The neuroanatomy and pneumaticity of Hamadasuchus (Crocodylomorpha, Peirosauridae) from the Cretaceous of Morocco and its paleoecological significance for altirostral forms

Size, not phylogeny, explains the morphology of the endosseous labyrinth in the crown clade Crocodylia

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.

Cranial morphology and phylogenetic reassessment of Barreirosuchus franciscoi (Crocodylomorpha, Notosuchia), a Peirosauria from the Late Cretaceous of Brazil

With nearly 30 living species of relatively similar ecological traits, Crocodylomorpha is represented today by only a small fraction of its past diversity. The well-documented crocodylomorph fossil record has revealed more than 500 taxa, with much higher ecological and morphological diversity than their extant counterparts. An example of such astonishing diversity is the Late Cretaceous rocks of the Bauru Group (southeast Brazil), from which numerous taxa are known, belonging to the clade Notosuchia. These were predominantly terrestrial taxa, some of which exhibited traits associated with omnivorous or even herbivorous feeding behaviors, such as Sphagesauridae, whereas others were adapted to a carnivore diet, such as Baurusuchidae and Peirosauridae. Among these is Barreirosuchus franciscoi, originally described as a neosuchian (Trematochampsidae) but later interpreted as a peirosaurid notosuchian. Even though included in recent morphological and phylogenetic analyses, B. franciscoi still lacked a more detailed description. Here, we provide an in-depth description of the cranial elements of B. franciscoi, using data from computed tomography and a broad sample of comparative material, including living and fossil crocodylomorphs. Also, the neuro-cavities, including the endocast, nasopharyngeal duct, and the olfactory region, were digitally reconstructed. Finally, a new phylogenetic analysis recovered B. franciscoi nested within Peirosauria, forming the Itasuchidae clade with other potentially semiaquatic species: Rukwasuchus yajabalajekundu, Pepesuchus deiseae, and Itasuchus jesuinoi. The morphological and phylogenetic reassessment of B. franciscoi indicates a semiaquatic form, highlighting the ecological diversity of notosuchians from the Bauru Group as well as the capacity of notosuchians to explore a myriad of environments.

A review of the non-semiaquatic adaptations of extinct crocodylomorphs throughout their fossil record

Crocodylomorphs constitute a clade of archosaurs that have thrived since the Mesozoic until today and have survived numerous major biological crises. Contrary to historic belief, their semiaquatic extant representatives (crocodylians) are not living fossils, and, during their evolutionary history, crocodylomorphs have evolved to live in a variety of environments. This review aims to summarize the non-semiaquatic adaptations (i.e., either terrestrial or fully aquatic) of different groups from different periods, highlighting how exactly those different lifestyles are inferred for those animals, with regard to their geographic and temporal distribution and phylogenetic relationships. The ancestral condition for Crocodylomorpha seems to have been a terrestrial lifestyle, linked with several morphological adaptations such as an altirostral skull, long limbs allowing a fully erect posture and a specialized dentition for diets based on land. However, some members of this clade, such as thalattosuchians and dyrosaurids display adaptations for an opposite, aquatic lifestyle, interestingly inferred from the same type of morphological observations. Finally, new techniques for inferring the paleobiology of those extinct animals have been put forward in the last decade, appearing as a complementary approach to traditional morphological descriptions and comparisons. Such is the case of paleoneuroanatomical (CT scan data), histological, and geochemical studies.

Life history and growth dynamics of a peirosaurid crocodylomorph (Mesoeucrocodylia; Notosuchia) from the Late Cretaceous of Argentina inferred from its bone histology

Notosuchia were a successful lineage of Crocodyliformes that achieved a remarkable diversity during the Cretaceous of Gondwana, particularly in South America. Although paleohistology has expanded our knowledge of the paleobiology of notosuchians, several clades of this lineage remain poorly understood in this aspect. Here we help to address this gap by conducting the first histological analysis of appendicular bones of a peirosaurid. To increase our knowledge about growth dynamics and examine intraeskeletal and interspecific histological variation, we analyze the microstructure of a tibia, fibula, phalanx, fragment of ornamented element (possible osteoderm or skull bone) and a possible long bone of an individual assigned to Peirosauridae indet. (MAU-Pv-437). The peirosaurid studied here appears to have reached sexual but not somatic maturity and the minimum age inferred from appendicular bones results in a lower estimated than the age inferred from osteoderms in a previous study on the same individual. The cortical bone in MAU-Pv 437 is formed by vascularized parallel fibered bone/lamellar bone which indicates that this individual experienced a moderate growth rate. This indicates different growth dynamics from what has been observed for other notosuchians specimens, suggesting a lack of a uniform growth pattern for this clade.

Neuroanatomy and pneumaticity of the extinct Malagasy "horned" crocodile Voay robustus and its implications for crocodylid phylogeny and palaeoecology

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.