A new look at the verticalization of the braincase of Crocodyliformes

PINHEIRO, A.E.; POL, D.

Buenos Aires

Jornada; XXVI Jornadas Argentinas de Paleontología de Vertebrados; 2012

One of the majors cranial changes recognized for Eusuchia is the verticalization of the basioccipital and basisphenoid. This transformation involves major changes in the eustachian and periotic ear sinus system as well as a rearrangement of the jaw adductor muscles. The verticali- zation of the braincase is not exclusive of Eusuchia, but in this clade reaches its maximum expression, with these elements forming an angle of almost 90 degrees respect to the horizontal plane. The fossil record and the distribution of characters has increased during the last twenty seven years and indicates that the verticalization of basicranium is not exclusive of eusuchians but is a widespread feature among crocodyli- forms. In fact, few lineages (e.g. Protosuchidae, Thalattosuchia, Baurusuchidae, Sphagesauridae) do not exhibit any degree of verticalization of the basicranium, showing the plesiomorphic archosaurian structure of ventro-posterior and oblique orientation of the basisphenoid and basioccipital that are well exposed in ventral view. Many other non-eusuchian crocodyliforms lineages (e.g. some sphenosuchians like Sphe- nosuchus acutus; most neosuchians; uruguaysuchids; peirosaurids; sebecids) exhibit this verticalization trend in different degrees and shapes (varying the element, the sutural contact, how marked is the verticalization, and the degree of ventral exposure). The verticalization trend is first recorded in basal forms from the Late Triassic (e.g. Hesperosuchus agilis) and had independent increases of verticalization in several lineages during the Jurassic (e.g. Goniopholis baryglyphaeus) and Cretaceous. This complex transformation of the braincase actually comprises different characters that evolve independently (many of which have several homoplastic instances), resulting in a mosaic evolutionary pattern within Crocodylomorpha.