The ontogenetic pattern of neurocentral suture closure in the axial skeleton of Hyperodapedontinae (Archosauromorpha, Rhynchosauria) and its evolutionary implications

Abstract

Understanding the ontogeny of a taxon is a crucial step to properly elucidate its taxonomy and evolution. However, aside from histological data, osteological criteria for assessing maturity are considered lineage specific or controversial. The sequence of neurocentral suture closure of the axial skeleton of extant crocodilians, which occurs in a posterior–anterior sequence, is a non-destructive method used to determine maturity in extinct reptiles. However, its use in extinct archosaurs not closely related to crocodilians is debatable, as the ancestral condition of Archosauria is unknown and variation occurs in timing and sequence orientation within the clade. We have assessed the pattern of neurocentral suture closure of Hyperodapedontinae rhynchosaurs, an early archosauromorph clade distantly related to archosaurs. Unlike extant crocodilians, they exhibit an anterior–posterior sequence neurocentral suture closure. Relative size and other ontogenetic markers suggest that neurocentral closure in the Hyperodapedontinae correlates with aging, although closed sutures were rare in the sample. A high number of open or partially open sutures in mature individuals indicate that they remained open during most of their life. Our study indicates that: (1) the delayed neurocentral closure may be a paedomorphic heterochronic process in Hyperodapedontinae, as it contrasts with the fully closed neurocentral sutures of early diverging non-hyperodapedontine rhynchosaurs; (2) the assumption that open neurocentral sutures indicates immaturity in extinct reptiles is not always correct; and (3) the delayed closure may have originated independently in several archosauromorph lineages, but the ancestral condition of Archosauria probably follows the crocodilian closure pattern.