The osteohistology of Orthosuchus stormbergi using synchrotron radiation microcomputed tomography

Orthosuchus stormbergi was a small-bodied crocodyliform, representative of a diverse assemblage of Early Jurassic, early branching crocodylomorph taxa from the upper Elliot Formation of South Africa. The life history of these early branching taxa remains poorly understood, with only sparse investigations into their osteohistology, yet species like Orthosuchus have potential to inform about the macroevolution of growth strategies on the stem leading to crown crocodilians. In order to elucidate the growth patterns of Orthosuchus, we used propagation phase contrast X-ray synchrotron micro-computed tomography to virtually image the osteohistology of the postcrania of two specimens, including multiple elements from the type (SAM-PK-K409), and the femur of a referred specimen (BP/1/4242). In total, we scanned nine mid-diaphyseal sections of the humerus, radius, ulna, radiale, femur, tibia, fibula, and a rib. We then compared our results to osteohistological sections of crocodylomorph taxa from the published literature. Our results show that the most predominant bone tissue type in Orthosuchus is lamellar, with a few patches of woven and parallel-fibred bone. The type specimen contains four to five lines of arrested growth and the hindlimb elements present outer circumferential lamellae, whereas the referred specimen contains six to seven. Both specimens grew at similar rates, reaching adult skeletal body size at year four or five. The sectioned bones, most notably the radius and ulna, are comparatively thick walled and compact. Our virtual osteohistological sections are one of the first for an early branching crocodyliform, and the broad sample of skeletal elements makes Orthosuchus a key anchor point for understanding the plesiomorphic life history traits of the clade.

A new small-sized predatory pseudosuchian archosaur from the Middle-Late Triassic of Southern Brazil

Before the rise of dinosaurs and pterosaurs, pseudosuchians-reptiles from the crocodilian lineage-dominated the Triassic land ecosystems. This lineage diversified into several less inclusive clades, resulting in a wide ecomorphological diversity during the Middle and Late Triassic. Some giant pseudosuchians occupied the top of the trophic webs, while others developed extensive bony armor as a defense mechanism, which later evolved as a convergence in the avemetatarsalian lineage. On the other hand, there were groups like the Gracilisuchidae, which was composed of carnivorous forms with lightweight build and less than 1 m in length. The fossil record of gracilisuchids is geographically restricted to China and Argentina, with one ambiguous record from Brazil. In the present study, the first unambiguous gracilisuchid from Brazil is described. Parvosuchus aurelioi gen. et sp. nov. comes from the Dinodontosaurus Assemblage Zone of the Santa Maria Formation, which is associated with the Ladinian-Carnian boundary. Composed of a complete cranium, vertebrae, pelvic girdle and hindlimbs, the new species nests with Gracilisuchus stipanicicorum and Maehary bonapartei in a phylogenetic analysis. Its discovery fills a taxonomic gap in Brazilian pseudosuchian fauna and reveals the smallest known member of this clade from the Dinodontosaurus Assemblage Zone, highlighting the diversity of pseudosuchians during the moment that preceded the dawn of dinosaurs.

Oldest record of Machimosaurini (Thalattosuchia, Teleosauroidea): teeth and scavenging traces from the Middle Jurassic (Bajocian) of Switzerland

The Jurassic period was a time of major diversification for Mesozoic marine reptiles, including Ichthyosauria, Plesiosauria and thalattosuchian Crocodylomorpha. The latter originated in the Early Jurassic and thrived during the Late Jurassic. Unfortunately, the Middle Jurassic, a crucial time in their evolution, has a poor fossil record. Here, we document the first evidence of macrophagous/durophagous Machimosaurini-tribe teleosauroid thalattosuchians from the late Bajocian (ca 169 Ma) in the form of three robust tooth crowns with conical blunt shapes and anastomosed pattern of thick enamel ridges towards the apex, associated with the skeleton of a large ichthyosaur lacking preserved tooth crowns. The tooth crowns were found on the posterior section of the lower jaw (left angular), a lacrimal and the axis neural arch of the ichthyosaur. In addition, some of the distal sections of the posterior dorsal ribs of the ichthyosaur skeleton exhibit rounded bite marks and some elongated furrows that fit in size and shape with the Machimosaurini teeth. These marks, together with the absence of healing in the rib bone are interpreted here as the indicators of peri- to post-mortem scavenging by a Machimosaurini teleosauroid after the large ichthyosaur carcass settled on the floor of a shallow ocean.

