POST-HATCHLING CRANIAL ONTOGENETIC VARIATION IN THE BASAL ARCHOSAURIFORM PROTEROSUCHUS FERGUSI FROM THE LOWER TRIASSIC OF SOUTH AFRICA

EZCURRA, M. D.

Barcelona

Congreso; 10th International Congress of Vertebrate Morphology; 2013

The basal archosauriform Proterosuchus fergusi is known from multiple specimens collected from the LowerTriassic of South Africa. This relatively large sample is important because ontogenetic studies of Mesozoicreptiles are restricted to a handful of well-known species. The phylogenetic position of Proterosuchus as themost basal archosauriform means that it is a critically important taxon for understanding the successfulevolutionary radiation of this group during the Mesozoic. Qualitative and quantitative analyses of cranialontogenetic variation in Proterosuchus were conducted based on 13 post-hatchling specimens. The smallestindividual is 37% of the size of the largest one and osteohistological evidence suggests that four specimenshad not reached sexual maturity at the time of death. Allometric coefficients were calculated fromstandardised major axis regressions of 68 log-transformed cranial measurements, using skull length as astandard measurement. Most of the recovered allometric coefficients showed isometric trends (73%).Positive allometric trends were mostly associated with measurements of cranial height (e.g. maximum heightof skull, minimum height of maxillary horizontal process and height of jugal anterior process andinfratemporal fenestra) and the length of the infratemporal fenestra. Negative allometric trends were foundmostly associated with dental measurements (e.g. number of maxillary and dentary teeth and height ofmaxillary teeth). Thus, through ontogeny the skull of Proterosuchus became proportionally taller, theinfratemporal fenestra larger and teeth more isodont and numerous but with apicobasally shorter crowns.Additional changes in medium to large specimens include less interdigitated sutures in the skull roof,proportionally narrower parietals between the supratemporal fenestrae and the appearance of a pinealfossa. The elongated snout and enlarged premaxilla that are characteristic of proterosuchids underwentisometric growth. This new information on Proterosuchus cranial ontogeny provides a basis for the futureidentification of heterochronic events during the early evolution of Archosauriformes.