the first one occurring during the latest Mississippian and the second one towards the end of the Pennsylvanian. The first extinction event distinctly correlates with the known diversity decline of other marine inhabitants and the second occurred during an unstable period of multiple phases of glaciation.

The Xenacanthiformes from Central and Eastern European deposits have been extensively studied, but the systematics of the species from the French Carboniferous–Permian Autun Basin (Saône-et-Loire) remains debated. Numerous xenacanthiform remains are still identified under the doubtful genus ‘Expleuracanthus’, and many of them consist of isolated dorsal spines which are difficult to identify. Numerous well-preserved specimens are still undescribed and the diversity of the xenacanthiform fauna from the Autun Basin is poorly understood. For example, specimens of the genus Triodus from the Muse oil-shale bed (OSB) of the Autun Basin have no specific attribution, whereas this genus is widely distributed across European Carboniferous–Permian basins. In this study, we describe new specimens of Triodus from the lowermost Permian of the Muse OSB. They allow the erection of a new species, Triodus aeduorum sp. nov., and discussion of the validity of several species from the same locality: ‘Expleuracanthus’ frossardi is considered as a nomen dubium and other Triodus specimens need for the time being to be left in open nomenclature as Triodus sp. These results highlight the endemism of the Triodus species in each European Carboniferous–Permian basin and raise the question of how they migrated from one to another.

Phoebodont-like teeth originally described as Phoebodus brodiei and Phoebodus keuperinus from the Upper Triassic of England and Germany, respectively, are attributed to a new genus Keuperodus of the family Jalodontidae. The characteristic features of all jalodontid genera are re-examined leading to the conclusion that this group formed a separate, distinct chondrichthyan evolutionary line for which a new order, the Jalodontiformes is established. The jalodontid dentition evolved from a homodont-like type to one which was markedly heterodont.

Basking sharks are presented in the modern fauna by a single species (Cetorhinus maximus), although they have been much more diverse in the past. This group of sharks first appeared in the fossil record in the middle Eocene of the Antarctic and the U.S.A., but most of the described extinct taxa are known from the Oligocene and lower Miocene of Europe. Gill rakers are the most abundant among basking shark remains and their morphological details play an important role in species diagnostics. The shape of isolated gill rakers from 16 Oligocene localities of Poland was analyzed using various morphological approaches, including geometric morphometrics. Results indicate that descriptive characters have a wide range of variation and low diagnostic value, and they are associated directly with the position of gill rakers on the gill arch. Morphological indices describe proportions by discrete structures and could be effective only in the identification of the stratigraphically most distant taxa. Geometric morphometrics revealed significant differences between all of the species designated earlier except for †Caucasochasma zherikhini and †Keasius parvus. At the same time, considering the obtained results on morphology along with the geographic distance, Oligocene basking sharks from Poland should be assigned to †K. parvus. Geometric morphometrics of gill rakers supports the taxonomic distinctness of both †K. rhenanus and †K. septemtrionalis. Study results indicate that reliable taxonomic attribution of extinct basking sharks by the shape of gill rakers, in the absence of other skeletal elements (teeth and/or vertebrae), should be sample-based using multivariate approaches.

The Miocene fossil fish †Pentaceros sakhalinicus , (Perciformes, Pentacerotidae) is redescribed based on the holotype and numerous additional materials collected on Sakhalin Island (western Pacific, Russian Far East) at the end of the 20th century. The general appearance and body proportions, as well as the presence of multiple (more than three) anal-fin spines, make the Miocene taxon similar to members of the family Pentacerotidae, whereas generalized morphological characters such as the presence of three supraneurals and the absence of rugosity of the superficial head bones differentiate this species greatly from any known pentacerotid genus. Thus, the new genus †Portentosoceros is erected for this fish, which was considered the only known fossil member of the family. Due to the fact that this fish lacks the main synapomorphic characters of the Pentacerotidae, †Portentosoceros gen. nov. is left incertae sedis within the Percoidei, though probably related to Recent armorheads. The observed variability in the number of analfin spines (from four to six) in †Portentosoceros sakhalinicus is extremely unusual.