Reassessment of the enigmatic "Prestosuchus" loricatus (Archosauria: Pseudosuchia) from the Middle-Late Triassic of southern Brazil

Our knowledge of the diversity and evolution of South American Triassic pseudosuchians has greatly improved in the past 15 years, due to new discoveries, but also to the revision of several historically important specimens. One of the earliest descriptions of pseudosuchians from the Triassic of Brazil stems from the classic work of Huene from the first half of the 20th century, who described several species, including such influential taxa as Rauisuchus tiradentes or Prestosuchus chiniquensis, which have recently been reviewed. The more poorly known proposed second species of Prestosuchus, P. loricatus, is the focus of the present work. The original material included some elements of the axial skeleton (cervical and caudal vertebrae, ribs, osteoderms) and the hindlimb (ischia, calcaneum, metatarsus), collected from the Dinodontosaurus Assemblage Zone of the Chiniquá area, west of São Pedro do Sul. "Prestosuchus" loricatus shows numerous differences to P. chiniquensis, including the shape of cervical neural spines, presence of epipophyses on the cervical vertebrae, presence of a pit in the iliac articulation of the ischium, lack of longitudinal furrows in the dorsolateral surface of the ischial shafts, the more slender calcaneal tuber and a less pronounced ventral pit in the calcaneum, and is thus referred to a new genus, Schultzsuchus gen. nov. Phylogenetic analysis indicates an early branching position within Poposauroidea for Schultzsuchus, making it the oldest known member of this clade in South America.

Decoupling speciation and extinction reveals both abiotic and biotic drivers shaped 250 million years of diversity in crocodile-line archosaurs

Whereas living representatives of Pseudosuchia, crocodylians, number fewer than 30 species, more than 700 pseudosuchian species are known from their 250-million-year fossil record, displaying far greater ecomorphological diversity than their extant counterparts. With a new time-calibrated tree of >500 species, we use a phylogenetic framework to reveal that pseudosuchian evolutionary history and diversification dynamics were directly shaped by the interplay of abiotic and biotic processes over hundreds of millions of years, supported by information theory analyses. Speciation, but not extinction, is correlated with higher temperatures in terrestrial and marine lineages, with high sea level associated with heightened extinction in non-marine taxa. Low lineage diversity and increased speciation in non-marine species is consistent with opportunities for niche-filling, whereas increased competition may have led to elevated extinction rates. In marine lineages, competition via increased lineage diversity appears to have driven both speciation and extinction. Decoupling speciation and extinction, in combination with ecological partitioning, reveals a more complex picture of pseudosuchian evolution than previously understood. As the number of species threatened with extinction by anthropogenic climate change continues to rise, the fossil record provides a unique window into the drivers that led to clade success and those that may ultimately lead to extinction.

Anatomy and relationships of the early diverging Crocodylomorphs Junggarsuchus sloani and Dibothrosuchus elaphros

The holotype of Junggarsuchus sloani, from the Shishugou Formation (early Late Jurassic) of Xinjiang, China, consists of a nearly complete skull and the anterior half of an articulated skeleton, including the pectoral girdles, nearly complete forelimbs, vertebral column, and ribs. Here, we describe its anatomy and compare it to other early diverging crocodylomorphs, based in part on CT scans of its skull and that of Dibothrosuchus elaphros from the Early Jurassic of China. Junggarsuchus shares many features with a cursorial assemblage of crocodylomorphs, informally known as "sphenosuchians," whose relationships are poorly understood. However, it also displays several derived crocodyliform features that are not found among most "sphenosuchians." Our phylogenetic analysis corroborates the hypothesis that Junggarsuchus is closer to Crocodyliformes, including living crocodylians, than are Dibothrosuchus and Sphenosuchus, but not as close to crocodyliforms as Almadasuchus and Macelognathus, and that the "Sphenosuchia" are a paraphyletic assemblage. D. elaphros and Sphenosuchus acutus are hypothesized to be more closely related to Crocodyliformes than are the remaining non-crocodyliform crocodylomorphs, which form several smaller groups but are largely unresolved.

