Assessing ontogenetic maturity in extinct saurian reptiles

Seis-ing up the Super-Morrison formation sauropods

The Upper Jurassic Morrison Formation sauropods Diplodocus (formerly "Seismosaurus") hallorum and Supersaurus vivianae are quantifiably the largest dinosaurian taxa from the formation, as well as being among the largest dinosaurs in the world. Their extreme body size (in particular body length, c. 50+ m) has fascinated the paleontological community since their discoveries and has sparked an ongoing discussion on the trends and limits of Morrison Formation sauropod body size. Although not an undeviating proxy, often the largest and skeletally most mature specimens are among the rarest (as exemplified in Triceratops). While their body size has no phylogenetic bearing, the extreme size and potential eco and biological significance of these two sauropod taxa are frequently discussed. Whether these rare and titanically proportioned sauropod specimens are large-bodied, senescent or both is an often-repeating rhetoric. To definitively make maturational inferences about these taxa, we osteohistologically sampled the holotype of D. hallorum (NMMNH P-25079) and the second known specimen of S. vivianae (WDC DMJ-021). Our age-determinant and maturational assessments indicate that both specimens were skeletally mature at their respective age of death. Retrocalculation methods for D. hallorum NMMNH P-25079 produce a maximum age-at-death estimation of 60 years, whereas S. vivianae WDC DMJ-021 lived well past skeletal maturity-so much so that reliable retrocalculated ages cannot be accurately determined at this time. Additionally, the rarity of such large sauropods within the Morrison Formation might be more parsimoniously explained as relating to their maturity as opposed to representing aberrant taxa on the Morrison landscape.

A new abelisaurid dinosaur from the end Cretaceous of Patagonia and evolutionary rates among the Ceratosauria

Gondwanan dinosaur faunae during the 20 Myr preceding the Cretaceous-Palaeogene (K/Pg) extinction included several lineages that were absent or poorly represented in Laurasian landmasses. Among these, the South American fossil record contains diverse abelisaurids, arguably the most successful groups of carnivorous dinosaurs from Gondwana in the Cretaceous, reaching their highest diversity towards the end of this period. Here we describe Koleken inakayali gen. et sp. n., a new abelisaurid from the La Colonia Formation (Maastrichtian, Upper Cretaceous) of Patagonia. Koleken inakayali is known from several skull bones, an almost complete dorsal series, complete sacrum, several caudal vertebrae, pelvic girdle and almost complete hind limbs. The new abelisaurid shows a unique set of features in the skull and several anatomical differences from Carnotaurus sastrei (the only other abelisaurid known from the La Colonia Formation). Koleken inakayali is retrieved as a brachyrostran abelisaurid, clustered with other South American abelisaurids from the latest Cretaceous (Campanian-Maastrichtian), such as Aucasaurus, Niebla and Carnotaurus. Leveraging our phylogeny estimates, we explore rates of morphological evolution across ceratosaurian lineages, finding them to be particularly high for elaphrosaurine noasaurids and around the base of Abelisauridae, before the Early Cretaceous radiation of the latter clade. The Noasauridae and their sister clade show contrasting patterns of morphological evolution, with noasaurids undergoing an early phase of accelerated evolution of the axial and hind limb skeleton in the Jurassic, and the abelisaurids exhibiting sustained high rates of cranial evolution during the Early Cretaceous. These results provide much needed context for the evolutionary dynamics of ceratosaurian theropods, contributing to broader understanding of macroevolutionary patterns across dinosaurs.

Osteohistological insight into the growth dynamics of early dinosaurs and their contemporaries

Dinosauria debuted on Earth's stage in the aftermath of the Permo-Triassic Mass Extinction Event, and survived two other Triassic extinction intervals to eventually dominate terrestrial ecosystems. More than 231 million years ago, in the Upper Triassic Ischigualasto Formation of west-central Argentina, dinosaurs were just getting warmed up. At this time, dinosaurs represented a minor fraction of ecosystem diversity. Members of other tetrapod clades, including synapsids and pseudosuchians, shared convergently evolved features related to locomotion, feeding, respiration, and metabolism and could have risen to later dominance. However, it was Dinosauria that radiated in the later Mesozoic most significantly in terms of body size, diversity, and global distribution. Elevated growth rates are one of the adaptations that set later Mesozoic dinosaurs apart, particularly from their contemporary crocodilian and mammalian compatriots. When did the elevated growth rates of dinosaurs first evolve? How did the growth strategies of the earliest known dinosaurs compare with those of other tetrapods in their ecosystems? We studied femoral bone histology of an array of early dinosaurs alongside that of non-dinosaurian contemporaries from the Ischigualasto Formation in order to test whether the oldest known dinosaurs exhibited novel growth strategies. Our results indicate that the Ischigualasto vertebrate fauna collectively exhibits relatively high growth rates. Dinosaurs are among the fastest growing taxa in the sample, but they occupied this niche alongside crocodylomorphs, archosauriformes, and large-bodied pseudosuchians. Interestingly, these dinosaurs grew at least as quickly, but more continuously than sauropodomorph and theropod dinosaurs of the later Mesozoic. These data suggest that, while elevated growth rates were ancestral for Dinosauria and likely played a significant role in dinosaurs' ascent within Mesozoic ecosystems, they did not set them apart from their contemporaries.

Incongruent ontogenetic maturity indicators in a Late Triassic archosaur (Aetosauria: Typothorax coccinarum)

Maximum individual body size in pseudosuchian archosaurs is not well constrained in the fossil record, but it may be influenced by a variety of factors including basal metabolic rate, evolutionary relationships, and environmental conditions. Body size varies among the Aetosauria in which estimated total length ranges between 1 m (e.g., Coahomasuchus kahleorum) and 5 m (e.g., Desmatosuchus spurensis). A new, very large specimen of the aetosaurian Typothorax coccinarum from Petrified Forest National Park in northeastern Arizona is nearly twice the size of all other known specimens of Typothorax and is the largest aetosaur specimen currently known worldwide. The specimen lacks co-ossified neurocentral sutures in the trunk vertebrae which may suggest that the individual had not yet reached skeletal maturity, yet smaller specimens of T. coccinarum exhibit partially or fully co-ossified neurocentral sutures in the same region. If body size correlates with skeletal maturity in aetosaurs, this discrepancy warns that timing of neurocentral suture co-ossification in aetosaurs may not be a reliable indicator of ontogenetic stage. Osteohistological observations of a trunk rib demonstrate that although PEFO 42506 shows a large body size, the specimen did not deposit an external fundamental system despite depositing as many as 19 growth lines, further indicating that it had not yet reached skeletal maturity. Thus, at least within Aetosauria, neurocentral suture co-ossification and skeletal maturity may correlate, whereas body size can be incongruent in comparison. Furthermore, this specimen indicates that non-desmatosuchin aetosaurs could exhibit large body sizes and suggests that some aetosaurs may have experienced indeterminate growth.

