A large new Middle Jurassic ichthyosaur shows the importance of body size evolution in the origin of the Ophthalmosauria.

The Middle Jurassic is an important time period for the evolutionary history of marine reptiles as it represented a transitional phase for many clades. Notably, in ichthyosaurs, many early parvipelvian taxa went extinct. The Middle Jurassic saw the emergence of the derived Ophthalmosauria, ultimately becoming the dominant ichthyosaurian clade by the end of the epoch. Even though this is an important period in the evolutionary history of Ophthalmosauria, our understanding remains limited in terms of morphology and taxonomy due to the scarcity of vertebrate-bearing strata. Here we present a large new ichthyosaur from the Bajocian of Switzerland, represented by an almost complete skull with 3D-preserved bones, the (inter)clavicles and a large portion of the postcranial skeleton. After CT- and surface scanning, we reconstructed the 3D in vivo morphology. Our morphological observations and phylogenetic analyses show that the new taxon named Argovisaurus martafernandezi is nested at the base of the Ophthalmosauria. The holotype and only known specimen of Argovisaurus likely represents an adult individual. Bajocian members of the Ophthalmosauria (Mollesaurus and Argovisaurus) were large-bodied animals, a trait typically associated with the more derived Platypterygiinae. This hints at the importance of a large body size early in ophthalmosaurian evolution.LSID: urn:lsid:zoobank.org:act:C3312628-1544-4B87-BBE3-B12346A30BE3LSID: urn:lsid:zoobank.org:act:23C2BD71-8CF0-4D99-848A-0D631518415B.

The postcranial skeleton of Teyujagua paradoxa (Reptilia: Archosauromorpha) from the early Triassic of South America.

Teyujagua paradoxa is a remarkable early archosauromorph from the Lower Triassic Sanga do Cabral Formation, Brazil. The species was originally described from an almost complete skull and a few associated cervical vertebrae, and no further postcranial elements were known at that time. Additional fieldwork in the Sanga do Cabral Formation, however, was successful in recovering a fairly complete postcranial skeleton attributable to the holotype. Here, we describe this new postcranial material, which is composed of cervical, dorsal, sacral and caudal vertebrae, limbs, pectoral and pelvic girdles, ribs, and gastralia. The description of its postcranial skeleton makes T. paradoxa one of the best-known early-diverging archosauromorphs. The cladistic analysis performed after the scoring of postcranial data recovered T. paradoxa in the same position initially described, close to the node that defines the Archosauriformes. Teyujagua paradoxa shares morphological features with representatives of early-diverging archosauromorphs and archosauriforms, with certain traits demonstrating a mosaic of plesiomorphic and apomorphic character states. We also performed partitioned morphospace and disparity analysis to elucidate the morphological disparity and evolutionary patterns among archosauromorphs. Teyujagua paradoxa occupies a notable position, suggesting an intermediate morphology between early archosauromorphs and proterosuchids. Disparity estimates highlighted Pseudosuchia and Avemetatarsalia as having the highest median disparity, reflecting their diverse cranial and postcranial morphologies, respectively. These findings offer valuable insights into archosauromorph macroevolution and adaptation.

Functional morphology of the Triassic apex predator Saurosuchus galilei (Pseudosuchia: Loricata) and convergence with a post-Triassic theropod dinosaur.

Pseudosuchian archosaurs, reptiles more closely related to crocodylians than to birds, exhibited high morphological diversity during the Triassic and are thus associated with hypotheses of high ecological diversity during this time. One example involves basal loricatans which are non-crocodylomorph pseudosuchians traditionally known as "rauisuchians." Their large size (5-8+ m long) and morphological similarities to post-Triassic theropod dinosaurs, including dorsoventrally deep skulls and serrated dentitions, suggest basal loricatans were apex predators. However, this hypothesis does not consider functional behaviors that can influence more refined roles of predators in their environment, for example, degree of carcass utilization. Here, we apply finite element analysis to a juvenile but three-dimensionally well-preserved cranium of the basal loricatan Saurosuchus galilei to investigate its functional morphology and to compare with stress distributions from the theropod Allosaurus fragilis to assess degrees of functional convergence between Triassic and post-Triassic carnivores. We find similar stress distributions and magnitudes between the two study taxa under the same functional simulations, indicating that Saurosuchus had a somewhat strong skull and thus exhibited some degree of functional convergence with theropods. However, Saurosuchus also had a weak bite for an animal of its size (1015-1885 N) that is broadly equivalent to the bite force of modern gharials (Gavialis gangeticus). We infer that Saurosuchus potentially avoided tooth-bone interactions and consumed the softer parts of carcasses, unlike theropods and other basal loricatans. This deduced feeding mode for Saurosuchus increases the known functional diversity of basal loricatans and highlights functional differences between Triassic and post-Triassic apex predators.

