古生物学：鱗竜類の起源

The skull of a lizard-like reptile from the Triassic of Argentina, dated to be around 231 million years old, sheds light on the origins of the group that gave rise to snakes, lizards and tuataras. The fossil is described in this week’s Nature.

Lepidosaurs are scaled reptiles. They include squamates (lizards and snakes) and sphenodontians, such as the tuatara (reptiles found in New Zealand). With more than 11,000 species, lepidosaurs are the most diverse group of terrestrial vertebrates alive, yet compared with their sister lineage (the archosauromorphs, which includes crocodiles, and avian and non-avian dinosaurs), little is known about their early evolution because the fossil record is fragmented.

A well-preserved reptile skull that predates the split between squamates and sphenodontians, described by Ricardo Martínez and colleagues, is close to the origin of lepidosaurs and may be one of the earliest known lepidosaurs. The skull shares features with modern tuataras, suggesting that several anatomical features, presumed exclusive to sphenodontians, must have originated early in lepidosaur evolution. The fossil is about 11 million years younger than the oldest known lepidosaurs from Europe, and approximately the same age as the oldest known South American lepidosaurs. The findings indicate that stem and crown lepidosaurs were contemporaries for at least 10 million years during the Triassic, and that early lepidosaurs had a much broader geographical distribution than was previously thought.