The osteology of Shuvosaurus inexpectatus, a shuvosaurid pseudosuchian from the Upper Triassic Post Quarry, Dockum Group of Texas, USA

A vast array of pseudosuchian body plans evolved during the diversification of the group in the Triassic Period, but few can compare to the toothless, long-necked, and bipedal shuvosaurids. Members of this clade possess theropod-like character states mapped on top of more plesiomorphic pseudosuchian character states, complicating our understanding of the evolutionary history of the skeleton. One taxon in this clade, Shuvosaurus inexpectatus has been assigned to various theropod dinosaur groups based on a partial skull and referred material and its postcranium was assigned to a different taxon in Pseudosuchia. After the discovery of a skeleton of a shuvosaurid with a Shuvosaurus-like skull and a pseudosuchian postcranial skeleton, it became clear Shuvosaurus inexpectatus was a pseudosuchian. Nevertheless, a number of questions have arisen about what skeletal elements belonged to Shuvosaurus inexpectatus, the identification of skull bones, and the resulting implication for pseudosuchian evolution. Here, we detail the anatomy of the skeleton Shuvosaurus inexpectatus through a critical lens, parse out the bones that belong to the taxon or those that clearly do not or may not belong to the taxon, rediagnose the taxon based on these revisions, and compare the taxon to other archosaurs. We find that Shuvosaurus inexpectatus possesses similar anatomy to other shuvosaurids but parts of the skeleton of the taxon clarifies the anatomy of the group given that they are preserved in Shuvosaurus inexpectatus but not in others. Shuvosaurus inexpectatus is represented by at least 14 individuals from the West Texas Post Quarry (Adamanian holochronozone) and all Shuvosaurus inexpectatus skeletal material from the locality pertains to skeletally immature individuals. All of the skeletons are missing most of the neural arches, ribs, and most of the forelimb. We only recognize Shuvosaurus inexpectatus from the Post Quarry and all other material assigned to the taxon previously is better assigned to the broader group Shuvosauridae.

Bone histology supports gregarious behavior and an early ontogenetic stage to Decuriasuchus quartacolonia (Pseudosuchia: Loricata) from the Middle-Late Triassic of Brazil

Decuriasuchus quartacolonia is a middle-sized basal "rauisuchian" (Pseudosuchia, Loricata) from the Triassic beds of Brazil, whose original description was based on 10 specimens of equivalent size found in aggregation. In this contribution, we explore the osteohistology of its appendicular bones and a rib, aiming to infer growth patterns and ontogeny. In all analyzed bones (except in the rib) from three individuals, we found fibrolamellar bone (FLB) throughout the cortices, a pattern shared with other histologically sampled basal loricatans. We found evidence that suggests an early ontogenetic stage for all studied specimens: absence of secondary osteons, lack of transition from an inner highly vascularized FLB matrix to a poor vascularized parallel-fibred bone, one line of arrested growth, open vascular canals in the external surface of the cortex and absence of an external fundamental system. In addition, we observed that the neurocentral sutures in the caudal to cervical vertebrae of the holotype are unfused, strongly suggesting that these individuals were not skeletally mature, as had been previously assumed. In addition, our data support the prior hypothesis that Decuriasuchus is the oldest-known archosaur to show evidence of gregarious behavior and adds a probable cause of this as a strategy to obtain food and avoid predation until becoming independent as adults. Furthermore, our results open the possibility that Decuriasuchus may represent an earlier growth stage of the larger Prestosuchus chiniquensis, with two specimens recovered from the same stratigraphic level and paleontological site. Since the adult form of Decuriasuchus remains unknown, a deeper anatomical study is needed to discuss the validity of the species. This study case exemplifies the importance of paleohistology as an essential complementary tool to describe a new morphospecies in vertebrate paleontology.

On a skeletally immature individual of Unaysaurus tolentinoi (Dinosauria: Sauropodomorpha) from the upper Triassic of southern Brazil

The lineage of sauropodomorph dinosaurs raised some of the most impressive animals that ever walked on Earth. However, the massive titans of the Mesozoic Era originated from far smaller dinosaurs. The Triassic beds from Brazil yielded the earliest part of this evolutionary history. Despite the diverse fossil record of early sauropodomorphs, juvenile specimens, as well as certain species are poorly sampled. This is the case for Unaysaurus tolentinoi, an unaysaurid sauropodomorph from Caturrita Formation (ca. 225 Ma; early Norian, Late Triassic). The holotype and only specimen of U. tolentinoi was excavated from the Água Negra Locality (São Martinho da Serra, Rio Grande do Sul, Brazil) in 1998. More than two decades later, no other fossil vertebrates have been reported from the same fossiliferous site. Here we describe a skeletally immature specimen which was found in association with the holotype of U. tolentinoi. The specimen was discovered after a first-hand examination of the holotype and comprises some isolated vertebrae and elements from the posterior autopodium. According to linear regressions, its metatarsal I is approximately 41.7 mm in length, compared to approximately 75.9 mm in the holotype. The repeated elements and reduced size indicates that it does not belong to the elements originally used to erect U. tolentinoi. Rather, the specimen is assigned to U. tolentinoi by topotypy and shared morphology. In addition to the reduced size, distinct lines of evidence (e.g., neurocentral sutures; bone texture) support its assignment to a skeletally immature individual. In sum, the new material expands the record of U. tolentinoi, and represents an additional juvenile dinosaur from the Caturrita Formation.

Cranial and postcranial anatomy of a juvenile baurusuchid (Notosuchia, Crocodylomorpha) and the taxonomical implications of ontogeny

Baurusuchidae comprises a clade of top-tier terrestrial predators and are among the most abundant crocodyliforms found in the Adamantina Formation, Bauru Basin, Brazil (Campanian-Maastrichtian). Here, we provide a detailed description of the cranial and postcranial osteology and myology of the most complete juvenile baurusuchid found to date. Although the preservation of juvenile individuals is somewhat rare, previously reported occurrences of baurusuchid egg clutches, a yearling individual, and larger, but skeletally immature specimens, comprise a unique opportunity to track anatomical changes throughout their ontogenetic series. Its cranial anatomy was resolved with the aid of a three-dimensional model generated by the acquisition of computed tomography data, and its inferred adductor mandibular musculature was compared to that of mature specimens in order to assess possible ontogenetic shifts. A subsequent phylogenetic analysis included the scoring of Gondwanasuchus scabrosus, the smallest baurusuchid species known to date, to evaluate its phylogenetic relations relative to a known juvenile. We find considerable differences between juveniles and adults concerning skull ornamentation and muscle development, which might indicate ontogenetic niche partitioning, and also anatomical and phylogenetic evidence that G. scabrosus corresponds to a young semaphoront lacking mature cranial features.

A juvenile bird with possible crown-group affinities from a dinosaur-rich Cretaceous ecosystem in North America

BACKGROUND

Living birds comprise the most speciose and anatomically diverse clade of flying vertebrates, but their poor early fossil record and the lack of resolution around the relationships of the major clades have greatly obscured extant avian origins.

RESULTS

Here, I describe a Late Cretaceous bird from North America based on a fragmentary skeleton that includes cranial material and portions of the forelimb, hindlimb, and foot and is identified as a juvenile based on bone surface texture. Several features unite this specimen with crown Aves, but its juvenile status precludes the recognition of a distinct taxon. The North American provenance of the specimen supports a cosmopolitan distribution of early crown birds, clashes with the hypothesized southern hemisphere origins of living birds, and demonstrates that crown birds and their closest relatives coexisted with non-avian dinosaurs that independently converged on avian skeletal anatomy, such as the alvarezsaurids and dromaeosaurids.

CONCLUSIONS

By revealing the ecological and biogeographic context of Cretaceous birds within or near the crown clade, the Lance Formation specimen provides new insights into the contingent nature of crown avian survival through the Cretaceous-Paleogene mass extinction and the subsequent origins of living bird diversity.