Turtle skull development unveils a molecular basis for amniote cranial diversity.

Amniote skulls display diverse architectural patterns including remarkable variations in the number of temporal arches surrounding the upper and lower temporal fenestrae. However, the cellular and molecular basis underlying this diversification remains elusive. Turtles are a useful model to understand skull diversity due to the presence of secondarily closed temporal fenestrae and different extents of temporal emarginations (marginal reduction of dermal bones). Here, we analyzed embryos of three turtle species with varying degrees of temporal emargination and identified shared widespread coexpression of upstream osteogenic genes Msx2 and Runx2 and species-specific expression of more downstream osteogenic genes Sp7 and Sparc in the head. Further analysis of representative amniote embryos revealed differential expression patterns of osteogenic genes in the temporal region, suggesting that the spatiotemporal regulation of Msx2, Runx2, and Sp7 distinguishes the temporal skull morphology among amniotes. Moreover, the presence of Msx2- and/or Runx2-positive temporal mesenchyme with osteogenic potential may have contributed to their extremely diverse cranial morphology in reptiles.

Reappraisal of the thalattosuchian crocodylomorph record from the Middle-Upper Jurassic Rosso Ammonitico Veronese of northeastern Italy: Age calibration, new specimens and taphonomic biases.

Despite their extremely rare and fragmentary record, aquatic crocodylomorphs from the Middle to Upper Jurassic (Bajocian-Tithonian) Rosso Ammonitico Veronese (RAV) of northeastern Italy have sparked interest since the late 18th century. Among marine reptiles, Thalattosuchia is by far one of the best represented groups from the RAV units, especially in the Middle Jurassic. Although some specimens have been the subject of multiple studies in recent times, most of them still lack precise stratigraphic assignment and taphonomic assessment, while others remain undescribed. Here we provide a comprehensive revision of the thalattosuchian record from the RAV, alongside the most up-to-date age determination, by means of calcareous nannofossils, when available. Three new metriorhynchoid specimens are described for the first time from the Middle Jurassic of Asiago Plateau (Vicenza province). While the taphonomy of the newly described specimens hampers any taxonomic attribution below superfamily/family level, all three were confidently assigned to a precise interval between the upper Bajocian and the upper Bathonian. This revised record has major paleobiogeographical implications: the new specimens confirm an early origin and distribution of Metriorhynchoidea in the Tethys area and suggest a fast colonization of the open-ocean environment since the upper Bajocian.

High phenotypic plasticity at the dawn of the eosauropterygian radiation.

The initial radiation of Eosauropterygia during the Triassic biotic recovery represents a key event in the dominance of reptiles secondarily adapted to marine environments. Recent studies on Mesozoic marine reptile disparity highlighted that eosauropterygians had their greatest morphological diversity during the Middle Triassic, with the co-occurrence of Pachypleurosauroidea, Nothosauroidea and Pistosauroidea, mostly along the margins of the Tethys Ocean. However, these previous studies quantitatively analysed the disparity of Eosauropterygia as a whole without focussing on Triassic taxa, thus limiting our understanding of their diversification and morphospace occupation during the Middle Triassic. Our multivariate morphometric analyses highlight a clearly distinct colonization of the ecomorphospace by the three clades, with no evidence of whole-body convergent evolution with the exception of the peculiar pistosauroid Wangosaurus brevirostris, which appears phenotypically much more similar to nothosauroids. This global pattern is mostly driven by craniodental differences and inferred feeding specializations. We also reveal noticeable regional differences among nothosauroids and pachypleurosauroids of which the latter likely experienced a remarkable diversification in the eastern Tethys during the Pelsonian. Our results demonstrate that the high phenotypic plasticity characterizing the evolution of the pelagic plesiosaurians was already present in their Triassic ancestors, casting eosauropterygians as particularly adaptable animals.

