Cranial evolution of the Late Triassic phytosaurs (Diapsida, Archosauria): preliminary observations from landmark-based morphometric analysis

Abstract

ABSTRACT Landmark-based Principal Component Analysis was performed on multiple phytosaur and crocodilian skulls. Multiple landmark points were selected defining suture points of bone tissues and regions of maximum or minimum curvature along the skull. Scatter plots of the principal component scores show that phytosaurs cluster according to their higher order taxonomic groups, and the convex hull polygons of the different clades occupy distinct morphospaces albeit with variable degrees of overlap. Disparity between the morphospaces of non-Mystriosuchinae parasuchids and Mystriosuchinae phytosaurs suggests marked differences in skull shape among basal and derived taxa. The vector deformation plots of phytosaur skulls showcase the evolution of skull shape from basal to advanced forms. The morphological changes imply evolution of the phytosaur skulls towards a wider and deeper snout, cage-like grip, tendency for increasing jaw strength, and faster-snapping action. In addition, heightening of the nostrils suggested that phytosaurs evolved into ambush predators, and increased their ability to remain submerged underwater with their nostrils high above the water surface, thereby enhancing their ability to ambush prey. The study supports prior assumptions of dietary preferences among the phytosaurs and maps the evolution and prevalence of cranial morphotypes, associated with such dietary preferences, across the different phytosaur clades.