A new basal crocodylomorph from the Late Jurassic of Patagonia and its implications for the evolution of the crocodyliform braincase

POL, D.; RAUHUT, OLIVER; LECUONA, A.; LEARDI, J. M.

Las Vegas

Congreso; Anual Meeting of the Society of Vertebrate Paleontology; 2011

Society of Vertebrate Paleontology

Crocodyliformes is a group of highly modified archosaurs that achieved a remarkable taxonomic and ecological diversity during the Mesozoic. The sequence of transformations leading from the typical basal archosaurian skulls of ?sphenosuchians? to the derived and akinetic skull of crocodyliforms is still poorly understood, with only the late Middle Jurassic Chinese form Junggarsuchusproviding some information. A new crocodylomorph from the Late Jurassic Cañadón Calcáreo Formation of Patagonia is reported here that is represented by the posterior region of the skull (including a well preserved braincase), and fragmentary remains of the rostrum, palate, mandible, and postcranium. The skull remains bear autapo-morphic features (e.g., laterally concave extermal margin of the squamosal and postorbital, large quadrate fenestra) combined with a unique combination of plesiomorphic characters of basal crocodylomorphs (e.g., paired frontals, supraoccipital participating from the foramen magnum, absence of well developed skull table) and apomorphies previously known only in Crocodyliformes (e.g., absence of basipterygoid process, highly pneumatic basisphenoid, posterior closure of otic notch by triple contact of squamosal, quadrate, and otoccipital). A phylogenetic analysis that includes both ?sphenosuchians? and representatives of the major lineages of Crocodyliformes depict the new taxon as the closest relative of Crocodyliformes, being closer to this clade than Junggarsuchus. The morphology of the braincase of the new taxon coupled with its phylogenetic position reveals that modifications in the articulations of the braincase with both the palate and the quadrate occurred before the origin of Crocodyli-formes and therefore predated the remarkable changes in the snout, jaws, and dentition of the phylogenetically more derived crocodyliforms.