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.

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.

The orbitalauricular chord of Alligator: The unusual functional linkage between the earflap and eyelid of Crocodylians

One of the distinctive features of the Crocodylia is the presence of a superficial meatal chamber the aperture of which is regulated by two earflaps. The movements of the upper earflap have been detailed by multiple workers, however, the mechanics of the lower earflap remain unresolved. The present study was undertaken to document the mechanics of the lower earflap in the American alligator, Alligator mississippiensis, and to explore the functional bases of coordinated movements between the lower earflap and lower eyelid in this species. This anatomical system was examined using a combination of fresh dissection, histology, and micro‐CT analyses applied to post‐embryonic specimens. The rostral margin of the lower earflap is tightly bound to a block of dense connective tissue herein termed the orbitalauricular chord. The orbitalauricular chord is anatomically distinct from both a ligament and a tendon. The dorsal surface of the orbitalauricular chord is attached to a slip of the levator palpebra, while the ventral surface is attached to a slip of the depressor palpebra. These attachments produce a simple mechanism for the elevation and depression of the lower earflap, and thus the opening and closing of the meatal aperture. The caudal surface of the orbitalauricular chord has connective tissue links to the rostral margin of the lower earflap. The morphology of the orbitalauricular chord appears to explain both the mechanics of the lower earflap and the functional coupling between the lower eyelid and lower earflap.

Walking-and Running and Jumping-with Dinosaurs and Their Cousins, Viewed Through the Lens of Evolutionary Biomechanics

Archosauria diversified throughout the Triassic Period before experiencing two mass extinctions near its end ∼201 Mya, leaving only the crocodile-lineage (Crocodylomorpha) and bird-lineage (Dinosauria) as survivors; along with the pterosaurian flying reptiles. About 50 years ago, the "locomotor superiority hypothesis" (LSH) proposed that dinosaurs ultimately dominated by the Early Jurassic Period because their locomotion was superior to other archosaurs'. This idea has been debated continuously since, with taxonomic and morphological analyses suggesting dinosaurs were "lucky" rather than surviving due to being biologically superior. However, the LSH has never been tested biomechanically. Here we present integration of experimental data from locomotion in extant archosaurs with inverse and predictive simulations of the same behaviours using musculoskeletal models, showing that we can reliably predict how extant archosaurs walk, run and jump. These simulations have been guiding predictive simulations of extinct archosaurs to estimate how they moved, and we show our progress in that endeavour. The musculoskeletal models used in these simulations can also be used for simpler analyses of form and function such as muscle moment arms, which inform us about more basic biomechanical similarities and differences between archosaurs. Placing all these data into an evolutionary and biomechanical context, we take a fresh look at the LSH as part of a critical review of competing hypotheses for why dinosaurs (and a few other archosaur clades) survived the Late Triassic extinctions. Early dinosaurs had some quantifiable differences in locomotor function and performance vs. some other archosaurs, but other derived dinosaurian features (e.g., metabolic or growth rates, ventilatory abilities) are not necessarily mutually exclusive from the LSH; or maybe even an opportunistic replacement hypothesis; in explaining dinosaurs' success.

