Tooth attachment in Silesauridae: understanding the ankylo-thecodont ontogenetic phase in the evolution of archosaur thecodonty

Author(s)

Gabriel Mestriner da Silva

• Document type: Master's Dissertation
• Press: Ribeirão Preto.
• Institution: Universidade de São Paulo (USP). Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto (PCARP/BC)
• Defense date: 2021-03-12

Examining board members

Max Cardoso Langer; Julio Cesar de Almeida Marsola; Jonathas de Souza Bittencourt Rodrigues; Juliana Manso Sayão

• Advisor: Max Cardoso Langer

Abstract

Contrary to the traditional view in which ankylo-thecodonty is recovered as a synapomorphic trait of Silesauridae, new histological data show that it merely represents the last stage of tooth development within the group. Those dinosauromorphs have neither the crocodilian/dinosaur \"permanent gomphosis\" nor the \"rapid ankylosis\" that is plesiomorphic for amniotes. Instead, all sampled silesaurids show \"delayed ankylosis\", a condition in which teeth pass through an initial gomphosis stage followed by final ankylosis. This suggests that, as already documented for synapsids, tooth fixation in Archosauria might have followed a paedomorphic evolutionary pattern, with the crocodile/dinosaur gomphosis representing the maintenance of an early ontogenetic stage, in which the alveolar bone does not calcify the periodontal ligament between the tooth root and the alveolus. \"Delayed ankylosis\" in Silesauridae results in accepting the dinosaur and crocodile \"permanent gomphosis\" as convergently acquired or, less likely, that the silesaurid condition represents a synapomorphic reversal. Moreover, if Silesauridae is nested within Ornitischia, \"permanent gomphosis\" could even be convergent between the two main dinosaurs lineages. In any case, a ankylo-thecodonty to thecodonty one-step transition appears even more as the oversimplification of a much more complex evolutionary history. In fact, characteristics involving tooth attachment must be evaluated in more detail when they are codified in archosaur phylogenies. An important emerging aspect of gomphosis as synapomorphic for dinosaurs, is that it may represent one of the key features that allowed their thriving on Earth for more than 150 million years. (AU)

• FAPESP's process: 18/24031-6 - Systematics of new Dinosauriformes specimens from the Upper Triassic (Carnian) of Cerro da Alemoa, Santa Maria-RS Formation and their importance for evolution and paleobiology of the first dinosaurs
• Grantee: Gabriel Mestriner da Silva
• Support Opportunities: Scholarships in Brazil - Master