Palaeobiological inferences of "rauisuchians" Fasolasuchus tenax (Los Colorados Fm., Argentina) and Prestosuchus chiniquensis (Santa Maria Super sequence, Brazil) from the Middle-Upper Triassic of South America based on microstructural analyses

"Rauisuchia" is a non-monophyletic group of quadrupedal and carnivorous pseudosuchians that inhabited the entire world during the Middle-Upper Triassic period (Anisian/Ladinian-Rhaetian). In South America, "rauisuchians" reached the largest sizes among continental carnivores. Despite their important ecological role, some aspects of their palaeobiology have been poorly examined. Here, we study appendicular bones, dorsal ribs and osteoderms of two genera, the Argentinean Fasolasuchus tenax (PVL 3850, holotype) and the Brazilian Prestosuchus chiniquensis (SNSB-BSPG AS XXV) respectively. The femur of F. tenax is formed by laminar fibrolamellar bone, which is composed of non-fully monorefringent woven-fibred matrix and primary osteons; the dorsal rib has a Haversian bone composition with an external fundamental system recorded and the osteoderm is formed by well-organised parallel-fibred bone. The femur, humerus and fibula of P. chiniquensis are mostly composed of strongly arranged parallel-fibred bone and a laminar vascularisation. The minimal ages obtained correspond to 9 years for F. tenax (based on the maximum number of growth marks in the osteoderm) and 4 years for P. chiniquensis (obtained from the highest count of growth marks in the femur and in the humerus). F. tenax attained somatic and skeletal maturity, while P. chiniquensis was near to reaching skeletal and sexual maturity, but it was somatically immature. The overall rapid growth rate and the high and uniform vascularisation seems to imply that these features are common in most of "rauisuchians", except in P. chiniquensis.

A Morrison stem gekkotan reveals gecko evolution and Jurassic biogeography

Geckos are a speciose and globally distributed clade of Squamata (lizards, including snakes and amphisbaenians) that are characterized by a host of modifications for nocturnal, scansorial and insectivorous ecologies. They are among the oldest divergences in the lizard crown, so understanding the origin of geckoes (Gekkota) is essential to understanding the origin of Squamata, the most species-rich extant tetrapod clade. However, the poor fossil record of gekkotans has obscured the sequence and timing of the assembly of their distinctive morphology. Here, we describe the first North American stem gekkotan based on a three-dimensionally preserved skull from the Morrison Formation of western North America. Despite its Late Jurassic age, the new species already possesses several key characteristics of the gekkotan skull along with retained ancestral features. We show that this new stem gekkotan, and several previously named species of uncertain phylogenetic relationships, comprise a widespread clade of early crown lizards, substantiating faunal homogeneity in Laurasia during the Late Jurassic that extended across disparate ecological, body-size and physiological classes.

Modified skulls but conservative brains? The palaeoneurology and endocranial anatomy of baryonychine dinosaurs (Theropoda: Spinosauridae)

The digital reconstruction of neurocranial endocasts has elucidated the gross brain structure and potential ecological attributes of many fossil taxa, including Irritator, a spinosaurine spinosaurid from the "mid" Cretaceous (Aptian) of Brazil. With unexceptional hearing capabilities, this taxon was inferred to integrate rapid and controlled pitch-down movements of the head that perhaps aided in the predation of small and agile prey such as fish. However, the neuroanatomy of baryonychine spinosaurids remains to be described, and potentially informs on the condition of early spinosaurids. Using micro-computed tomographic scanning (μCT), we reconstruct the braincase endocasts of Baryonyx walkeri and Ceratosuchops inferodios from the Wealden Supergroup (Lower Cretaceous) of England. We show that the gross endocranial morphology is similar to other non-maniraptoriform theropods, and corroborates previous observations of overall endocranial conservatism amongst more basal theropods. Several differences of unknown taxonomic utility are noted between the pair. Baryonychine neurosensory capabilities include low-frequency hearing and unexceptional olfaction, whilst the differing morphology of the floccular lobe tentatively suggests less developed gaze stabilisation mechanisms relative to spinosaurines. Given the morphological similarities observed with other basal tetanurans, baryonychines likely possessed comparable behavioural sophistication, suggesting that the transition from terrestrial hypercarnivorous ancestors to semi-aquatic "generalists" during the evolution of Spinosauridae did not require substantial modification of the brain and sensory systems.

Cranial anatomy of the mekosuchine crocodylian Trilophosuchus rackhami Willis, 1993

One of the best-preserved crocodylian fossil specimens from the Cenozoic of Australia is the holotype of the mekosuchine Trilophosuchus rackhami, from the middle Miocene (13.56 ± 0.67 Ma) Ringtail Site at Riversleigh, northwestern Queensland. Although lacking most of the snout, the holotype skull of T. rackhami (QMF16856) has an exceptionally well-preserved cranium. Micro-CT scanning of the holotype has allowed for all the preserved cranial bones to be digitally disarticulated, facilitating an unprecedented insight into the cranial anatomy of not just T. rackhami, but any mekosuchine. Trilophosuchus rackhami was a small-bodied crocodylian and one of the most morphologically distinct mekosuchines, characterized by a unique combination of cranial characteristics several of which are exclusive to the species. Fossil material that is definitively referrable to the species T. rackhami is currently known solely from the middle Miocene Ringtail Site. However, an isolated parietal from Hiatus Site at Riversleigh demonstrates that Trilophosuchus also occurred during the late Oligocene (~25 Ma), extending the range of the genus by more than 10 million years. The new description of T. rackhami also allowed for a reevaluation of its phylogenetic relationships. Our results reaffirm the placement of T. rackhami as a member of Mekosuchinae within the subclade Mekosuchini. In all analyses, Mekosuchinae was consistently found to be monophyletic and part of the larger crocodylian clade Longirostres. However, the assignment of Mekosuchinae as a subset of Crocodylidae is brought into question, suggesting that the status of Mekosuchinae as a subfamily should be reconsidered.

Evolutionary origins of the prolonged extant squamate radiation

Squamata is the most diverse clade of terrestrial vertebrates. Although the origin of pan-squamates lies in the Triassic, the oldest undisputed members of extant clades known from nearly complete, uncrushed material come from the Cretaceous. Here, we describe three-dimensionally preserved partial skulls of two new crown lizards from the Late Jurassic of North America. Both species are placed at the base of the skink, girdled, and night lizard clade Pan-Scincoidea, which consistently occupies a position deep inside the squamate crown in both morphological and molecular phylogenies. The new lizards show that several features uniting pan-scincoids with another major lizard clade, the pan-lacertoids, in trees using morphology were convergently acquired as predicted by molecular analyses. Further, the palate of one new lizard bears a handful of ancestral saurian characteristics lost in nearly all extant squamates, revealing an underappreciated degree of complex morphological evolution in the early squamate crown. We find strong evidence for close relationships between the two new species and Cretaceous taxa from Eurasia. Together, these results suggest that early crown squamates had a wide geographic distribution and experienced complicated morphological evolution even while the Rhynchocephalia, now solely represented by the tuatara, was the dominant clade of lepidosaurs.