New reptile shows dinosaurs and pterosaurs evolved among diverse precursors.

Dinosaurs and pterosaurs have remarkable diversity and disparity through most of the Mesozoic Era1-3. Soon after their origins, these reptiles diversified into a number of long-lived lineages, evolved unprecedented ecologies (for example, flying, large herbivorous forms) and spread across Pangaea4,5. Recent discoveries of dinosaur and pterosaur precursors6-10 demonstrated that these animals were also speciose and widespread, but those precursors have few if any well-preserved skulls, hands and associated skeletons11,12. Here we present a well-preserved partial skeleton (Upper Triassic, Brazil) of the new lagerpetid Venetoraptor gassenae gen. et sp. nov. that offers a more comprehensive look into the skull and ecology of one of these precursors. Its skull has a sharp, raptorial-like beak, preceding that of dinosaurs by around 80 million years, and a large hand with long, trenchant claws that firmly establishes the loss of obligatory quadrupedalism in these precursor lineages. Combining anatomical information of the new species with other dinosaur and pterosaur precursors shows that morphological disparity of precursors resembles that of Triassic pterosaurs and exceeds that of Triassic dinosaurs. Thus, the 'success' of pterosaurs and dinosaurs was a result of differential survival among a broader pool of ecomorphological variation. Our results show that the morphological diversity of ornithodirans started to flourish among early-diverging lineages and not only after the origins of dinosaurs and pterosaurs.

Thalattosuchian crocodylomorphs from European Russia, and new insights into metriorhynchid tooth serration evolution and their palaeolatitudinal distribution.

From the Middle Jurassic to the Early Cretaceous, metriorhynchid crocodylomorphs inhabited marine ecosystems across the European archipelago. Unfortunately, European metriorhynchids are only well known from Germany, France, and the UK, with the Eastern European fossil record being especially poor. This hinders our understanding of metriorhynchid biodiversity across these continuous seaways, and our ability to investigate provincialism. Here we describe eleven isolated tooth crowns and six vertebrae referable to Metriorhynchidae from the Callovian, Oxfordian, Volgian (Tithonian), and Ryazanian (Berriasian) or Valanginian of European Russia. We also describe an indeterminate thalattosuchian tooth from the lower Bajocian of the Volgograd Oblast, the first discovery of a marine reptile from the Bajocian strata of European Russia. These rare fossils, along with previous reports of Russian thalattosuchians, indicate that thalattosuchians have been common in the Middle Russian Sea since it was formed. Palaeolatitude calculations for worldwide metriorhynchid-bearing localities demonstrate that the occurrences in European Russia are the most northern, located mainly between 44-50 degrees north. However, metriorhynchids appear to be rare at these palaeolatitudes, and are absent from palaeolatitudes higher than 50°. These observations support the hypothesis that metriorhynchids evolved an elevated metabolism but were not endo-homeothermic, especially as endo-homeothermic marine reptiles (ichthyosaurs and plesiosaurs) remained abundant at much higher palaeolatitudes.

An armoured marine reptile from the Early Triassic of South China and its phylogenetic and evolutionary implications.