Paratympanic sinuses in juvenile Alligator

Crocodylia has an extensive epithelial pneumatic space in the middle ear, paratympanic sinus system. Although fossil and extant crocodylian paratympanic sinus systems have been studied recently using the computed tomography (CT) and three-dimensional (3D) reconstruction data, due to the soft tissue nature of the pneumatic system and presence of its surrounding soft tissue structures, some boundaries, and definitions of each extension remain ambiguous. We describe the comprehensive paratympanic sinus system in posthatched alligator using soft tissue enhanced CT data with 3D reconstructions. The data are compared to the available data to discuss the ontogenetic pattern in alligator. We introduce further divided entities of the pneumatic system based on their associated bony and soft tissue structures and epithelial membrane and clarify the pneumatic terminologies. We then re-visit the potential homology of the paratympanic sinus in Archosauria. Epithelial boundaries of the ventral portion of the pneumatic system from the histological data suggest that the dual origin of the basioccipital diverticulum derived from the tympanic sinus and basicranial diverticulum medially. The presence of the epithelial boundary and pneumatic changes in ontogeny suggests that the middle ear may function differently in developmental stages. Lastly, a morphogenetic tree is constructed to help future work of comparative developmental studies of the paratympanic sinus system between crocodiles and birds.

Reevaluation of the cranial osteology and phylogenetic position of the early crocodyliform Eopneumatosuchus colberti, with an emphasis on its endocranial anatomy

Eopneumatosuchus colberti Crompton and Smith, 1980, known from a single partial skull, is an enigmatic crocodylomorph from the Lower Jurassic Kayenta Formation. In spite of its unique morphology, an exceptionally pneumatic braincase, and presence during a critical time period of crocodylomorph evolution, relatively little is known about this taxon. Here, we redescribe the external cranial morphology of E. colberti, present novel information on its endocranial anatomy, evaluate its phylogenetic position among early crocodylomorphs, and seek to better characterize its ecology. Our examination clarifies key aspects of cranial suture paths and braincase anatomy. Comparisons with related taxa (e.g., Protosuchus haughtoni) demonstrate that extreme pneumaticity of the braincase may be more widespread in protosuchids than previously appreciated. Computed tomography scans reveal an endocranial morphology that resembles that of other early crocodylomorphs, in particular the noncrocodyliform crocodylomorph Almadasuchus figarii. There are, however, key differences in olfactory bulb and cerebral hemisphere morphology, which demonstrate the endocranium of crocodylomorphs is not as conserved as previously hypothesized. Our phylogenetic analysis recovers E. colberti as a close relative of Protosuchus richardsoni and Edentosuchus tienshanensis, contrasting with previous hypotheses of a sister group relationship with Thalattosuchia. Previous work suggested the inner ear has some similarities to semi-aquatic crocodyliforms, but the phylogenetic placement of E. colberti among protosuchids with a terrestrial postcranial skeletal morphology complicates paleoecological interpretation.

Extinction and dawn of the modern world in the Carnian (Late Triassic)

The Carnian Pluvial Episode (Late Triassic) was a time of global environmental changes and possibly substantial coeval volcanism. The extent of the biological turnover in marine and terrestrial ecosystems is not well understood. Here, we present a meta-analysis of fossil data that suggests a substantial reduction in generic and species richness and the disappearance of 33% of marine genera. This crisis triggered major radiations. In the sea, the rise of the first scleractinian reefs and rock-forming calcareous nannofossils points to substantial changes in ocean chemistry. On land, there were major diversifications and originations of conifers, insects, dinosaurs, crocodiles, lizards, turtles, and mammals. Although there is uncertainty on the precise age of some of the recorded biological changes, these observations indicate that the Carnian Pluvial Episode was linked to a major extinction event and might have been the trigger of the spectacular radiation of many key groups that dominate modern ecosystems.

Inner ear sensory system changes as extinct crocodylomorphs transitioned from land to water

