進化学：ジュラ紀のトカゲの頭蓋骨化石が爬虫類の進化に関する新たな手掛かりに

The partially preserved skulls of two new lizard species from the Late Jurassic period (around 145 million years ago) of North America are described in a paper in Nature Communications. Both fossils represent crown squamates, a group containing living lizards, and provides insights into the relationships between groups within this clade.

Fossils from early squamate evolution are limited and usually poorly preserved, with the oldest uncrushed skeletal material dating back to the Cretaceous period (approximately 145.5–65.5 million years ago). While potential snakes, gekkos, and skinks have been reported from the Jurassic (around 200–145.5 million years ago), the variability in their preservation has meant that their placement in relation to more recent fossils is unclear. In general, the state of the fossil record has obscured the radiation and diversification of this group, and limited hypotheses about the geographic distribution of squamates.

Chase Brownstein and colleagues describe two new lizards, Eoscincus ornatus and Microteras borealis, from the Late Jurassic period of North America. Anatomical analyses of the newly discovered partially preserved Eoscincus ornatus skull, and the Microteras borealis skull, show that these fossils belong to a group encompassing skinks, girdled lizards, night lizards, spectacled lizards, whiptail lizards, wall lizards, and amphisbaenians. These fossils have ancestral traits, not seen in more recent examples of this group, and highlight differences between squamate evolutionary trees based on shape and genetic data. Contemporaneous expansion of dinosaurs, mammals, and turtles combined with these findings suggests that the expansion of the Atlantic Ocean was associated with terrestrial vertebrate biogeography.

This work supports our understanding of episodes of morphological innovation in reptiles, and illustrates how early squamates have evolved into lizards living today.