A gigantic bizarre marine turtle (Testudines: Chelonioidea) from the Middle Campanian (Late Cretaceous) of South-western Europe

Marine turtles were common in the subtropical Upper Cretaceous epi-continental seas that once washed the coasts of the ancient European archipelago. But unlike its contemporaneous faunas from North America, in Europe no taxon surpassed the 1.5 m shell-length. Here, the remains of a new large marine turtle, Leviathanochelys aenigmatica gen. et sp. nov., from the Middle Campanian of the Southern Pyrenees are described. Anatomical and histological evidence concur in identifying the specimen as a basal chelonioid. The new taxon autapomorphically differs from other marine turtles by possessing an additional process on the anteromedial side of the pelvis, and an acetabulum directed strongly ventrally. Based on the pelvis size, it is likely that Leviathanochelys was as large as Archelon, thus becoming one of the largest marine turtles found to ever exist. The large body size of the new taxon could have evolved as a response to the unique habitat conditions of the European Cretaceous archipelago seas. The presence of the accessory pubic process further suggests the occurrence of an additional insertion point of the Musculus rectus abdominis, which together with the paleohistologic evidences support the hypothesis that the new taxon had an open marine pelagic lifestyle.

Ontogenetic growth in the crania of Exaeretodon argentinus (Synapsida: Cynodontia) captures a dietary shift

Background

An ontogenetic niche shift in vertebrates is a common occurrence where ecology shifts with morphological changes throughout growth. How ecology shifts over a vertebrate's lifetime is often reconstructed in extant species-by combining observational and skeletal data from growth series of the same species-because interactions between organisms and their environment can be observed directly. However, reconstructing shifts using extinct vertebrates is difficult and requires well-sampled growth series, specimens with relatively complete preservation, and easily observable skeletal traits associated with ecologies suspected to change throughout growth, such as diet.

Methods

To reconstruct ecological changes throughout the growth of a stem-mammal, we describe changes associated with dietary ecology in a growth series of crania of the large-bodied (∼2 m in length) and herbivorous form, Exaeretodon argentinus (Cynodontia: Traversodontidae) from the Late Triassic Ischigualasto Formation, San Juan, Argentina. Nearly all specimens were deformed by taphonomic processes, so we reconstructed allometric slope using a generalized linear mixed effects model with distortion as a random effect.

Results

Under a mixed effects model, we find that throughout growth, E. argentinus reduced the relative length of the palate, postcanine series, orbits, and basicranium, and expanded the relative length of the temporal region and the height of the zygomatic arch. The allometric relationship between the zygomatic arch and temporal region with the total length of the skull approximate the rate of growth for feeding musculature. Based on a higher allometric slope, the zygoma height is growing relatively faster than the length of the temporal region. The higher rate of change in the zygoma may suggest that smaller individuals had a crushing-dominated feeding style that transitioned into a chewing-dominated feeding style in larger individuals, suggesting a dietary shift from possible faunivory to a more plant-dominated diet. Dietary differentiation throughout development is further supported by an increase in sutural complexity and a shift in the orientation of microwear anisotropy between small and large individuals of E. argentinus. A developmental transition in the feeding ecology of E. argentinus is reflective of the reconstructed dietary transition across Gomphodontia, wherein the earliest-diverging species are inferred as omnivorous and the well-nested traversodontids are inferred as herbivorous, potentially suggesting that faunivory in immature individuals of the herbivorous Traversodontidae may be plesiomorphic for the clade.

Intraspecific variation in the cranial osteology of Diplometopon zarudnyi (Squamata: Amphisbaenia: Trogonophidae)

A snake-like body plan and burrowing lifestyle characterize numerous vertebrate groups as a result of convergent evolution. One such group is the amphisbaenians, a clade of limbless, fossorial lizards that exhibit head-first burrowing behavior. Correlated with this behavior, amphisbaenian skulls are more rigid and coossified than those of nonburrowing lizards. However, due to their lifestyle, there are many gaps in our understanding of amphisbaenian anatomy, including how their cranial osteology varies among individuals of the same species and what that reveals about constraints on the skull morphology of head-first burrowing taxa. We investigated intraspecific variation in the cranial osteology of amphisbaenians using seven individuals of the trogonophid Diplometopon zarudnyi. Variation in both skull and individual skull element morphology was examined qualitatively and quantitatively through three-dimensional (3D) models created from microcomputed tomography data. Qualitative examination revealed differences in the number and position of foramina, the interdigitation between the frontals and parietal, and the extent of coossification among the occipital complex, fused basioccipital and parabasisphenoid ("parabasisphenoid-basioccipital complex"), and elements X. We performed 3D landmark-based geometric morphometrics for the quantitative assessment, revealing shape differences in the skull, premaxilla, maxilla, frontal, and parietal. The observed intraspecific variation may be the result of different stages of ontogenetic development or biomechanical optimization for head-first burrowing. For example, variation in the coossification of the occipital region suggests a potential ontogenetic coossification sequence. Examination of these areas of variation across other head-first burrowing taxa will help determine if the variation is clade-specific or part of a broader macroevolutionary pattern of head-first burrowing.

Interelemental osteohistological variation in Massospondylus carinatus and its implications for locomotion

Massospondylus carinatus Owen, 1854 is an iconic basal sauropodomorph dinosaur from the Early Jurassic of southern Africa. Over 200 specimens have been referred to this taxon, spanning the entire ontogenetic series from embryo to adult. Consequently, it provides an ideal sample for investigating dinosaur developmental biology, including growth patterns and growth rates, through osteohistological analysis. Massospondylus carinatus was the first early-branching sauropodomorph dinosaur for which a femoral growth series was sampled. Since then, growth series of other non-avian dinosaur taxa have shown that growth plasticity, interelemental variation, and ontogenetic locomotory shifts can complicate our understanding of growth curves and patterns. To investigate these questions further, it is necessary to sample multiple skeletal elements from multiple individuals across a large range of sizes, something that is often hindered by the incompleteness of the fossil record. Here, we conducted a broad, multielement osteohistological study of long bones (excluding metapodials) from 27 specimens of Massospondylus carinatus that span its ontogenetic series. Our study reveals substantial variations in growth history. A cyclical woven-parallel complex is the predominant bone tissue pattern during early and mid-ontogeny, which transitions to slower forming parallel-fibred bone during very late ontogeny. The bone tissue is interrupted by irregularly spaced cyclical growth marks (CGMs) including lines of arrested growth indicating temporary cessations in growth. These CGMs show that the previously recorded femoral growth plasticity is also visible in other long bones, with a poor correlation between body size (measured by midshaft circumference) and CGM numbers. Furthermore, we found that the growth trajectory for an individual can vary depending on which limb element is studied. This makes the establishment of an accurate growth curve and determination of the onset of reproductive maturity difficult for this taxon. Finally, we found no evidence of differential growth rates in forelimb vs hindlimb samples from the same individual, providing further evidence falsifying hypothesised ontogenetic postural shifts in Massospondylus carinatus.

Africa's oldest dinosaurs reveal early suppression of dinosaur distribution

The vertebrate lineages that would shape Mesozoic and Cenozoic terrestrial ecosystems originated across Triassic Pangaea^1-11. By the Late Triassic (Carnian stage, ~235 million years ago), cosmopolitan 'disaster faunas' (refs. ^12-14) had given way to highly endemic assemblages^12,13 on the supercontinent. Testing the tempo and mode of the establishment of this endemism is challenging-there were few geographic barriers to dispersal across Pangaea during the Late Triassic. Instead, palaeolatitudinal climate belts, and not continental boundaries, are proposed to have controlled distribution^15-18. During this time of high endemism, dinosaurs began to disperse and thus offer an opportunity to test the timing and drivers of this biogeographic pattern. Increased sampling can test this prediction: if dinosaurs initially dispersed under palaeolatitudinal-driven endemism, then an assemblage similar to those of South America^4,19-21 and India^19,22-including the earliest dinosaurs-should be present in Carnian deposits in south-central Africa. Here we report a new Carnian assemblage from Zimbabwe that includes Africa's oldest definitive dinosaurs, including a nearly complete skeleton of the sauropodomorph Mbiresaurus raathi gen. et sp. nov. This assemblage resembles other dinosaur-bearing Carnian assemblages, suggesting that a similar vertebrate fauna ranged high-latitude austral Pangaea. The distribution of the first dinosaurs is correlated with palaeolatitude-linked climatic barriers, and dinosaurian dispersal to the rest of the supercontinent was delayed until these barriers relaxed, suggesting that climatic controls influenced the initial composition of the terrestrial faunas that persist to this day.