Sauropterygia was a taxonomically and ecomorphologically diverse clade of Mesozoic marine reptiles spanning the Early Triassic to the Late Cretaceous. Sauropterygians are traditionally divided into two groups representing two markedly different body plans - the short-necked, durophagous Placodontia and the long-necked Eosauropterygia - whereas Saurosphargidae, a small clade of armoured marine reptiles, is generally considered as the sauropterygian sister-group. However, the early evolutionary history of sauropterygians and their phylogenetic relationships with other groups within Diapsida are still incompletely understood. Here, we report a new saurosphargid from the Early Triassic (Olenekian) of South China - Prosaurosphargis yingzishanensis gen. et sp. nov. - representing the earliest known occurrence of the clade. An updated phylogenetic analysis focussing on the interrelationships among diapsid reptiles recovers saurosphargids as nested within sauropterygians, forming a clade with eosauropterygians to the exclusion of placodonts. Furthermore, a clade comprising Eusaurosphargis and Palatodonta is recovered as the sauropterygian sister-group within Sauropterygomorpha tax. nov. The phylogenetic position of several Early and Middle Triassic sauropterygians of previously uncertain phylogenetic affinity, such as Atopodentatus, Hanosaurus, Majiashanosaurus, and Corosaurus, is also clarified, elucidating the early evolutionary assembly of the sauropterygian body plan. Finally, our phylogenetic analysis supports the placement of Testudines and Archosauromorpha within Archelosauria, a result strongly corroborated by molecular data, but only recently recovered in a phylogenetic analysis using a morphology-only dataset. Our study provides evidence for the rapid diversification of sauropterygians in the aftermath of the Permo-Triassic mass extinction event and emphasises the importance of broad taxonomic sampling in reconstructing phylogenetic relationships among extinct taxa.

Around 252 million years ago, just before the start of a period of time known as the Triassic, over 90% of animals, plants and other species on Earth went extinct in what was the worst mass extinction event in the planet's history. It is thought to have happened because of an increase in volcanic eruptions that led to global warming, acid rain and other catastrophic changes in the environment. The loss of so many species caused ecosystems to restructure as the surviving species evolved to fill niches left by those that had gone extinct. On land, reptiles diversified to give rise to dinosaurs, the flying pterosaurs, and the ancestors of modern crocodiles, lizards, snakes and turtles. Some of these land-based animals evolved to live in water, resulting in many species of marine reptiles emerging during the Triassic period. This included the saurosphargids, a group of marine reptiles that lived in the Middle Triassic around 247­237 million years ago. They were 'armoured' with a shield made of broadened ribs superficially similar to that of turtles, and a covering of bony plates. However, it is unclear how the saurosphargids evolved and how closely they are related to other marine reptiles. Here, Wolniewicz et al. studied a new species of saurosphargid named Prosaurosphargis yingzishanensis that was found fossilized in a quarry in South China. The animal was around 1.5 metres long and had a chest shield and armoured plates like other saurosphargids. The characteristics of the rock surrounding the fossil suggest that this individual lived in the Early Triassic, several million years before other saurosphargid species. The team used a phylogenetic approach to infer the evolutionary relationships between P. yingzishanensis and numerous other land-based and marine reptiles based on over 220 anatomical characteristics of the animals. The resulting evolutionary tree indicated that the saurosphargids represented an early stage in the evolution of a larger group of marine reptiles known as the sauropterygians. The analysis also identified the closest land-based relatives of sauropterygians. These findings provide evidence that marine reptiles rapidly diversified in the aftermath of the mass extinction event 252 million years ago. Furthermore, they contribute to our understanding of how ecosystems recover after a major environmental crisis.

Late Jurassic teeth of plesiosauroid origin from the Owadów-Brzezinki Lägerstatte, Central Poland.

Owadów-Brzezinki is currently one of the most promising Upper Jurassic sites in Central Poland, with a wide array of both vertebrate and invertebrate fossil fauna present. The discoveries of large-bodied marine reptiles fossils such as ichthyosaurs, turtles, and marine crocodylomorphs attracted attention to the location. A particular Mesozoic marine group, plesiosaurs, remained to be found, and in this report, we note four isolated teeth with distinguishing apicobasal ridging pattern and elongated, conical shape characteristic for plesiosaurians. The outcomes of the Principal Coordinates Analysis (PCoA) of the largest and most complete tooth specimen ZPAL R.11/OB/T4 enabled us to confirm its classification as Plesiosauroidea. This discovery affirms the importance of the site as the area of mixing between Boreal and Tethyan faunas, expanding the broad spectrum of fossil taxa found in this location. Together with previous findings of plesiosaur material in a nearby region, it provides the evidence for the presence of Plesiosauroidea in Owadów-Brzezinki Lägerstatte.

Description and phylogenetic relationships of a new species of Torvoneustes (Crocodylomorpha, Thalattosuchia) from the Kimmeridgian of Switzerland.