Coelacanths are represented today by a single genus, Latimeria. They are known to be ‘living fossils’, because their evolutionary history dates back into the Devonian. Through the Upper Triassic, coelacanths are represented by two families, the Latimeriidae and Mawsoniidae. While latimeriids represent their marine lineage, mawsoniids were, until recently (), thought to be exclusively freshwater inhabitants. Through the Late Triassic, mawsoniids originated in freshwater environments of America, and only little is known about their evolution in Europe during this time period. Here we report on two morphotypes of mawsoniid basisphenoids from the marine Rhaetian (Upper Triassic) Bonenburg locality (Kreis Höxter, North Rhine-Westphalia, Germany) showing that (1) Late Triassic coelacanths from Germany can be assigned to the family level, specifically to Mawsoniidae, (2) mawsoniid coelacanths were already present in Europe in the Late Triassic, and (3) these inhabited marine environments at the end of the Triassic, supporting the affinities of habitat preference of previous finds from France (). The results strengthen the hypothesis an intermediate marine phase between the origin of Mawsoniidae in freshwater environments of nowadays North America during the Late Triassic and their final occurrence in continental deposits of Western Gondwana (Africa and South America) and nowadays Europe in the Late Cretaceous ().

The Early Carboniferous stem tetrapod Whatcheeria deltae is among the earliest-branching limbed tetrapods represented by multiple near-complete specimens, making it an important taxon in understanding the vertebrate water-to-land transition. However, all preserved skulls of Whatcheeria suffer from post-mortem crushing and lateral compression, which has made cranial reconstruction problematic. In this study, computed tomography data of three Whatcheeria specimens were segmented using visualization software to digitally separate each individual skull bone from matrix. Digital methods were used to repair and retrodeform the bones and produce the first complete three-dimensional skull reconstruction of Whatcheeria. We provide a revised description of the cranial and lower jaw anatomy of Whatcheeria based on CT data, focusing on sutural morphology and previously unknown anatomical details. Our findings suggest that Whatcheeria had one of the narrowest skulls of any known early tetrapod, a gap between the nasals, and significant overlap of the lacrimal onto the nasal and prefrontal. Sutural morphology is used to infer loading regime in the skull during feeding and suggests the skull of Whatcheeria was well adapted to resist stresses induced by biting large prey with its enlarged anterior fangs.

Baenidae are the most diverse clade of turtles from the latest Cretaceous and earliest Paleogene of North America. Palatobaena species have a distinctive cranial bauplan within baenid turtles and we herein describe a new species, P. knellerorum, based on a complete cranium from the lower Paleocene Denver Formation in the Denver Basin, Colorado. Palatobaena knellerorum differs from other Palatobaena species in the extreme degree of its dorsally oriented orbits, as well as the presence of a broad pentagonal midline crest formed by the supraoccipital and parietals. It can be further diagnosed from other species of Palatobaena by the following unique combination of characters: having a broadly rounded cranium, presence of upper temporal emargination that extends well anterior to the otic chamber, a broad exposure of the supraoccipital on the dorsal skull roof, a broadly rounded ‘tongue groove’ between the maxillae and premaxillae, and a deeply emarginated nasal region that exposes the underlying premaxillae in dorsal view. Palatobaena knellerorum most closely resembles the Maastrichtian/Danian taxon Palatobaena cohen and the Danian taxon P. bairdi. A phylogenetic analysis places P. knellerorum as sister to P. bairdi, and these taxa are sister to P. cohen. Palatobaena knellerorum overlaps in time with P. cohen, but the two are separated geographically with P. knellerorum restricted to the Denver Basin and P. cohen restricted to the Williston Basin. Cranial anatomical data and sedimentological data suggest Palatobaena spp. were bottom dwelling turtles living in shallow water environments that used a putative proboscis to prey upon freshwater-shelled invertebrates.