Major evolutionary transitions, in which animals develop new body plans and adapt to dramatically new habitats and lifestyles, have punctuated the history of life. The origin of cetaceans from land-living mammals is among the most famous of these events. Much earlier, during the Mesozoic Era, many reptile groups also moved from land to water, but these transitions are more poorly understood. We use computed tomography to study changes in the inner ear vestibular system, involved in sensing balance and equilibrium, as one of these groups, extinct crocodile relatives called thalattosuchians, transitioned from terrestrial ancestors into pelagic (open ocean) swimmers. We find that the morphology of the vestibular system corresponds to habitat, with pelagic thalattosuchians exhibiting a more compact labyrinth with wider semicircular canal diameters and an enlarged vestibule, reminiscent of modified and miniaturized labyrinths of other marine reptiles and cetaceans. Pelagic thalattosuchians with modified inner ears were the culmination of an evolutionary trend with a long semiaquatic phase, and their pelagic vestibular systems appeared after the first changes to the postcranial skeleton that enhanced their ability to swim. This is strikingly different from cetaceans, which miniaturized their labyrinths soon after entering the water, without a prolonged semiaquatic stage. Thus, thalattosuchians and cetaceans became secondarily aquatic in different ways and at different paces, showing that there are different routes for the same type of transition.

First evidence for a latitudinal body mass effect in extant Crocodylia and the relationships of their reproductive characters

Relationships between distribution patterns and body size have been documented in many endothermic taxa. However, the evidence for these trends in ectotherms generally is equivocal, and there have been no studies of effects in crocodylians specifically. Here, we examine the relationship between latitudinal distribution and body mass in 20 extant species of crocodylians, as well as the relationships between seven important reproductive variables. Using phylogenetically independent contrasts to inform generalized linear models, we provide the first evidence of a latitudinal effect on adult female body mass in crocodylians. In addition, we explore the relationships between reproductive variables including egg mass, hatchling mass and clutch size. We report no correlation between egg mass and clutch size, upholding previously reported within-species trends. We also find no evidence of a correlation between measures of latitudinal range and incubation temperature, contrasting with the trends found in turtles.

The evolution of dermal shield vascularization in Testudinata and Pseudosuchia: phylogenetic constraints versus ecophysiological adaptations

Studies on living turtles have demonstrated that shells are involved in the resistance to hypoxia during apnea via bone acidosis buffering; a process which is complemented with cutaneous respiration, transpharyngeal and cloacal gas exchanges in the soft-shell turtles. Bone acidosis buffering during apnea has also been identified in crocodylian osteoderms, which are also known to employ heat transfer when basking. Although diverse, many of these functions rely on one common trait: the vascularization of the dermal shield. Here, we test whether the above ecophysiological functions played an adaptive role in the evolutionary transitions between land and aquatic environments in both Pseudosuchia and Testudinata. To do so, we measured the bone porosity as a proxy for vascular density in a set of dermal plates before performing phylogenetic comparative analyses. For both lineages, the dermal plate porosity obviously varies depending on the animal lifestyle, but these variations prove to be highly driven by phylogenetic relationships. We argue that the complexity of multi-functional roles of the post-cranial dermal skeleton in both Pseudosuchia and Testudinata probably is the reason for a lack of obvious physiological signal, and we discuss the role of the dermal shield vascularization in the evolution of these groups. This article is part of the theme issue 'Vertebrate palaeophysiology'.

Cranial anatomy of Allosaurus jimmadseni, a new species from the lower part of the Morrison Formation (Upper Jurassic) of Western North America

Allosaurus is one of the best known theropod dinosaurs from the Jurassic and a crucial taxon in phylogenetic analyses. On the basis of an in-depth, firsthand study of the bulk of Allosaurus specimens housed in North American institutions, we describe here a new theropod dinosaur from the Upper Jurassic Morrison Formation of Western North America, Allosaurus jimmadseni sp. nov., based upon a remarkably complete articulated skeleton and skull and a second specimen with an articulated skull and associated skeleton. The present study also assigns several other specimens to this new species, Allosaurus jimmadseni, which is characterized by a number of autapomorphies present on the dermal skull roof and additional characters present in the postcrania. In particular, whereas the ventral margin of the jugal of Allosaurus fragilis has pronounced sigmoidal convexity, the ventral margin is virtually straight in Allosaurus jimmadseni. The paired nasals of Allosaurus jimmadseni possess bilateral, blade-like crests along the lateral margin, forming a pronounced nasolacrimal crest that is absent in Allosaurus fragilis.

The evolution and role of the hyposphene-hypantrum articulation in Archosauria: phylogeny, size and/or mechanics?

