A Triassic crown squamate

Abstract

Mammals, birds, and squamates (lizards, snakes, and relatives) are key living vertebrates, and thus understanding their evolution underpins important questions in biodiversity science. Whereas the origins of mammals and birds are relatively well understood, the roots of squamates have been obscure. Here, we report a modern-type lizard from the Late Triassic of England [202 million years (Ma)], comprising a partial skeleton, skull, and mandibles. It displays at least 15 unique squamate traits and further shares unidentatan and anguimorph apomorphies. The new discovery fixes the origin of crown Squamata as much older than had been thought, and the revised dating shows substantial diversification of modern-type squamates following the Carnian Pluvial Episode, 232 Ma ago.

The affinities of the Late Triassic Cryptovaranoides and the age of crown squamates

“…For instance, in dataset 1, the median ages for Archosauromorpha, Crocopoda, and Allokotosauria all differ by less than approximately 1.3 Myr from previous estimates [28]. This result indicates the phylogenetic hypothesis provided here for † C. microlanius (as an archosauromorph) is largely compatible with the specimen age (contrary to its original phylogenetic hypothesis [24]). Furthermore, divergence times for the major clades of squamates using total evidence dating of dataset 2 (even when including † C. microlanius ) are estimated to occur during the Middle and Late Jurassic (figure 7; electronic supplementary material, figure S21), in agreement with previous estimates [5,7,9,13–15,26].…”

“…The data matrices used by Whiteside et al [24] to assess the phylogenetic affinities of †C. microlanius are not ideal, as nearly all the non-lepidosaur species included in the original versions of those matrices were excluded.…”

“…One of these datasets (dataset 2 herein: Simões et al [7] and its subsequent expansions, (e.g. [32,33])) is the same as the one used by Whiteside et al [24] for their initial phylogenetic assessment of †C. microlanius, allowing for direct comparisons to the results presented 24]: electronic supplementary material).…”

“…The Triassic-Jurassic fissure fill deposits of Gloucestershire and Somerset in the United Kingdom are known for producing an important fossil record of early lepidosaurs [20][21][22][23]. Recently, Whiteside et al [24] described a surprising addition to this fauna, †Cryptovaranoides microlanius (figures 1-5), which they hypothesized as having phylogenetic affinities to Anguimorpha (monitors, Gila monsters, slow worms, alligator lizards, etc. ), thus being deeply nested within the squamate crown-some analyses even placed †C.…”

“…), thus being deeply nested within the squamate crown-some analyses even placed †C. microlanius as the sister taxon to the living genus Xenosaurus [24]. Anguimorphs are otherwise universally inferred to have diverged from other squamates between the Early Jurassic and the Late Cretaceous based on total evidence dating (morphological and molecular) analyses [7,9,10] or genomic timetrees [5,14,25,26].…”