Gwendal Perrichon

‘Thoracosaurs’ are Late Cretaceous to early Palaeogene longirostrine eusuchian crocodyliforms from coastal marine deposits in Europe and North America. Their phylogenetic placement is debated; whereas most morphological studies place them within Gavialidae, implying extensive ghost lineages inconsistent with molecular‐based divergence estimates, analyses applying tip‐dated Bayesian inference in a total evidence framework retrieve thoracosaurs outside of Crocodylia. Recent advances in computed tomography (CT) scanning of eusuchian skulls have allowed exploration of endocranial anatomy, an additional morphological resource that could contribute to resolving conflicting phylogenetic hypotheses, and shed light on their ecology. Here, we use CT scanning to reconstruct the endocranial cavities of three thoracosaur skulls: Thoracosaurus isorhynchus from the Maastrichtian of France and Danian of Sweden, and Eothoracosaurus mississippiensis from the Maastrichtian of the USA. Dorsal expansions of the olfactory region within the lacrimals and prefrontals, enlarged semicircular canals of the inner ear, and the absence of a bony wall between the vestibule and the encephalic endocast support a marine ecology for thoracosaurs, consistent with their depositional record and biogeographic distribution. Expansive nasopharyngeal ducts indicate that thoracosaurs were probably incapable of deep diving; instead, they may have used them for acoustic signalling. Comparison of endocranial features of thoracosaurs to extinct and extant crocodyliforms suggests that many of these characteristics are affected by longirostry and/or ecological adaptation. Our results do not clearly support either phylogenetic hypothesis for the position of ‘thoracosaurs’. Rather, our study highlights the need for increased taxon sampling to clarify the phylogenetic distribution of endocranial features across Crocodyliformes.

Basal eusuchians diversified during the Early Cretaceous and paved the way for the radiation of modern crocodylians. While several neosuchians and eusuchians (e.g. Allodaposuchidae, Bernissartidae, Susisuchidae) already display the classic semi-aquatic bauplan, Hylaeochampsa vectiana OWEN 1874 from the Early Cretaceous of the Isle of Wight (UK) displays an extremely derived morphology, questioning the stem condition for the Eusuchia as well as its paleoecological preferences. Here, we investigate the endocranial morphology of Hylaeochampsa vectiana using microtomographic data and reconstruct its brain endocast, inner ear, associated nerves and blood vessels, and paratympanic and paranasal sinuses. Using these new data, we independently confirm the subadult pedomorphic condition of this taxon. Hylaeochampsa vectiana retains characters with some taxa lying at the neosuchian-eusuchian transition, including a flat endocast and the retention of a prooticparietal connection in the paratympanic sinus system as in Paralligator and Koumpiodontosuchus. Although previously considered closely related, we also recognize important endocranial differences between Hylaeochampsa vectiana and Allodaposuchidae, inviting to reevaluate their phylogenetic affinities. Given the high number of derived characters as well as the shared morphology of the paratympanic sinus with Caimaninae, we recommend not to use Hylaeochampsa vectiana to root characters in phylogenetic analyses of the Eusuchia. Further investigation of basal eusuchian phylogenies should await a wider sampling of other taxa such as Acynodon spp. and Iharkutosuchus makadii in order to better understand endocranial character variability and how to incorporate those into phylogenies. Hylaeochampsa vectiana was a small crocodyliform with endocranial adaptations suggesting a semi-aquatic lifestyle in a freshwater habitat.

La détermination des liens de parenté entre les crocodiliens actuels et fossiles est rendue compliqué par de nombreuses convergences évolutives au niveau du crâne. La recherche de nouveaux caractères liés aux organes internes tel que les sinus, le cerveau et les oreilles internes pourraient permettre de résoudre les conflits entre les phylogénies moléculaires et morphologiques. En particulier, le système complexe de sinus paratympaniques des crocodyliens présente une diversité et une disparité de forme sans pareille tant à l'actuel que dans le fossile. Etablir le lien entre l’allongement du crâne et la forme de ces structures internes permettraient de discriminer la part du rôle de la phylogénie, du développement, des contraintes de conformation crânienne et de l’écologie dans l’évolution de leurs formes au cours du temps. Pour cela, une description détaillée des sinus paratympaniques des espèces actuelles sera réalisée, en comparant chaque espèce au cours de leur développement post-éclosion. Ces descriptions seront accompagnées d’analyse statistiques en morphométrie géométrique sur la forme de ces structures, qui seront mis en perspective avec les différentes hypothèses de la littérature sur la phylogénie et l’écologie des spécimens étudiés. Le choix de nouveaux caractères internes et leur inclusion dans des matrices phylogénétiques permettra de tester leur valeur pour améliorer la détermination des liens de parenté entre les espèces actuelles. Ces résultats seront ensuite comparé avec des structures endocrâniennes observées chez des espèces fossiles dont les morphologies sont particulières ou dont les liens de parenté sont discutés, pour tenter de comprendre l'évolution des sinus au cours de l'évolution des crocodiliens.

This contribution contains the 3D models of the paratympanic sinus system, the endocast and the neurovascular bony canal of the maxilla, premaxilla and the jugal of Leidyosuchus canadensis and Stangerochampsa mccabei described and figured in the following publication: G. Donz é, G.