Living members of Archosauria, the reptile clade containing Crocodylia and Aves, have a wide range of skeletal morphologies, ecologies and body size. The range of body size greatly increases when extinct archosaurs are included, because extinct Archosauria includes the largest members of any terrestrial vertebrate group (e.g. 70-tonne titanosaurs, 20-tonne theropods). Archosaurs evolved various skeletal adaptations for large body size, but these adaptations varied among clades and did not always appear consistently with body size or ecology. Modification of intervertebral articulations, specifically the presence of a hyposphene-hypantrum articulation between trunk vertebrae, occurs in a variety of extinct archosaurs (e.g. non-avian dinosaurs, pseudosuchians). We surveyed the phylogenetic distribution of the hyposphene-hypantrum to test its relationship with body size. We found convergent evolution among large-bodied clades, except when the clade evolved an alternative mechanism for vertebral bracing. For example, some extinct lineages that lack the hyposphene-hypantrum articulation (e.g. ornithischians) have ossified tendons that braced their vertebral column. Ossified tendons are present even in small taxa and in small-bodied juveniles, but large-bodied taxa with ossified tendons reached those body sizes without evolving the hyposphene-hypantrum articulation. The hyposphene-hypantrum was permanently lost in extinct crownward members of both major archosaur lineages (i.e. Crocodylia and Aves) as they underwent phyletic size decrease, changes in vertebral morphology and shifts in ecology.

The multi-peak adaptive landscape of crocodylomorph body size evolution

BACKGROUND

Little is known about the long-term patterns of body size evolution in Crocodylomorpha, the > 200-million-year-old group that includes living crocodylians and their extinct relatives. Extant crocodylians are mostly large-bodied (3-7 m) predators. However, extinct crocodylomorphs exhibit a wider range of phenotypes, and many of the earliest taxa were much smaller (< 1.2 m). This suggests a pattern of size increase through time that could be caused by multi-lineage evolutionary trends of size increase or by selective extinction of small-bodied species. Here, we characterise patterns of crocodylomorph body size evolution using a model fitting-approach (with cranial measurements serving as proxies). We also estimate body size disparity through time and quantitatively test hypotheses of biotic and abiotic factors as potential drivers of crocodylomorph body size evolution.

RESULTS

Crocodylomorphs reached an early peak in body size disparity during the Late Jurassic, and underwent an essentially continual decline since then. A multi-peak Ornstein-Uhlenbeck model outperforms all other evolutionary models fitted to our data (including both uniform and non-uniform), indicating that the macroevolutionary dynamics of crocodylomorph body size are better described within the concept of an adaptive landscape, with most body size variation emerging after shifts to new macroevolutionary regimes (analogous to adaptive zones). We did not find support for a consistent evolutionary trend towards larger sizes among lineages (i.e., Cope's rule), or strong correlations of body size with climate. Instead, the intermediate to large body sizes of some crocodylomorphs are better explained by group-specific adaptations. In particular, the evolution of a more aquatic lifestyle (especially marine) correlates with increases in average body size, though not without exceptions.

CONCLUSIONS

Shifts between macroevolutionary regimes provide a better explanation of crocodylomorph body size evolution on large phylogenetic and temporal scales, suggesting a central role for lineage-specific adaptations rather than climatic forcing. Shifts leading to larger body sizes occurred in most aquatic and semi-aquatic groups. This, combined with extinctions of groups occupying smaller body size regimes (particularly during the Late Cretaceous and Cenozoic), gave rise to the upward-shifted body size distribution of extant crocodylomorphs compared to their smaller-bodied terrestrial ancestors.

The crocodylian skull and osteoderms: A functional exaptation to ectothermy?