Moon B, Yan C, Motani R, Jiang D, An Z, Fang Z. The oldest record of Saurosphargiformes (Diapsida) from South China could fill an ecological gap in the Early Triassic biotic recovery

Diversification following the end-Permian mass extinction marks the initiation of Mesozoic reptile dominance and of modern marine ecosystems, yet major clades are best known from the Middle Triassic suggesting delayed recovery, while Early Triassic localities produce poorly preserved specimens or have restricted diversity. Here we describe Pomolispondylus biani gen. et sp. nov. from the Early Triassic Nanzhang-Yuan'an Fauna of China assigned to Saurosphargiformes tax. nov., a clade known only from the Middle Triassic or later, which includes Saurosphargidae, and likely is the sister taxon to Sauropterygia. Pomolispondylus biani is allied to Saurosphargidae by the extended transverse processes of dorsal vertebrae and a low, table-like dorsal surface on the neural spine; however, it does not have the typical extensive osteoderms. Rather an unusual tuberous texture on the dorsal neural spine and rudimentary ossifications lateral to the gastralia are observed. Discovery of Pomolispondylus biani extends the known range of Saurosphargiformes and increases the taxic and ecological diversity of the Nanzhang-Yuan'an Fauna. Its small size fills a different ecological niche with respect to previously found species, but the overall food web remains notably different in structure to Middle Triassic and later ecosystems, suggesting this fauna represents a transitional stage during recovery rather than its endpoint.

Radial porosity profiles: a new bone histological method for comparative developmental analysis of diametric limb bone growth

In fossil tetrapods, limb bone histology is considered the most reliable tool not only for inferring skeletal maturity-a crucial assessment in palaeobiological and evolutionary studies-but also for evaluating the growth dynamics within the ontogenetic window represented by the primary bone cortex. Due to its complex relationship with bone growth and functional maturation, primary cortical vascularity is an indispensable osteohistological character for reconstructing growth dynamics, especially in the context of various developmental strategies along the precocial-altricial spectrum. Using this concept as our working hypothesis, we developed a new quantitative osteohistological parameter, radial porosity profile (RPP), that captures relative cortical porosity changes in limb bones as trajectories. We built a proof-of-concept RPP dataset on extant birds, then added fossil paravian dinosaurs and performed a set of trajectory-grouping analyses to identify potential RPP categories and evaluate them in the context of our ontogeny-developmental strategy working hypothesis. We found that RPPs, indeed, reflect important developmental features within and across elements, specimens and taxa, supporting their analytical power. Our RPPs also revealed unexpected potential osteohistological correlates of growth and functional development of limb bones. The diverse potential applications of RPPs open up new research directions in the evolution of locomotor ontogeny.

A new juvenile Yamaceratops (Dinosauria, Ceratopsia) from the Javkhlant Formation (Upper Cretaceous) of Mongolia

Here we report a new articulated skeleton of Yamaceratops dorngobiensis (MPC-D 100/553) from the Khugenetjavkhlant locality at the Shine Us Khudag (Javkhlant Formation, ?Santonian-Campanian) of the eastern Gobi Desert, Mongolia, which represents the first substantially complete skeleton and the first juvenile individual of this taxon. The specimen includes a nearly complete cranium and large portions of the vertebral column and appendicular skeleton. Its skull is about 2/3 the size of the holotype specimen, based on mandibular length. Its juvenile ontogenetic stage is confirmed by multiple indicators of skeletal and morphological immaturity known in ceratopsians, such as the long-grained surface texture on the long bones, the smooth external surface on the postorbital, open neurocentral sutures of all caudal vertebrae, a large orbit relative to the postorbital and jugal, the low angle of the lacrimal ventral ramus relative to the maxillary teeth row, narrow frontal, and straight ventral edge of the dentary. Osteohistological analysis of MPC-D 100/553 recovered three lines of arrested growth, implying around 3 years of age when it died, and verified this specimen's immature ontogenetic stage. The specimen adds a new autapomorphy of Yamaceratops, the anteroventral margin of the fungiform dorsal end of the lacrimal being excluded from the antorbital fossa. Furthermore, it shows a unique combination of diagnostic features of some other basal neoceratopsians: the ventrally hooked rostral bone as in Aquilops americanus and very tall middle caudal neural spines about or more than four times as high as the centrum as in Koreaceratops hwaseongensis, Montanoceratops cerorhynchus, and Protoceratops andrewsi. The jugal with the subtemporal ramus deeper than the suborbital ramus as in the holotype specimen is also shared with A. americanus, Liaoceratops yanzigouensis, and juvenile P. andrewsi. Adding 38 new scorings into the recent comprehensive data matrix of basal Neoceratopsia and taking into account the ontogenetically variable characters recovered Y. dorngobiensis as the sister taxon to Euceratopsia (Leptoceratopsidae plus Coronosauria). A second phylogenetic analysis with another matrix for Ceratopsia also supported this position. The new phylogenetic position of Y. dorngobiensis is important in ceratopsian evolution, as this taxon represents one of the basalmost neoceratopsians with a broad, thin frill and hyper-elongated middle caudal neural spines while still being bipedal.

Subaqueous foraging among carnivorous dinosaurs

Secondary aquatic adaptations evolved independently more than 30 times from terrestrial vertebrate ancestors^1,2. For decades, non-avian dinosaurs were believed to be an exception to this pattern. Only a few species have been hypothesized to be partly or predominantly aquatic^3-11. However, these hypotheses remain controversial^12,13, largely owing to the difficulty of identifying unambiguous anatomical adaptations for aquatic habits in extinct animals. Here we demonstrate that the relationship between bone density and aquatic ecologies across extant amniotes provides a reliable inference of aquatic habits in extinct species. We use this approach to evaluate the distribution of aquatic adaptations among non-avian dinosaurs. We find strong support for aquatic habits in spinosaurids, associated with a marked increase in bone density, which precedes the evolution of more conspicuous anatomical modifications, a pattern also observed in other aquatic reptiles and mammals^14-16. Spinosaurids are revealed to be aquatic specialists with surprising ecological disparity, including subaqueous foraging behaviour in Spinosaurus and Baryonyx, and non-diving habits in Suchomimus. Adaptation to aquatic environments appeared in spinosaurids during the Early Cretaceous, following their divergence from other tetanuran theropods during the Early Jurassic¹⁷.