The cranial skeleton of the enigmatic gliding neodiapsid reptile Coelurosauravus elivensis (Lower Sakamena Formation, Lopingian, Southwestern Madagascar) is re-described in detail. All previously referred specimens are reexamined under both direct observations and Reflectance Transformation Imaging. Their exquisite preservation yields detailed three-dimensional information on the outline of individual bones and their osteological relationships, which are missing in the Laurasian remains. In contrast to previous studies, the ontogenetic maturity of all specimens is re-affirmed. Previously unidentified elements of the palate, braincase and mandible are described, and a novel reconstruction is proposed, including the first palatal reconstruction in a weigeltisaurid reptile. C. elivensis has the smallest skull of all weigeltisaurids and differs from other species in its facial ornamentation, parietosquamosal frill and larger anterior maxillary dentition. We also provide extensive comparisons with contemporaneous reptiles, possibly closely related taxa and more recent analogs, as well as a preliminary discussion of the functional anatomy of the peculiar cranial morphology of weigeltisaurids. The cranial skeleton is a truss construction with large orbits and temporal fenestrae. By analogy with extant chamaeleonids, the elongate parietosquamosal frill is associated with an increase in length and diameter of the temporal jaw adductors, resulting in an increased gape and/or bite force and speed. Additionally, the spikes and frills of weigeltisaurids most likely served as a display and defensive structure.

The Middle Triassic was an important time in the evolutionary history of reptiles because it was during this time that many modern groups originated and initially diversified. In this context, the rich fossiliferous deposits of the Ladinian-age Erfurt Formation in southern Germany play an important role in elucidating reptilian diversity during this interval. Here we present an interesting record of a reptile with an unusual dentition. The material comprises a fragment of a left dentary. The teeth are tightly spaced and have chisel-shaped crowns, with the posterolingually extending apices abutting those of the successive teeth and forming a continuous cutting edge. Micro-computed tomography scanning of the jaw fragment revealed the teeth have expanded roots and sit on an open, shallow groove. The Meckelian canal is partially open lingually and a second canal for the inferior alveolar nerve and artery extends parallel to it. Although the preserved anatomical features of the material are insufficient for precise phylogenetic placement, the structure of the robust dentary and its teeth resembles those of some lepidosaurs and certain early Mesozoic marine diapsids. The distinctive morphology of the teeth, combined with the apparent lack of tooth wear, makes it difficult to assess their functional significance. It is interesting that extinct tetrapods with similar tooth morphologies come from strata representing freshwater and brackish-water paleoenvironments, which may serve as an additional line of evidence for future Morpho-functional assessments.

The holotype specimen of Alexandronectes zealandiensis is analyzed using digital reconstruction based on CT scans. Additional information regarding internal anatomy or obscured details are added. Additional features include: ectopterygoid rhombic in shape with posterior end pointed, a feature shared only with Aristonectes quiriquinensis; pterygoid shows a high dorsal crest located anteriorly and laterally to the level of the basipterygoid process; posterior margin of parabasisphenoid medially notched in ventral view and surrounding anteriorly and laterally a midline pit. Two canals for XII nerve are present in the right exoccipital-opisthothic and only one in the left one. Supraoccipital with two medially curved ridges on its posterior surface. Additionally, the presence of a stapes is described for the first time in an aristonectine elasmosaurid. The inner ear labyrinth is described and compared with that of other plesiosaurs, and the floccular recess (osseous correlate of the floccular lobe of the cerebellum) is described for first time among elasmosaurids. This feature is probably related to the presence of a long neck and with predatory behavior as the floccular lobe (housed in the floccular recess) stabilized the head via the cervical musculature, and stabilized the retinal image during rotational head movements.

