A Triassic Stem Lepidosaur Illuminates the Origin of Lizard-Like Reptiles

Abstract

The early evolution of diapsid reptiles is marked by a deep contrast between our knowledge on the origin and early evolution of archosauromorphs (crocodiles, avian and non-avian dinosaurs) to that of lepidosauromorphs—squamates (lizards, snakes) and sphenodontians (tuataras). Whereas the former includes hundreds of fossil species across various lineages during the Triassic1, the latter is highly obscured by an extremely patchy early fossil record comprised by only a handful of fragmentary fossils, most with uncertain phylogenetic affinities and confined to Europe1-3. Here we report on the discovery of a three-dimensionally preserved reptile skull from the Late Triassic of Argentina robustly inferred phylogenetically as the earliest evolving lepidosauromorph in the world using various data types and optimality criteria. Micro CT-scans of Taytalura alcoberi, gen & sp. nov. reveal unprecedented details onto the origin of the lepidosaurian skull from early diapsids, suggesting that several traits traditionally associated with sphenodontians in fact evolved much earlier in lepidosauromorph evolution. Taytalura suggests that the strongly conserved evolutionary skull architecture of sphenodontians represents the plesiomorphic condition for all lepidosaurs, that stem and crown lepidosaurs were contemporaries for at least 10myr during the Triassic, and that early lepidosauromorphs had a much broader geographic distribution than previously thought.