Metriorhynchids are marine crocodylomorphs found across Jurassic and Lower Cretaceous deposits of Europe and Central and South America. Despite being one of the oldest fossil families named in paleontology, the phylogenetic relationships within Metriorhynchidae have been subject to many revisions over the past 15 years. Herein, we describe a new metriorhynchid from the Kimmeridgian of Porrentruy, Switzerland. The material consists of a relatively complete, disarticulated skeleton preserving pieces of the skull, including the frontal, prefrontals, right postorbital, nasals, maxillae, right premaxillae and nearly the entire mandible, and many remains of the axial and appendicular skeleton such as cervical, dorsal, and caudal vertebrae, ribs, the left ischium, the right femur, and the right fibula. This new specimen is referred to the new species Torvoneustes jurensis sp. nov. as part of the large-bodied macrophagous tribe Geosaurini. Torvoneustes jurensis presents a unique combination of cranial and dental characters including a smooth cranium, a unique frontal shape, acute ziphodont teeth, an enamel ornamentation made of numerous apicobasal ridges shifting to small ridges forming an anastomosed pattern toward the apex of the crown and an enamel ornamentation touching the carina. The description of this new species allows to take a new look at the currently proposed evolutionary trends within the genus Torvoneustes and provides new information on the evolution of this clade.

Decapitation in the long-necked Triassic marine reptile Tanystropheus.

Extreme neck elongation was a common evolutionary strategy among Mesozoic marine reptiles, occurring independently in several lineages1,2. Despite its evolutionary success, such an elongate neck might have been particularly susceptible to predation1, but direct evidence for this possibility has been lacking. Composed of only 13 hyperelongate vertebrae and associated strut-like ribs, the configuration of the long neck of the Triassic archosauromorph Tanystropheus is unique among tetrapods. It was probably stiffened and used to catch prey through an ambush-strategy2. Here, we show that the neck was completely severed in two Tanystropheus specimens (Figure 1), most likely due to a predatory attack, providing vivid evidence of predator-prey interactions among Mesozoic marine reptiles that are rarely preserved in the fossil record. The recurring incidence of decapitation suggests that the elongate neck was a functional weak spot in Tanystropheus, and possibly the long-necked marine reptile bauplan more generally.

Heads or tails first? Evolution of fetal orientation in ichthyosaurs, with a scrutiny of the prevailing hypothesis.

According to a longstanding paradigm, aquatic amniotes, including the Mesozoic marine reptile group Ichthyopterygia, give birth tail-first because head-first birth leads to increased asphyxiation risk of the fetus in the aquatic environment. Here, we draw upon published and original evidence to test two hypotheses: (1) Ichthyosaurs inherited viviparity from a terrestrial ancestor. (2) Asphyxiation risk is the main reason aquatic amniotes give birth tail-first. From the fossil evidence, we conclude that head-first birth is more prevalent in Ichthyopterygia than previously recognized and that a preference for tail-first birth likely arose in derived forms. This weakens the support for the terrestrial ancestry of viviparity in Ichthyopterygia. Our survey of extant viviparous amniotes indicates that fetal orientation at birth reflects a broad diversity of factors unrelated to aquatic vs. terrestrial habitat, further undermining the asphyxiation hypothesis. We propose that birth preference is based on parturitional mechanics or carrying efficiency rather than habitat.

Palaeohistology reveals an unusual periodontium and tooth implantation in a filter-feeding pterodactyloid pterosaur, Pterodaustro guinazui, from the Lower Cretaceous of Argentina.

Pterosaurs are an extinct group of Mesozoic flying reptiles, which exhibited high diversity with regard to their dentition. Although morphological features of pterosaur dentition have been described in detail in several contributions, the histology of tooth and tooth attachment tissues (i.e. periodontium) has been scarcely analysed to date for this clade. Here we describe and interpret the microstructure of the tooth and periodontium attachment tissues of Pterodaustro guinazui, a filter-feeding pterodactyloid pterosaur from the Lower Cretaceous of Argentina. The histological analysis of the lower jaw and its filamentous teeth verifies that the geometry of the implantation corresponds to an aulacodont condition (i.e. teeth are set in a groove with no interdental separation). This pattern departs from that recorded in other archosaurs, being possibly also present in other, non-closely related, pterosaurs. Regarding tooth attachment, in contrast to other pterosaurs, there is no direct evidence for gomphosis in Pterodaustro (i.e. the absence of cementum, mineralized periodontal ligamentum and alveolar bone). Nevertheless, the current evidence for ankylosis is still not conclusive. Contrary to that reported for other archosaurs, replacement teeth are absent in Pterodaustro, which is interpreted as evidence for monophyodonty or diphyodonty in this taxon. Most of the microstructural features are possibly related to the complex filter-feeding apparatus of Pterodaustro and does not appear to represent the general pattern of pterosaurs.