A new pseudosuchian archosaur, Mambawakale ruhuhu gen. et sp. nov., from the Middle Triassic Manda Beds of Tanzania

The Manda Beds of southwest Tanzania have yielded key insights into the early evolutionary radiation of archosaurian reptiles. Many key archosaur specimens were collected from the Manda Beds in the 1930s and 1960s, but until recently, few of these had been formally published. Here, we describe an archosaur specimen collected in 1963 which has previously been referred to informally as Pallisteria angustimentum. We recognize this specimen as the type of a new taxon, Mambawakale ruhuhu gen. et sp. nov. The holotype and only known specimen of M. ruhuhu comprises a partial skull of large size (greater than 75 cm inferred length), lower jaws and fragments of the postcranium, including three anterior cervical vertebrae and a nearly complete left manus. Mambawakale ruhuhu is characterized by several cranial autapomorphies that allow it to be distinguished with confidence from all other Manda Beds archosaurs, with the possible exception of Stagonosuchus nyassicus for which comparisons are highly constrained due to very limited overlapping material. Phylogenetic analysis suggests that M. ruhuhu is an early diverging pseudosuchian, but more precise resolution is hampered by missing data. Mambawakale ruhuhu is one of the largest known pseudosuchians recovered to date from the Middle Triassic.

Ontogenetic variation in the skull of Stenopterygius quadriscissus with an emphasis on prenatal development

The availability of a large sample size from a range of ontogenetic stages makes Stenopterygius quadriscissus a good model to study ontogenetic variation in a fossil sauropsid. We qualitatively examined pre- and postnatal ontogenetic changes in the cranium of S. quadriscissus. The prenatal ossification sequence is similar to other diapsids, exhibiting delayed chondrocranial ossification compared to the dermatocranium. In the dermatocranium, the circumorbital area is more ossified earlier in development relative to other elements, especially those of the skull roof where ossification is comparatively weaker across prenatal stages. Perinatally all cranial elements are ossified, and many scarf and step joints are already closed. We propose four prenatal and three postnatal stages in S. quadriscissus on the basis of relative ossification, size and qualitative cranial characters pertaining to the jugal, parietal, frontal, pterygoid and surangular. These will provide a basis for determining ontogenetic stages in other ichthyosaurs. Moreover, our postnatal observations aid in refining ontogenetic characters for phylogenetic studies. Lastly, we observed that the antimeric sutures of the midline of the skull roof are open perinatally and that fusion of the midline only appears in the adult stage. We hypothesize that the loose connection of the midline functions as a fontanelle, limiting potential damage during birth.

Forty new specimens of Ichthyornis provide unprecedented insight into the postcranial morphology of crownward stem group birds

Ichthyornis has long been recognized as a pivotally important fossil taxon for understanding the latest stages of the dinosaur-bird transition, but little significant new postcranial material has been brought to light since initial descriptions of partial skeletons in the 19^th Century. Here, we present new information on the postcranial morphology of Ichthyornis from 40 previously undescribed specimens, providing the most complete morphological assessment of the postcranial skeleton of Ichthyornis to date. The new material includes four partially complete skeletons and numerous well-preserved isolated elements, enabling new anatomical observations such as muscle attachments previously undescribed for Mesozoic euornitheans. Among the elements that were previously unknown or poorly represented for Ichthyornis, the new specimens include an almost-complete axial series, a hypocleideum-bearing furcula, radial carpal bones, fibulae, a complete tarsometatarsus bearing a rudimentary hypotarsus, and one of the first-known nearly complete three-dimensional sterna from a Mesozoic avialan. Several pedal phalanges are preserved, revealing a remarkably enlarged pes presumably related to foot-propelled swimming. Although diagnosable as Ichthyornis, the new specimens exhibit a substantial degree of morphological variation, some of which may relate to ontogenetic changes. Phylogenetic analyses incorporating our new data and employing alternative morphological datasets recover Ichthyornis stemward of Hesperornithes and Iaceornis, in line with some recent hypotheses regarding the topology of the crownward-most portion of the avian stem group, and we establish phylogenetically-defined clade names for relevant avialan subclades to help facilitate consistent discourse in future work. The new information provided by these specimens improves our understanding of morphological evolution among the crownward-most non-neornithine avialans immediately preceding the origin of crown group birds.

An Italian dinosaur Lagerstätte reveals the tempo and mode of hadrosauriform body size evolution

During the latest Cretaceous, the European Archipelago was characterized by highly fragmented landmasses hosting putative dwarfed, insular dinosaurs, claimed as fossil evidence of the “island rule”. The Villaggio del Pescatore quarry (north-eastern Italy) stands as the most informative locality within the palaeo-Mediterranean region and represents the first, multi-individual Konservat-Lagerstätte type dinosaur-bearing locality in Italy. The site is here critically re-evaluated as early Campanian in age, thus preceding the final fragmentation stages of the European Archipelago, including all other European localities preserving hypothesized dwarfed taxa. New skeletal remains allowed osteohistological analyses on the hadrosauroid Tethyshadros insularis indicating subadult features in the type specimen whereas a second, herein newly described, larger individual is likely somatically mature. A phylogenetic comparative framework places the body-size of T. insularis in range with other non-hadrosaurid Eurasian hadrosauroids, rejecting any significant evolutionary trend towards miniaturisation in this clade, confuting its ‘pygmy’ status, and providing unmatched data to infer environmentally-driven body-size trends in Mesozoic dinosaurs.

The first edentulous ceratosaur from South America

The recognition of ontogenetic edentulism in the Jurassic noasaurid Limusaurus inextricabilis shed new light on the dietary diversity within Ceratosauria, a stem lineage of non-avian theropod dinosaurs known for peculiar craniomandibular adaptations. Until now, edentulism in Ceratosauria was exclusive to adult individuals of Limusaurus. Here, an exceptionally complete skeleton of a new toothless ceratosaur, Berthasaura leopoldinae gen. et sp. nov., is described from the Cretaceous aeolian sandstones of the Bauru Basin, Southern Brazil. The specimen resembles adult individuals of Limusaurus by the absence of teeth but based on the unfused condition of several elements (e.g., skull, vertebral column) it clearly represents an ontogenetically immature individual, indicating that it might never have had teeth. The phylogenetic analysis performed here has nested Berthasaura leopoldinae as an early-divergent Noasauridae, not closely related to Limusaurus. It represents the most complete non-avian theropod from the Brazilian Cretaceous and preserves the most complete noasaurid axial series known so far. Moreover, the new taxon exhibits many novel osteological features, uncommon in non-avian theropods, and unprecedented even among South American ceratosaurs. These include not only toothless jaws but also a premaxilla with cutting occlusal edge, and a slightly downturned rostral tip. This indicate that B. leopoldinae unlikely had the same diet as other ceratosaurs, most being regarded as carnivorous. As the ontogenetically more mature specimens of Limusaurus, Berthasaura might have been herbivorous or at least omnivorous, corroborating with an early evolutionary divergence of noasaurids from the ceratosaurian bauplan by disparate feeding modes.

Living fast in the Triassic: New data on life history in Lystrosaurus (Therapsida: Dicynodontia) from northeastern Pangea

Lystrosaurus was one of the few tetrapods to survive the Permo-Triassic mass extinction, the most profound biotic crisis in Earth’s history. The wide paleolatitudinal range and high abundance of Lystrosaurus during the Early Triassic provide a unique opportunity to investigate changes in growth dynamics and longevity following the mass extinction, yet most studies have focused only on species that lived in the southern hemisphere. Here, we present the long bone histology from twenty Lystrosaurus skeletal elements spanning a range of sizes that were collected in the Jiucaiyuan Formation of northwestern China. In addition, we compare the average body size of northern and southern Pangean Triassic-aged species and conduct cranial geometric morphometric analyses of southern and northern taxa to begin investigating whether specimens from China are likely to be taxonomically distinct from South African specimens. We demonstrate that Lystrosaurus from China have larger average body sizes than their southern Pangean relatives and that their cranial morphologies are distinctive. The osteohistological examination revealed sustained, rapid osteogenesis punctuated by growth marks in some, but not all, immature individuals from China. We find that the osteohistology of Chinese Lystrosaurus shares a similar growth pattern with South African species that show sustained growth until death. However, bone growth arrests more frequently in the Chinese sample. Nevertheless, none of the long bones sampled here indicate that maximum or asymptotic size was reached, suggesting that the maximum size of Lystrosaurus from the Jiucaiyuan Formation remains unknown.

