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

Most living reptile diversity is concentrated in Squamata (lizards, including snakes), which have poorly known origins in space and time. Recently, †Cryptovaranoides microlanius from the Late Triassic of the United Kingdom was described as the oldest crown squamate. If true, this result would push back the origin of all major lizard clades by 30-65 Myr and suggest that divergence times for reptile clades estimated using genomic and morphological data are grossly inaccurate. Here, we use computed tomography scans and expanded phylogenetic datasets to re-evaluate the phylogenetic affinities of †Cryptovaranoides and other putative early squamates. We robustly reject the crown squamate affinities of †Cryptovaranoides, and instead resolve †Cryptovaranoides as a potential member of the bird and crocodylian total clade, Archosauromorpha. Bayesian total evidence dating supports a Jurassic origin of crown squamates, not Triassic as recently suggested. We highlight how features traditionally linked to lepidosaurs are in fact widespread across Triassic reptiles. Our study reaffirms the importance of critically choosing and constructing morphological datasets and appropriate taxon sampling to test the phylogenetic affinities of problematic fossils and calibrate the Tree of Life.

Evolutionary origins of the prolonged extant squamate radiation.

Squamata is the most diverse clade of terrestrial vertebrates. Although the origin of pan-squamates lies in the Triassic, the oldest undisputed members of extant clades known from nearly complete, uncrushed material come from the Cretaceous. Here, we describe three-dimensionally preserved partial skulls of two new crown lizards from the Late Jurassic of North America. Both species are placed at the base of the skink, girdled, and night lizard clade Pan-Scincoidea, which consistently occupies a position deep inside the squamate crown in both morphological and molecular phylogenies. The new lizards show that several features uniting pan-scincoids with another major lizard clade, the pan-lacertoids, in trees using morphology were convergently acquired as predicted by molecular analyses. Further, the palate of one new lizard bears a handful of ancestral saurian characteristics lost in nearly all extant squamates, revealing an underappreciated degree of complex morphological evolution in the early squamate crown. We find strong evidence for close relationships between the two new species and Cretaceous taxa from Eurasia. Together, these results suggest that early crown squamates had a wide geographic distribution and experienced complicated morphological evolution even while the Rhynchocephalia, now solely represented by the tuatara, was the dominant clade of lepidosaurs.

Re-description of the early Triassic diapsid Palacrodon from the lower Fremouw formation of Antarctica.

The rapid radiation and dispersal of crown reptiles following the end-Permian mass extinction characterizes the earliest phase of the Mesozoic. Phylogenetically, this early radiation is difficult to interpret, with polytomies near the crown node, long ghost lineages, and enigmatic origins for crown group clades. Better understanding of poorly known taxa from this time can aid in our understanding of this radiation and Permo-Triassic ecology. Here, we describe an Early Triassic specimen of the diapsid Palacrodon from the Fremouw Formation of Antarctica. While Palacrodon is known throughout the Triassic and exhibits a cosmopolitan geographic range, little is known of its evolutionary relationships. We recover Palacrodon outside of crown reptiles (Sauria) but more crownward than Youngina capensis and other late Permian diapsids. Furthermore, Palacrodon possesses anatomical features that add clarity to the evolution of the stapes within the reptilian lineage, as well as incipient adaptations for arboreality and herbivory during the earliest phases of the Permo-Triassic recovery.