Crocodylians evolved a high variety of rostral morphologies during their evolutionary history, highlighting the strong links between morphological plasticity and environmental and ecological parameters. Two Late Cretaceous alligatoroids, the mesorostrine Leidyosuchus canadensi s Lambe, 1907, and the brevirostrine Stangerochampsa mccabei Wu et al., 1996, from Alberta, Canada, preserve a large groove‐shaped recess on the posterior part of the maxilla that has not been documented in other alligatoroids. Despite the potential phylogenetic and paleoecological significance of this neurovascular feature, internal and endocranial structures remain under‐explored among stem alligatoroids. The endocranial morphology, including the paratympanic sinus system of Leidyosuchus canadensis and Stangerochampsa mccabei , was compared to those of extant crocodylians and of the extinct alligatoroid Diplocynodon ratelii based on computed tomography data. The Cretaceous alligatoroids share endocranial features, such as a posteroventral neurovascular projection of the labiolateral canal that connects to the groove‐like recess at the posterior edge of the maxilla and a paratympanic sinus system most similar to those of small‐bodied and young extant crocodylians, suggesting that these pedomorphic features may reflect the ancestral crocodylian condition. Future phylogenetic studies should consider internal and endocranial characters alike to improve our understanding on the relationships among crocodylians.

The endosseous labyrinths are associated with several functions, including hearing and spatial orientation. Throughout their evolutionary history, crocodylomorphs have thrived in diverse environments, and the morphology of their endosseous labyrinths has been suggested as a proxy for inferring their lifestyle. However, the relationships between the shape of their endosseous labyrinths and ontogenetic and phylogenetic factors are difficult to interpret and have rarely been investigated in depth previously, particularly in terms of dataset size. Here, we present the most complete dataset to date on the endosseous labyrinths of extant crocodylians, including 111 specimens covering 22 species of different ontogenetic status (from hatchlings to adults). Using 3D geometric morphometrics, we show that allometry constitutes a major contributor of the shape variation of the crocodylian endosseous labyrinths and that the development of this structure is likely linked to the braincase conformation, in all extant genera. We also find a moderate phylogenetic signal, but only without considering the size effect, so it could not be translated into relevant discrete morphological characters. Based on these results, we discuss several remaining problems that prevent the inclusion of fossil forms with highly divergent lifestyles to study how ecological differences shaped the endosseous labyrinths of crocodylomorphs.

Eosuchus lerichei is a gavialoid crocodylian from late Paleocene marine deposits of northwestern Europe, known from a skull and lower jaws, as well as postcrania. Its sister taxon relationship with the approximately contemporaneous species Eosuchus minor from the east coast of the USA has been explained through transoceanic dispersal, indicating a capability for salt excretion that is absent in extant gavialoids. However, there is currently no anatomical evidence to support marine adaptation in extinct gavialoids. Furthermore, the placement of Eosuchus within Gavialoidea is labile, with some analyses supporting affinities with the Late Cretaceous to early Paleogene “thoracosaurs.” Here we present novel data on the internal and external anatomy of the skull of E. lerichei that enables a revised diagnosis, with 6 autapormorphies identified for the genus and 10 features that enable differentiation of the species from Eosuchus minor. Our phylogenetic analyses recover Eosuchus as an early diverging gavialid gavialoid that is not part of the “thoracosaur” group. In addition to thickened semi‐circular canal walls of the endosseous labyrinth and paratympanic sinus reduction, we identify potential osteological correlates for salt glands in the internal surface of the prefrontal and lacrimal bones of E. lerichei. These salt glands potentially provide anatomical evidence for the capability of transoceanic dispersal within Eosuchus, and we also identify them in the Late Cretaceous “thoracosaur” Portugalosuchus. Given that the earliest diverging and stratigraphically oldest gavialoids either have evidence for a nasal salt gland and/or have been recovered from marine deposits, this suggests the capacity for salt excretion might be ancestral for Gavialoidea. Mapping osteological and geological evidence for marine adaptation onto a phylogeny indicates that there was probably more than one independent loss/reduction in the capacity for salt excretion in gavialoids.

The present 3D Dataset contains the 3D models analyzed in

Voay robustus, the extinct Malagasy “horned” crocodile, was originally considered to be the only crocodylian representative in Madagascar during most part of the Holocene. However, Malagasy crocodylian remains have had confused taxonomic attributions and recent studies have underlined that Crocodylus and Voay populations coexisted on the island for at least 7500 years. Here, we describe the inner braincase anatomy of Voay robustus using x‐ray computed tomography on four specimens, to provide new anatomical information that distinguishes Voay from Crocodylus, especially features of the brain endocast and the paratympanic sinuses. Geometric morphometric analyses are performed on 3D models of the internal organs to compare statistically Voay with a subset of extant Crocodylidae. Following these comparisons, we build an endocranial morphological matrix to discuss the proposed phylogenetic affinities of Voay with Osteolaeminae from an endocranial point of view. Additionally, we discuss the use of internal characters in systematic studies and find that they can have a major impact on morphological analyses. Finally, new radiocarbon data on Voay and subfossil Crocodylus specimens are recovered between 2010 and 2750 cal BP, which confirm the cohabitation of the two species in the same area for a long period of time. We thus assess several extinction scenarios, and propose a slightly different ecology of Voay compared to Crocodylus, which could have allowed habitat partitioning on the island. Our approach complements information obtained from previous molecular and morphological phylogenies, as well as previous radiocarbon dating, together revealing past diversity and faunal turnovers in Madagascar.