Juan Martín Leardi

Postcranial Anatomy of Sebecus icaeorhinus (Crocodyliformes, Sebecidae) from the Eocene of Patagonia

ABSTRACT We describe postcranial remains of new specimens referred to Sebecus icaeorhinus found in the lower section of the Sarmiento Formation at Cañadón Hondo (central Patagonia, Argentina), commonly regarded as part of the Casamayoran South American Land Mammal Age (middle Eocene). The new specimens include a partially articulated postcranium associated with teeth and fragmentary remains of the mandible that allows their identification as S. icaeorhinus. This taxon was almost exclusively known from skull remains from the same stratigraphic unit and was characterized by unique cranial features such as a long, high, and narrow rostrum bearing serrated teeth. The new material reveals numerous details on the postcranial anatomy of this crocodyliform, including the presence of proportionately long limbs and 10 autapomorphies in the vertebrae, forelimb, and pelvic girdle (some of which are interpreted as adaptations to terrestriality and an erect limb posture). These features depict a highly modified postcranial anatomy for S. icaeorhinus in comparison with that of neosuchian crocodyliforms, paralleling the uniqueness of its skull anatomy. The new information is also phylogenetically informative and incorporated into a cladistic analysis that corroborates not only the close affinities of Sebecidae with Baurusuchidae (sebecosuchian monophyly), but also the deeply nested position of this clade within Notosuchia. The incorporation of postcranial characters to the phylogenetic analysis also results in a novel arrangement of the basal mesoeucrocodylians recorded in the Cretaceous—Cenozoic of Gondwana, clustering all of these species into a large monophyletic clade.

A new fossil from the Jurassic of Patagonia reveals the early basicranial evolution and the origins of Crocodyliformes

Extant crocodylians have a limited taxonomic and ecological diversity but they belong to a lineage (Crocodylomorpha) that includes basal and rather generalized species and a highly diverse clade, Crocodyliformes. The latter was among the most successful groups of Mesozoic tetrapods, both in terms of taxonomic and ecological diversity. Crocodyliforms thrived in terrestrial, semiaquatic, and marine environments, and their fossil diversity includes carnivorous, piscivorous, insectivorous, and herbivorous species. This remarkable ecological and trophic diversity is thought only to occur in forms with a completely akinetic skull, characterized by a functionally integrated and tightly sutured braincase‐quadrate‐palate complex. However, the patterns of evolutionary change that led to the highly modified skull of crocodyliforms and that likely enabled their diversification remain poorly understood. Herein, a new basal crocodylomorph from the Late Jurassic of Patagonia is described, Almadasuchus figarii gen. et sp. nov. The new taxon is known from a well‐preserved posterior region of the skull as well as other craniomandibular and postcranial remains. Almadasuchus figarii differs from all other crocodylomorphs in the presence of six autapomorphic features, including the presence of a large lateral notch on the upper temporal bar, an otic shelf of the squamosal that is wider than long, a deep subtriangular concavity on the posterolateral surface of the squamosal, and an elongated pneumatopore on the ventral surface of the quadrate. Phylogenetic analysis focused on the origin of Crocodyliformes places Almadasuchus as the sister group of Crocodyliformes, supported by synapomorphic features of the skull (e.g. subtriangular basisphenoid, absence of basipterygoid process, absence of a sagittal ridge on the frontal, and a flat anterior skull roof with an ornamented dorsal surface). New braincase information provided by Almadasuchus and other crocodylomorphs indicates that most of the modifications on the posterior region of the skull of crocodyliforms, including the strongly sutured braincase, quadrate, and the extensive secondary palate appeared in a stepwise manner, and pre‐dated the evolutionary changes in the snout, jaws, and dentition. This indicates that the progressively increased rigidity of the skull provided the structural framework that allowed the great ecological diversification of crocodyliforms during the course of the Mesozoic. The phylogenetic pattern of character acquisition inferred for the strongly sutured (akinetic) skull and the appearance of more diverse feeding behaviours that create high mechanical loads on the skull provides another interesting parallel between the evolution of Mesozoic crocodyliforms and the evolutionary origins of mammals.

The antorbital fenestra of Metriorhynchidae (Crocodyliformes, Thalattosuchia): Testing Its homology within a phylogenetic framework

The antorbital fenestra of Metriorhynchidae (Crocodyliformes, Thalattosuchia): Testing Its homology within a phylogenetic framework Juan Martin Leardi a , Diego Pol b & Marta S. Fernandez c a CONICET, IDEAN, Departamento de Ciencias Geologicas, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Intendente Guiraldes 2160, Ciudad Universitaria (C1428RGA), Buenos Aires, Argentina b CONICET, Museo Paleontologico Egidio Feruglio, Av. Fontana 140 (9100), Trelew, Chubut, Argentina c CONICET, Departamento de Paleontologia Vertebrados, Museo de La Plata, Paseo del Bosque s/n (1900) La Plata, Buenos Aires, Argentina

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 vagans Marsh, 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.