Earliest Triassic ichthyosaur fossils push back oceanic reptile origins.

Reptiles first radiated into oceanic environments after the cataclysmic end-Permian mass extinction (EPME)1, 251.9 million years (Ma) ago. The geologically oldest fossils evincing this adaptive transition have been recovered from upper-Lower Triassic (lower Spathian) strata, ∼248.8 Ma2, and postdate a landmark turnover of amphibian-dominated to reptile-dominated marine ecosystems spanning the late Smithian crisis (LSC)3, ∼249.6 Ma4 -less than ∼2.3 Ma after the EPME. Here, we report ichthyopterygian (the group including 'fish-shaped' ichthyosaurians1) remains from the Arctic island of Spitsbergen that predate the LSC in later-middle to early-late Smithian5 deposits up to ∼250 Ma. Unexpectedly, however, their large size and spongy internal bone structure indicate a fully pelagic ichthyopterygian1,6. Given this unambiguous occurrence ∼2 Ma after the EPME, these pioneering seagoing tetrapods can now be feasibly recast as mass extinction survivors instead of ecological successors2,3 within the earliest Mesozoic marine predator communities.

Neuroanatomy of the crocodylomorph Portugalosuchus azenhae from the late cretaceous of Portugal.

We present the first detailed braincase anatomical description and neuroanatomical study of Portugalosuchus azenhae, from the Cenomanian (Late Cretaceous) of Portugal. This eusuchian crocodylomorph was originally described as a putative Crocodylia and one of the oldest representatives of this clade; however, its phylogenetic position remains controversial. Based on new data obtained from high resolution Computed Tomography images (by micro-CT scan), this study aims to improve the original description of this taxon and also update the scarce neuroanatomical knowledge of Eusuchia and Crocodylia from this time interval, a key period to understand the origin and evolution of these clades. The resulting three-dimensional models from the CT data allowed a detailed description of its well-preserved neurocranium and internal cavities. Therefore, it was possible to reconstruct the cavities of the olfactory region, nasopharyngeal ducts, brain, nerves, carotid arteries, blood vessels, paratympanic sinus system and inner ear, which allowed to estimate some neurosensorial capabilities. By comparison with other crocodylomorphs, these analyses showed that Portugalosuchus, back in the Cenomanian, already displayed an olfactive acuity, sight, hearing and cognitive skills within the range of that observed in other basal eusuchians and crocodylians, including extant species. In addition, and in order to test its disputed phylogenetic position, these new anatomical data, which helped to correct and complete some of the original observations, were included in one of the most recent morphology-based phylogenies. The position of Portugalosuchus differs slightly from the original publication since it is now located as a "thoracosaurid" within Gavialoidea, but still as a crocodylian. Despite all this, to better contrast these results, additional phylogenetic analyses including this new morphological character coding together with DNA data should be performed.

Locomotor and postural diversity among reptiles viewed through the prism of femoral microanatomy: Palaeobiological implications for some Permian and Mesozoic taxa.