The crocodylians are ectothermic semi-aquatic vertebrates which are assessed to have evolved from endothermic terrestrial forms during the Mesozoic. Such a physiological transition should have involved modifications in their cardio-vascular system allowing to increase the heat transfers with the surrounding environment by growing a peripheral vascularization which would be mainly located in the dermal skeleton: the dermatocranium and the osteoderms. In order to assess the implication of these anatomical regions in thermal exchanges, we have recorded the temperature above a set of representative skin areas in order to draw comparisons between the skull, the osteoderms, and the rest of the body parts which present either none or residual dermal ossification. We computed the data after the specimens were successively laid in different stereotyped environmental conditions which involved significant variations in the environmental temperature. Our results show that the osteoderms collect the external heat during the basking periods as they become significantly warmer than the surrounding skin; they further release the heat into the core of the organism as they turn out to be colder than the surrounding skin after a significant cooling period. In disregard of the environmental temperature variations, the skull table (which encloses the braincase) remains warmer than the rest of the cranial regions and shows less temperature variations than the osteoderms; a result which has lead us to think that the braincase temperature is monitored and controlled by a thermoregulatory system. Therefore, as hypothesized by previous authors regarding the ectothermic diapsids, we assume that the crocodylian skull possesses shunting blood pathways which tend to maintain both the braincase and the main sensory organs at the nearest to the optimal physiological temperature depending on the external temperature variations. Concerning the skin vascularization, the study of an albino Alligator mississippiensis specimen permitted to observe the repartition of the superficial blood vessels by transparency through the skin. We thus testify that the skin which covers either the skull or the osteoderms is more vascularized than the skin which does not present any subjacent dermal ossification. We consequently deduce that the significant contrast in the thermal behavior between the dermal skeleton and the rest of the body is indeed correlated with a difference in the relative degree of skin vascularization. This last assessment confirms that the development of the dermal skeleton should have played a functional role in the crocodylian transition from endothermy to ectothermy through the set-up of a peripheral vessel network.

The palate and choanae structure of the Susisuchus anatoceps (Crocodyliformes, Eusuchia): phylogenetic implications

Crocodyliformes is a group with a broad fossil record, in which several morphological changes have been documented. Among known transformations the most iconic is perhaps the series of changes seen in the structural evolution of the choanae. The change in the position of the choanae was important during the evolutionary history of the Crocodyliformes. This structure is relevant in the phylogenetic position of many crocodyliforms. The new skull of Susisuchus anatoceps from the Crato Formation of the Santana Group (Lower Cretaceous) is described and the preservation in the ventral view allows character encoding not yet observed for the species. The new specimen shows a typical eusuchian palate for Susisuchus anatoceps, in which the choana is fully enclosed by the pterygoid. The Susisuchidae clade has been placed in different phylogenetic positions: as a sister group of Eusuchia, advanced Neosuchia and in Eusuchia. In Isisfordia there are reports that the choana of this taxon is or is not fully enclosed by the pterygoid. The encoding of the ventral characters of S. anatoceps places Susisuchidae in Eusuchia. However, this position must be further studied, since the matrices showed fragility in the reconstitution of the Neosuchia–Eusuchia transition.

Vascularization in Ornamented Osteoderms: Physiological Implications in Ectothermy and Amphibious Lifestyle in the Crocodylomorphs?

Vascularization in the core of crocodylian osteoderms, and in their superficial pits has been hypothesized to be a key feature involved in physiological thermoregulation and/or acidosis buffering during anoxia (apnea). However, up to now, there have been no quantitative data showing that the inner, or superficial, blood supply of the osteoderms is greater than that occurring in neighboring dermal tissues. We provide such data: our results clearly indicate that the vascular networks in both the osteoderms and the pits forming their superficial ornamentation are denser than in the overlying dermis. These results support previous physiological assumptions and indicate that vascularization in pseudosuchian (crocodylians and close relatives) ornamented osteoderms could be part of a broad eco-physiological adaptation towards ectothermy and aquatic ambush predation acquired by the crocodylomorphs during their post-Triassic evolution. Moreover, regressions demonstrate that the number of enclosed vessels is correlated with the sectional area of the cavities housing them (superficial pits and inner cavities). These regressions can be used to infer the degree of vascularization on dry and fossilized osteoderms and thus document the evolution of the putative function of the osteoderms in the Pseudosuchia. Anat Rec, 2017. © 2017 Wiley Periodicals, Inc. Anat Rec, 301:175-183, 2018. © 2017 Wiley Periodicals, Inc.