A new specimen of Sinopterus dongi (Pterosauria, Tapejaridae) from the Jiufotang Formation (Early Cretaceous, China)

The Tapejarinae are edentulous pterosaurs that are relatively common in Cretaceous continental deposits in South America, North Africa, Europe, and China (mostly Early Cretaceous). The Chinese Jiufotang Formation is particularly rich in tapejarine specimens, having yielded over 10 described specimens and dozens of undescribed ones. For the Jiufotang Formation, a total of seven nominal tapejarid species and two genera have been proposed. Some debate exists over how many of those are valid or, alternatively, sexual or ontogenetic morphs of fewer (or even a single) species. Despite the abundance of specimens and the relevant taxonomic problems involved, detailed revisions of the matter are still lacking. This is partly due to the relatively scarce knowledge on the comparative osteology of the Sinopterus complex, which is hampered by the fact that most specimens have been only preliminarily described. In this contribution, we present a new postcranial specimen, D3072, which we attribute to the type-species of the genus, Sinopterus dongi. This new specimen helps shed some new light in the osteology of Sinopterus dongi, hopefully serving as a basis for future comparative studies involving further specimens and other proposed species and, subsequently, taxonomic revisions.

A second peirosaurid crocodyliform from the Mid-Cretaceous Kem Kem Group of Morocco and the diversity of Gondwanan notosuchians outside South America

Notosuchians are an extinct clade of terrestrial crocodyliforms with a particularly rich record in the late Early to Late Cretaceous (approx. 130-66 Ma) of Gondwana. Although much of this diversity comes from South America, Africa and Indo-Madagascar have also yielded numerous notosuchian remains. Three notosuchian species are currently recognized from the early Late Cretaceous (approx. 100 Ma) Kem Kem Group of Morocco, including the peirosaurid Hamadasuchus rebouli. Here, we describe two new specimens that demonstrate the presence of at least a fourth notosuchian species in this fauna. Antaeusuchus taouzensis n. gen. n. sp. is incorporated into one of the largest notosuchian-focused character-taxon matrices yet to be compiled, comprising 443 characters scored for 63 notosuchian species, with an increased sampling of African and peirosaurid species. Parsimony analyses run under equal and extended implied weighting consistently recover Antaeusuchus as a peirosaurid notosuchian, supported by the presence of two distinct waves on the dorsal dentary surface, a surangular which laterally overlaps the dentary above the mandibular fenestra, and a relatively broad mandibular symphysis. Within Peirosauridae, Antaeusuchus is recovered as the sister taxon of Hamadasuchus. However, it differs from Hamadasuchus with respect to several features, including the ornamentation of the lateral surface of the mandible, the angle of divergence of the mandibular rami, the texture of tooth enamel and the shape of the teeth, supporting their generic distinction. We present a critical reappraisal of the non-South American Gondwanan notosuchian record, which spans the Middle Jurassic-late Eocene. This review, as well as our phylogenetic analyses, indicate the existence of at least three approximately contemporaneous peirosaurid lineages within the Kem Kem Group, alongside other notosuchians, and support the peirosaurid affinities of the 'trematochampsid' Miadanasuchus oblita from the Maastrichtian of Madagascar. Furthermore, the Cretaceous record demonstrates the presence of multiple lineages of approximately contemporaneous notosuchians in several African and Madagascan faunas, and supports previous suggestions regarding an undocumented pre-Aptian radiation of Notosuchia. By contrast, the post-Cretaceous record is depauperate, comprising rare occurrences of sebecosuchians in north Africa prior to their extirpation.

Osteology and relationships of Revueltosaurus callenderi (Archosauria: Suchia) from the Upper Triassic (Norian) Chinle Formation of Petrified Forest National Park, Arizona, United States

Once known solely from dental material and thought to represent an early ornithischian dinosaur, the early-diverging pseudosuchian Revueltosaurus callenderi is described from a minimum of 12 skeletons from a monodominant bonebed in the upper part of the Chinle Formation of Arizona. This material includes nearly the entire skeleton and possesses a combination of plesiomorphic and derived character states that help clarify ingroup relationships within Pseudosuchia. A phylogenetic analysis recovers R. callenderi in a clade with Aetosauria and Acaenasuchus geoffreyi that is named Aetosauriformes. Key autapomorphies of R. callenderi include a skull that is longer than the femur, a complete carapace of dermal armor including paramedian and lateral rows, as well as ventral osteoderms, and a tail end sheathed in bone. Histology of the femur and associated osteoderms demonstrate that R. callenderi was slow growing and that the individuals from the bonebed were not young juveniles but had not ceased growing. A review of other material assigned to Revueltosaurus concludes that the genus cannot be adequately diagnosed based on the type materials of the three assigned species and that only R. callenderi can be confidently referred to Revueltosaurus.

New spinosaurids from the Wessex Formation (Early Cretaceous, UK) and the European origins of Spinosauridae

Spinosaurids are among the most distinctive and yet poorly-known of large-bodied theropod dinosaurs, a situation exacerbated by their mostly fragmentary fossil record and competing views regarding their palaeobiology. Here, we report two new Early Cretaceous spinosaurid specimens from the Wessex Formation (Barremian) of the Isle of Wight. Large-scale phylogenetic analyses using parsimony and Bayesian techniques recover the pair in a new clade within Baryonychinae that also includes the hypodigm of the African spinosaurid Suchomimus. Both specimens represent distinct and novel taxa, herein named Ceratosuchops inferodios gen. et sp. nov. and Riparovenator milnerae gen. et sp. nov. A palaeogeographic reconstruction suggests a European origin for Spinosauridae, with at least two dispersal events into Africa. These new finds provide welcome information on poorly sampled areas of spinosaurid anatomy, suggest that sympatry was present and potentially common in baryonychines and spinosaurids as a whole, and contribute to updated palaeobiogeographic reconstructions for the clade.

Intraspecific variation in the axial skeleton of Aetosauroides scagliai (Archosauria: Aetosauria) and its implications for the aetosaur diversity of the Late Triassic of Brazil

Aetosauria represents a remarkable clade of armored pseudosuchians in which some of its oldest members are recovered from late Carnian units of Brazil. Three species are known: the mid-sized aetosaur Aetosauroides scagliai, which also occurs in Argentina, and two small-sized species, Aetobarbakinoides brasiliensis and Polesinesuchus aurelioi. We provide a detailed description and comparative analysis of the axial skeleton of Aetosauroides, identifying some diagnostic features as variable. These include the deep pocket pit lateral to the base of the neural spine, the presence of the infradiapophyseal laminae and the lateral fossa ventral to the neurocentral suture. These features are not found in smaller and immature Aetosauroides specimens, resembling the condition found in Polesinesuchus, which is based solely on a juvenile individual, as revealed by osteoderm microstructure analysis. As Polesinesuchus cannot be anatomically differentiated from other small individuals of Aetosauroides, we propose it as a junior synonym of Aetosauroides scagliai. Our results shrink the number of putative 'dwarf' aetosaurs, indicating that morphological variation related to ontogeny affects aetosaur taxonomy and phylogeny.