The water-to-land transition by the first tetrapod vertebrates represents a key stage in their evolution. Selection pressures exerted by this new environment on animals led to the emergence of new locomotor and postural strategies that favoured access to different ecological niches and contributed to their evolutionary success. Today, amniotes show great locomotor and postural diversity, particularly among Reptilia, whose extant representatives include parasagittally locomoting erect and crouched bipeds (birds), sub-parasagittal 'semi-erect' quadrupeds (crocodylians) and sprawling quadrupeds (squamates and turtles). But the different steps leading to such diversity remain enigmatic and the type of locomotion adopted by many extinct species raises questions. This is notably the case of certain Triassic taxa such as Euparkeria and Marasuchus. The exploration of the bone microanatomy in reptiles could help to overcome these uncertainties. Indeed, this locomotor and postural diversity is accompanied by great microanatomical disparity. On land, the bones of the appendicular skeleton support the weight of the body and are subject to multiple constraints that partly shape their external and internal morphology. Here we show how microanatomical parameters measured in cross-section, such as bone compactness or the position of the medullocortical transition, can be related to locomotion. We hypothesised that this could be due to variations in cortical thickness. Using statistical methods that take phylogeny into account (phylogenetic flexible discriminant analyses), we develop different models of locomotion from a sample of femur cross-sections from 51 reptile species. We use these models to infer locomotion and posture in 7 extinct reptile taxa for which they remain debated or not fully clear. Our models produced reliable inferences for taxa that preceded and followed the quadruped/biped and sprawling/erect transitions, notably within the Captorhinidae and Dinosauria. For taxa contemporary with these transitions, such as Terrestrisuchus and Marasuchus, the inferences are more questionable. We use linear models to investigate the effect of body mass and functional ecology on our inference models. We show that body mass seems to significantly impact our model predictions in most cases, unlike the functional ecology. Finally, we illustrate how taphonomic processes can impact certain microanatomical parameters, especially the eccentricity of the section, while addressing some other potential limitations of our methods. Our study provides insight into the evolution of enigmatic locomotion in various early reptiles. Our models and methods could be used by palaeontologists to infer the locomotion and posture in other extinct reptile taxa, especially when considered in combination with other lines of evidence.

The femora of Drepanosauromorpha (Reptilia: Diapsida): Implications for the functional evolution of the thigh of Sauropsida.

The femora of diapsids have undergone morphological changes related to shifts in postural and locomotor modes, such as the transition from plesiomorphic amniote and diapsid taxa to the apomorphic conditions related to a more erect posture within Archosauriformes. One remarkable clade of Triassic diapsids is the chameleon-like Drepanosauromorpha. This group is known from numerous articulated but heavily compressed skeletons that have the potential to further inform early reptile femoral evolution. For the first time, we describe the three-dimensional osteology of the femora of Drepanosauromorpha, based on undistorted fossils from the Upper Triassic Chinle Formation and Dockum Group of North America. We identify apomorphies and a combination of character states that link these femora to those in crushed specimens of drepanosauromorphs and compare our sample with a range of amniote taxa. Several characteristics of drepanosauromorph femora, including a hemispherical proximal articular surface, prominent asymmetry in the proximodistal length of the tibial condyles, and a deep intercondylar sulcus, are plesiomorphies shared with early diapsids. The femora contrast with those of most diapsids in lacking a crest-like, distally tapering internal trochanter. They bear a ventrolaterally positioned tuberosity on the femoral shaft, resembling the fourth trochanter in Archosauriformes. The reduction of an internal trochanter parallels independent reductions in therapsids and archosauriforms. The presence of a ventrolaterally positioned trochanter is also similar to that of chameleonid squamates. Collectively, these features demonstrate a unique femoral morphology for drepanosauromorphs, and suggest an increased capacity for femoral adduction and protraction relative to most other Permo-Triassic diapsids.

A conserved tooth resorption mechanism in modern and fossil snakes.

Whether snakes evolved their elongated, limbless bodies or their specialized skulls and teeth first is a central question in squamate evolution. Identifying features shared between extant and fossil snakes is therefore key to unraveling the early evolution of this iconic reptile group. One promising candidate is their unusual mode of tooth replacement, whereby teeth are replaced without signs of external tooth resorption. We reveal through histological analysis that the lack of resorption pits in snakes is due to the unusual action of odontoclasts, which resorb dentine from within the pulp of the tooth. Internal tooth resorption is widespread in extant snakes, differs from replacement in other reptiles, and is even detectable via non-destructive µCT scanning, providing a method for identifying fossil snakes. We then detected internal tooth resorption in the fossil snake Yurlunggur, and one of the oldest snake fossils, Portugalophis, suggesting that it is one of the earliest innovations in Pan-Serpentes, likely preceding limb loss.