The phylogenetic relationships of Pseudosuchia, the crocodile-line of Archosauria, are still poorly resolved, in part, due to the lack of crucial braincase information for several key taxa. Recently, erpetosuchids and ornithosuchids have been recovered as close relatives to Aetosauria, sharing several braincase features. Here we provide the description of the first braincase of the basal aetosaur Aetosauroides based on specimens from the Upper Triassic Candelária Sequence of Brazil. Our study revealed the presence of an exoccipital lateral ridge and a medial ridge on the supraoccipital (both shared with all aetosaurs and erpetosuchids, but absent in ornithosuchids) and an anterolateral exit for the internal carotids (shared with all aetosaurs and ornithosuchids, but not with erpetosuchids). Aetosauroides lacks a medial contact between the exoccipitals (shared with the aetosaurs Desmatosuchus smalli and Tecovasuchus) and possesses a single hypoglossal foramen (contrasting with Stagonolepis robertsoni and Desmatosuchus spurensis). It also differs from the putative Argentinian Aetosauroides (PVSJ 326) by the presence of a ridge connecting the basal tubera medially (contrasting also with all stagonolepidoideans) and by a bulbous and ventrolaterally recurved basipterygoid process (contrasting with Paratypothorax). These features show that the braincase of aetosaurs is suitable for providing further phylogenetic information and may contribute to resolving controversies within Pseudosuchia relationships.

Proterochampsids were a group of stem archosaurs from the Middle–Late Triassic of South America. Using for the first time in proterochampsids quantitative microanatomical and morphological data and an inference model, we analyze the microstructure of postcranial bones of proterochampsids to infer life-history traits dealing with growth dynamics, ontogenetic changes, dermal armor histogenesis and lifestyle. We studied proterochampsid individuals from the Middle–Late Triassic Chañares Formation: three specimens of Chanaresuchus bonapartei: PVL 4575 (osteoderms and femur), CRILAR-Pv 80 (osteoderm) and CRILAR-Pv 81 (humerus), and a femur of a non-identified Rhadinosuchinae specimen (CRILAR-Pv 488). The osteoderms show a compact-uniform composition of parallel-fibered bone and lines of arrested growth (LAGs). The long bones are composed mainly of highly vascularized, parallel-fibered bone and isolated areas of woven-fibered bone in the innermost portion of cortex (only in C. bonapartei PVL 4575). The absence of an external fundamental system (EFS) indicates that all of the sampled individuals were somatically immature at time of death. The absence of mineralized bundles of collagenous fibers supports intramembranous ossification as the developmental origin for the osteoderms. With an inference model used for the first time in proterochampsids, the lifestyle inferred for C. bonapartei is terrestrial. Histological evidence indicates that sexual maturity is reached before both somatic and skeletal maturity. The growth patterns among proterochampsids exhibit some degree of variation even within a single species. Such variation can be due to different causes, including possibly phenotypic plasticity.

The first unequivocal records of teratornithid birds from the Pleistocene of South America are here described, adding a new member, and the largest, to this highly diversified guild of large carnivorous flying birds that lived during these times in the Americas. The new specimens come from four fossiliferous localities of Central Argentina that range in age from the late middle to the early late Pleistocene, and agree with other known Teratornithidae taxa in size and morphology. We updated the taxonomy of the family and analyzed its fossil record in the Pleistocene of both Americas. The available evidence suggests that forms related to Teratornis lived in the South American Pampas around the time of the Last Interglacial (MIS 5), but they were restricted to North America during the latest Pleistocene (late MIS 3–early MIS 1). The contrasting latest Pleistocene record of teratorns between North and South America is not easy to understand, especially because the supposed flight capacity of these birds did not prevent them from crossing large geographical barriers. Although a bias in the fossil record cannot be ruled out, it is possible that the teratorns were limited in South America by paleoclimatic–paleoecological factors as yet undetermined, and/or that the northern and southern Pleistocene species had very dissimilar specializations. In relation to the latter, the previous inferences on the teratorn paleobiology without phylogenetic support are preliminarily questioned here.

A partial skull (BP/1/7976) of a very large cynodont from the Middle Triassic Cynognathus Assemblage Zone (Cricodon-Ufudocyclops subzone) of South Africa is described. The specimen represents a new gomphodont taxon, Impidens hancoxi, gen. et sp. nov., diagnosed by five sectorial teeth constituting just over half of the length of the upper postcanine tooth row, and enlarged canine and incisor teeth. BP/1/8123, a skull fragment also from the Cricodon-Ufudocyclops subzone, and AMNH FARB 24421, a partial skull from the upper Fremouw Formation of Antarctica, are also referred to the new species. The presence of this taxon in both the upper Fremouw Formation and Cricodon-Ufudocyclops subzone strengthens evidence of biostratigraphic correlation between these units. Impidens hancoxi, with an inferred skull length of up to 460 mm, was a large-bodied and likely omnivorous gomphodont, and may have played the role of apex predator within the tetrapod fauna of the Cricodon-Ufudocyclops subzone.

