The geometry of synapsid skull disparity

Abstract

ABATRACT Synapsida represents a rich lineage of tetrapods including a mixture of morphotypes, from reptilian-like ‘pelycosaurs’ to crown group mammals, spanning more than 325 My. Although such a morphological diversity peaked several times across three eras, little is known about the constraints underlying such evolutionary patterns. Using theoretical morphology rationales, we assessed the distribution of skull disparity by measuring three functional partitions (i.e. rostrum, orbit and braincase) in a sample encompassing most major clades of the lineage (n = 169). To broaden our macroevolutionary scope, we compared this pattern with that of the archosaurian sauropsids (Diapsida). Results show that despite being less diverse, the disparity of synapsid skulls almost doubles that of the archosaurs. The synapsids span an outstanding range of facial proportions, involving short faces (newly labelled parvirostral) and extreme shortening of the face in some primates, including humans, contributing less than 10% to skull geometry (microrostral). We also found that synapsids changed their geometric rules of skull organisation; in the Permo-Trias, the orbit and the braincase changed concomitantly, as in the diapsids, hence being more ‘reptilian’. Thereafter, depleting the orbit from craniofacial variation at the transition to therian mammals, in the Mesozoic, marked an evolutionary pathway that endured throughout the Cenozoic.