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

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.

KEYWORDS:

• Cranium
• evolution
• landmarks
• Late Triassic
• phytosaur

Acknowledgments

We thank Nigel Hughes, Department of Earth Sciences, University of California, for help and advice on landmark-based morphometrics. We thank S. Bandyopadhyay and D. P. Sengupta, Geological Studies Unit of the Indian Statistical Institute, Kolkata for allowing access to the phytosaur specimens under their care. We sincerely acknowledge Gareth Dyke, William Parker, David Gillette and an anonymous reviewer for their valuable inputs. The Science and Engineering Research Board, Department of Science and Technology, New Delhi, India and the Indian Institute of Technology, Kharagpur for financial support and infrastructural facilities, respectively.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

The work was funded by the Science and Engineering Research Board, Department of Science and Technology, New Delhi, India [SERB/CRG/2019/000388].