This paper describes, for the first time, the stapedes of several non-marsupial species of the metatherian clade Sparassodonta (Sipalocyon gracilis, Arctodictis sinclairi, and Borhyaena tuberata) which were fortuitously encountered during routine microtomography. To augment our comparative set we also scanned and reconstructed single examples of the stapedes of the fossil taxa Sparassocynus bahiai and Thylophorops cf. T. chapalmalensis (Didelphimorphia), Argyrolagus scagliai (?Paucituberculata), as well as single examples drawn from extant members of Caenolestidae (Caenolestes sp.) and Microbiotheriidae (Dromiciops gliroides). The sparassodont, didelphid, and microbiotherian samples exhibit a common bauplan (stapes triangular, with intracrural foramen), whereas the paucituberculatan samples differ in possessing columelliform, imperforate stapes as also previously reported for various australidelphians. The stapedial footplate is rounded in sparassodonts (stapedial ratio, ∼1.6), in both of the fossil didelphimorphians (∼1.7), and in the fossil ?paucituberculatan (∼1.5). According to our optimization of our results of mammalian phylogeny, and in contrast to some other reconstructions, a triangular stapes with intracrural foramen and rounded footplate is likely the ancestral condition for Marsupialia. No particular function can be correlated with possession of the intracrural foramen as opposed to an imperforate stapes, apart from accommodation of the proximal stapedial artery (as seen also in many eutherians). The frequent presence of the intracrural foramen in members of both infraclasses suggests that the ontogenies of the second arch's blood supply (stapedial artery) and its main skeletal element (stapes) have remained strongly integrated throughout therian evolution, even in cases in which the proximal part of the vessel involutes.

Late Oligocene caviomorphs represent one of the most important early adaptive radiations of this group of rodents. They show a high taxonomic diversity and a wide geographic distribution, with those found in the upper Oligocene of Salla (Bolivia) and Patagonia (Argentina) being particularly diverse. However, caviomorphs from the Deseadan South American Land Mammal Age in other latitudes are still scarcely known. In this contribution, we describe new caviomorph rodents from the Deseadan levels of Quebrada Fiera locality (Mendoza Province; central-western Argentina) and evaluate their systematic and paleobiogeographic significance. The taxonomic composition of the caviomorphs from Quebrada Fiera reveals a complex biogeographic history, as we recognize one endemic Cavioidea (Asteromys puelche sp. nov.) and two Chinchilloidea: one with Patagonian affinities (Cephalomys sp.) and another with a wide distribution in South America (Incamys bolivianus). Asteromys puelche sp. nov., shows closer phylogenetic affinities with the Bolivian A. bolivianus than with the Patagonian A. punctus. These rodents, in common with the whole mammalian association from Quebrada Fiera, show a mix of endemic taxa and others shared with higher and lower latitudes.

We report on a nearly complete cranium of Ekweeconfractus amorui gen. et sp. nov. (Hyaenodonta, Teratodontinae), from the early Miocene of Moruorot, Kenya. The cranium is dorsoventrally compressed, but sufficiently intact to allow for digital reconstruction of the neurocranium, resulting in a digital endocast that gives us a first glimpse into teratodontine brain morphology. The virtual endocast is one of the most well preserved of any hyaenodont known to date, with many of the cranial nerves and blood vessels identifiable. Endocasts are known from only a handful of hyaenodont species and little work has been done on hyaenodont brains in recent decades. To better understand the evolution of the brain in these animals, we place the endocast of E. amorui gen. et sp. nov., as well as several other previously described endocasts, in the latest phylogenetic framework. This analysis suggests that the expansion of the neocortex occurred convergently in several clades of Hyaenodonta. Our study provides a basis for future research on brain evolution in Hyaenodonta.