The Origin, Early History and Diversification of Lepidosauromorph Reptiles

Abstract

The reptilian group Lepidosauria diversified through the Mesozoic, survived the end-Cretaceous extinction relatively unscathed, and has more than 7,000 living species. Although originally constituted as a “waste-bin” for non-archosaurian diapsids, modern definitions limit Lepidosauria to its two constituent groups, Rhynchocephalia and Squamata, and their most recent common ancestor. To date, the earliest known lepidosaurs are from the Late Triassic (Carnian) of Europe and India, but their derived morphology provides indirect evidence of a longer, unrecorded, history. Rhynchocephalians and squamates probably diverged in the Early-Middle Triassic, and new material from the Early Triassic of Poland sheds some light on their common ancestor. The roots of Lepidosauria may extend into the Palaeozoic, but there are critical gaps in the fossil record.

Today Rhynchocephalia is limited to the New Zealand Tuatara, Sphenodon, but rhynchocephalians may have been the first lepidosaurs to undergo a major radiation. They had a global distribution in the Late Triassic, and diversified through the Jurassic and Early Cretaceous, expanding into new terrestrial, aquatic, and trophic niches. However, the group apparently went extinct in Asia during the first half of the Jurassic and then disappeared from Euramerica records in the mid-Cretaceous. This contrasts with Gondwanan records, as evidenced by recent discoveries in the Late Cretaceous of South America.

In many ways, our knowledge of Mesozoic squamate history is the reverse of that for Rhynchocephalia. With the exception of the Indian Tikiguana, there is no record of Triassic squamates, and only in the Early Cretaceous does the record improve significantly. Even so, most data comes from Laurasia. Very little is known of contemporaneous assemblages in Gondwana, making it difficult to reconstruct centres of origin for major squamate clades, although the relative abundance of snakes in Late Cretaceous of Gondwana is suggestive. For many years, our understanding of squamate evolution was obfuscated by an over reliance on characters of the temporal region, like the absence of a lower temporal bar. As a result, the fully diapsid skull of Sphenodon was considered primitive, and several small diapsids were mistakenly attributed either to Squamata or to its immediate stem. In fact, the fossil evidence demonstrates that the ancestral lepidosauromorph lacked a lower temporal bar. Rhynchocephalians and squamates each modified this ancestral condition in different ways.