At the feet of the dinosaurs: the early history and radiation of lizards

Abstract

Lizards, snakes and amphisbaenians together constitute the Squamata, the largest and most diverse group of living reptiles. Despite their current success, the early squamate fossil record is extremely patchy. The last major survey of squamate palaeontology and evolution was published 20 years ago. Since then, there have been major changes in systematic theory and methodology, as well as a steady trickle of new fossil finds. This review examines our current understanding of the first 150 million years of squamate evolution in the light of the new data and changing ideas. Contrary to previous reports, no squamate fossils are currently documented before the Jurassic. Nonetheless, indirect evidence predicts that squamates had evolved by at least the middle Triassic, and had diversified into existing major lineages before the end of this period. There is thus a major gap in the squamate record at a time when key morphological features were evolving. With the exception of fragmentary remains from Africa and India, Jurassic squamates are known only from localities in northern continents (Laurasia). The situation improves in the Early Cretaceous, but the southern (Gondwanan) record remains extremely poor. This constrains palaeobiogeographic discussion and makes it difficult to predict centres of origin for major squamate clades on the basis of fossil evidence alone. Preliminary mapping of morphological characters onto a consensus tree demonstrates stages in the sequence of acquisition for some characters of the skull and postcranial skeleton, but many crucial stages--most notably those relating to the acquisition of squamate skull kinesis--remain unclear.

The oldest known snakes from the Middle Jurassic-Lower Cretaceous provide insights on snake evolution

“…The new early snakes add to those hypotheses by extending the fossil record of snakes by 70 million years (Fig. 1m-p) coincident with the radiation of other mid-Jurassic squamate groups 12,13 and with molecular clock predictions 14 .…”

“…An extension of the fossil record of snakes to the Middle Jurassic is not surprising, as it coincides with the radiation of other major groups of squamates 12,13 , even though these new records of diverse and broadly distributed midMesozoic (B167-145 Myr ago) snake assemblages (Fig. 1m-p) create a significant temporal gap with the previous early records of snakes (B100 Myr ago) from Africa 1 , North America 2 , Brazil 32 and Europe 33 .…”

Mass extinction of lizards and snakes at the Cretaceous–Paleogene boundary

“…The history of this radiation extends deep into the Mesozoic. After the appearance of crown squamates in the Jurassic (3), lizards and snakes underwent a Cretaceous radiation (4,5), and by the late Cretaceous most major groups had appeared, including iguanians, geckos, skinks, anguids, and platynotans (3,4), as well as many snake lineages (5). However, with the exception of the marine mosasaurs, all major squamate lineages are thought to have survived the end of the Cretaceous.…”

Large-scale phylogeny of chameleons suggests African origins and Eocene diversification

“…The sister group to chameleons (Agamidae) is completely absent from Madagascar, Comoros and Seychelles, although it is distributed across most other East Gondwanan and Laurasian landmasses, which is a pattern common to multiple faunal groups, particularly within mammals and reptiles [5]. The fossil record suggests that the probable common ancestors of chameleons and agamids were distributed across Laurasia in the Mesozoic period [6][7][8][9]. Chameleons themselves are not abundant in the fossil record (and stem chameleons are completely absent), but they do have a presence in Africa and Europe in the Miocene, and on Madagascar in the Holocene [10][11][12][13].…”