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

Abstract

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.

Joint histology inAlligator mississippiensischallenges the identification of synovial joints in fossil archosaurs and inferences of cranial kinesis

“…The otic joint persists throughout Dinosauria and ultimately evolved into a highly mobile joint in birds. On the other hand, as crocodylomorphs sutured the quadrate to the braincase, the otic joint diminished in size and theoretically mobility, resulting in only a vestigial articulation that persists in Crocodylia [3,13,14].…”

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

“…This process forms the posterior border of the otic aperture and contacts through its medial surface the ventrolateral margin of the otoccipital. The quadrate-otoccipital contact is also present in Junggarsuchus , Almadasuchus and crocodyliforms, but not in more basal “sphenosuchians” (Pol et al, 2013). However, the morphology of this region of the quadrate is unknown in some of the other non-crocodyliform crocodylomorphs.…”

“…Here we apply this taxon to the clade found by our phylogenetic analysis comprising Hallopus victor (Marsh, 1890), Macelognathus vagans , and Almadasuchus figarii figarii Pol et al, 2013. We redefine it here as all taxa more closely related to Hallopus victor (Marsh, 1890) than to Protosuchus richardsoni Brown, 1933 or to Dibothrosuchus elaphros Simons, 1965.…”

“…However, given the incomplete nature of the remains, no clear relationship among basal crocodylomorphs was claimed. Later on, Pol et al (2013) described a new taxon from the Late Jurassic of Argentina ( Almadasuchus ) and noted some similarities between the femur of this new taxon and Macelognathus , but the large amount of missing data precluded the finding of a stable phylogenetic placement for the latter.…”

“…2A–2E). This contribution represents the first study of CT scans from a non-crocodyliform crocodylomorph, a group whose internal braincase anatomy is highly relevant to their evolution (Clark, 1986; Walker, 1990; Wu & Chatterjee, 1993; Pol et al, 2013). …”