New specimens provide insights into the anatomy of the dinosauriform Lewisuchus admixtus Romer, 1972 from the upper Triassic levels of the Chañares Formation, NW Argentina

Lewisuchus admixtus is an early dinosauriform described by Alfred Romer in 1972 on the basis of a single, incomplete skeleton, collected in lower Upper Triassic rocks of the renowned Chañares Formation, at the Los Chañares type-locality, La Rioja Province, north-western Argentina. Recent field explorations to the type-locality resulted in the discovery of two partial articulated skeletons, which provide significant novel information. The cranial bones, presacral series, femur, tibia, and proximal tarsals of the new specimens match the preserved overlapping anatomy of the holotype and previously referred specimens of L. admixtus, including the presence of unique combination of character states among dinosauriforms (anterior presacral column with additional ossification on the top of neural spines, dorsal neural spines fan-shaped, anterior surface of the astragalus with a dorsally curved groove, and an inflated area on the anterior portion of the medial surface of this bone). This new information improves our understanding of the anatomy and taxonomy of early dinosauriforms and reinforces the role of Argentinean beds on the study of the origin of dinosaurs.

Anatomy and systematics of the diplodocoid Amphicoelias altus supports high sauropod dinosaur diversity in the Upper Jurassic Morrison Formation of the USA

Sauropod dinosaurs were an abundant and diverse component of the Upper Jurassic Morrison Formation of the USA, with 24 currently recognized species. However, some authors consider this high diversity to have been ecologically unviable and the validity of some species has been questioned, with suggestions that they represent growth series (ontogimorphs) of other species. Under this scenario, high sauropod diversity in the Late Jurassic of North America is greatly overestimated. One putative ontogimorph is the enigmatic diplodocoid Amphicoelias altus, which has been suggested to be synonymous with Diplodocus. Given that Amphicoelias was named first, it has priority and thus Diplodocus would become its junior synonym. Here, we provide a detailed re-description of A. altus in which we restrict it to the holotype individual and support its validity, based on three autapomorphies. Constraint analyses demonstrate that its phylogenetic position within Diplodocoidea is labile, but it seems unlikely that Amphicoelias is synonymous with Diplodocus. As such, our re-evaluation also leads us to retain Diplodocus as a distinct genus. There is no evidence to support the view that any of the currently recognized Morrison sauropod species are ontogimorphs. Available data indicate that sauropod anatomy did not dramatically alter once individuals approached maturity. Furthermore, subadult sauropod individuals are not prone to stemward slippage in phylogenetic analyses, casting doubt on the possibility that their taxonomic affinities are substantially misinterpreted. An anatomical feature can have both an ontogenetic and phylogenetic signature, but the former does not outweigh the latter when other characters overwhelmingly support the affinities of a taxon. Many Morrison Formation sauropods were spatio-temporally and/or ecologically separated from one another. Combined with the biases that cloud our reading of the fossil record, we contend that the number of sauropod dinosaur species in the Morrison Formation is currently likely to be underestimated, not overestimated.

Dinosaur senescence: a hadrosauroid with age-related diseases brings a new perspective of "old" dinosaurs

Senile vertebrates are extremely rare in the fossil record, making their recognition difficult. Here we present the largest known representative of the Late Cretaceous hadrosauriform Gobihadros mongoliensis showing features of cessation of growth indicating attainment of the terminal size. Moreover, this is the first non-avian dinosaur with an age-related pathology recognized as primary calcium pyrophosphate deposition disease indicating its advanced age. Because senile dinosaurs are so rare and thus "senescence" in dinosaurs is unclear, we also propose a new unified definition of a senile dinosaur: an individual which achieved the terminal size as revealed by the presence of the external fundamental system and closed transcortical channels, has completely secondary remodeled weight-bearing bones and possesses non-traumatic, non-contagious bone pathologies correlated with advanced age.

Including Distorted Specimens in Allometric Studies: Linear Mixed Models Account for Deformation

Allometry—patterns of relative change in body parts—is a staple for examining how clades exhibit scaling patterns representative of evolutionary constraint on phenotype, or quantifying patterns of ontogenetic growth within a species. Reconstructing allometries from ontogenetic series is one of the few methods available to reconstruct growth in fossil specimens. However, many fossil specimens are deformed (twisted, flattened, and displaced bones) during fossilization, changing their original morphology in unpredictable and sometimes undecipherable ways. To mitigate against post burial changes, paleontologists typically remove clearly distorted measurements from analyses. However, this can potentially remove evidence of individual variation and limits the number of samples amenable to study, which can negatively impact allometric reconstructions. Ordinary least squares (OLS) regression and major axis regression are common methods for estimating allometry, but they assume constant levels of residual variation across specimens, which is unlikely to be true when including both distorted and undistorted specimens. Alternatively, a generalized linear mixed model (GLMM) can attribute additional variation in a model (e.g., fixed or random effects). We performed a simulation study based on an empirical analysis of the extinct cynodont, Exaeretodon argentinus, to test the efficacy of a GLMM on allometric data. We found that GLMMs estimate the allometry using a full dataset better than simply using only non-distorted data. We apply our approach on two empirical datasets, cranial measurements of actual specimens of E. argentinus (n = 16) and femoral measurements of the dinosaur Tawa hallae (n = 26). Taken together, our study suggests that a GLMM is better able to reconstruct patterns of allometry over an OLS in datasets comprised of extinct forms and should be standard protocol for anyone using distorted specimens.

A shift in ontogenetic timing produced the unique sauropod skull

Sauropod dinosaurs include the largest terrestrial vertebrates that have ever lived. Virtually every part of the sauropod body is heavily modified in association with gigantic size and associated physiological alterations. Sauropod skulls are no exception: they feature elongated, telescoped facial regions connected to tilted neurocrania and reoriented jaw adductor muscles. Several of these cranial features have been suggested to be adaptations for feeding on the one hand and the result of paedomorphic transformation near the base of Sauropoda on the other. However, the scarcity of sauropodomorph ontogenetic series has impeded further investigation of these hypotheses. We re-evaluated the cranial material attributed to the early sauropodomorph Anchisaurus, which our phylogenetic analyses confirm to be closely related to sauropods. Digital assembly of μCT-scanned skulls of the two known specimens, a juvenile and an adult, permitted us to examine the detailed ontogeny of cranial elements. The skull anatomy of Anchisaurus is distinguished by a mosaic of ancestral saurischian and sauropod-like characters. Sauropod-like characters of the braincase and adductor chamber appear late in ontogeny, suggesting that these features first evolved by the developmental mechanism of terminal addition. Shape analyses and investigation of allometric evolution demonstrate that cranial characters that appear late in the ontogeny of sauropodomorphs closely related to sauropods are already present in the embryos and juveniles of sauropods, suggesting a predisplacement-type shift in developmental timing from the ancestral anchisaurian condition. We propose that this developmental shift relaxed prior constraints on skull morphology, allowing sauropods to explore a novel range of phenotypes and enabling specializations of the feeding apparatus. The shift in timing occurred in concert with the evolution of gigantism and physiological and locomotory innovations.