Detailed anatomy of the braincase of Macelognathus vagans Marsh, 1884 (Archosauria, Crocodylomorpha) using high resolution tomography and new insights on basal crocodylomorph phylogeny

Background

Macelognathus vagansMarsh, 1884 from the Late Jurassic Morrison Fm. of Wyoming was originally described as a dinosaur by Marsh and in 1971 Ostrom suggested crocodilian affinities. In 2005, Göhlich and collaborators identified new material of this species from Colorado as a basal crocodylomorph. However, a partial skull found in association with mandibular and postcranial remains was not described.

Methods

Due to the small size and delicate structures within the braincase, micro CT studies were performed on this specimen. The new anatomical information was incorporated in a phylogenetic dataset, expanding both character and taxon sampling.

Results

This new material reinforces the non-crocodyliform crocodylomorph affinities of Macelognathusas it bears a large otic aperture, unfused frontals and lacks ornamentation on the dorsal cranial bones. The internal structures also support these affinities as this specimen bears traits (i.e., heavily pneumatized and expanded basisphenoid; the presence of additional pneumatic features on the braincase; and the otoccipital-quadrate contact) not present in most basal crocodylomorphs. Furthermore, the presence of a wide supraoccipital and a cranioquadrate passage are traits shared with Almadasuchus from the early Late Jurassic of Argentina. Macelognathus was recovered as one of the closest relatives of crocodyliforms, forming a clade (Hallopodidae) with two other Late Jurassic taxa (Almadasuchus and Hallopus).

Discussion

The clade formed by Almadasuchus + Hallopus + Macelognathus, the Hallopodidae, is characterized by a higher degree of suturing of the braincase, posteriorly closed otic aperture (paralleled in mesoeucrocodylians) and cursorial adaptations. Also, the phylogenetic position of this lineage of derived crocodylomorphs as the sister group of Crocodyliformes implies a large amount of unsampled record (ghost lineage), at least 50 million years.

A juvenile subfossil crocodylian from Anjohibe Cave, Northwestern Madagascar

Madagascar’s subfossil record preserves a diverse community of animals including elephant birds, pygmy hippopotamus, giant lemurs, turtles, crocodiles, bats, rodents, and carnivorans. These fossil accumulations give us a window into the island’s past from 80,000 years ago to a mere few hundred years ago, recording the extinction of some groups and the persistence of others. The crocodylian subfossil record is limited to two taxa, Voay robustus and Crocodylus niloticus, found at sites distributed throughout the island. V. robustus is extinct while C. niloticus is still found on the island today, but whether these two species overlapped temporally, or if Voay was driven to extinction by competing with Crocodylus remains unknown. While their size and presumed behavior was similar to each other, nearly nothing is known about the growth and development of Voay, as the overwhelming majority of fossil specimens represent mature adult individuals. Here we describe a nearly complete juvenile crocodylian specimen from Anjohibe Cave, northwestern Madagascar. The specimen is referred to Crocodylus based on the presence of caviconchal recesses on the medial wall of the maxillae, and to C. niloticus based on the presence of an oval shaped internal choana, lack of rostral ornamentation and a long narrow snout. However, as there are currently no described juvenile specimens of Voay robustus, it is important to recognize that some of the defining characteristics of that genus may have changed through ontogeny. Elements include a nearly complete skull and many postcranial elements (cervical, thoracic, sacral, and caudal vertebrae, pectoral elements, pelvic elements, forelimb and hindlimb elements, osteoderms). Crocodylus niloticus currently inhabits Madagascar but is locally extinct from this particular region; radiometric dating indicates an age of ∼460–310 years before present (BP). This specimen clearly represents a juvenile based on the extremely small size and open sutures/detached neural arches; total body length is estimated to be ∼1.1 m (modern adults of this species range from ∼4–6 m). This fossil represents the only juvenile subfossil crocodylian specimen reported from Madagascar.