Research press release



Palaeontology: Ancient reptile revamps lizard family tree



今回、Tiago Simoesたちの研究グループは、アルプス山脈のイタリア側で発見されていたMegachirella wachtleriの化石を再び調べて、有鱗目も含まれるより広範な分類群である鱗竜亜綱に分類し直した。また、高解像度のCATスキャナーを用い、骨格化石の中にこれまで気付かなかった特徴(有鱗目に特有の小さな下顎骨など)を発見した。これと並行して、Simoesたちは、これまでで最大の化石と現生爬虫類のデータセットをまとめて、有鱗目の進化史上のMegachirellaの位置を評価した。


The oldest-known member of the evolutionary lineage that led to the squamates (the group of animals that today includes snakes and lizards) lived in the Middle Triassic, some 240 million years ago, reports a paper in Nature this week. This finding dates both the origin of squamates and the split of the wider diapsid reptile lineages to before the Permian/Triassic mass-extinction, 252 million years ago. This event could have created new opportunities for diversification within the reptile lineages, rather than their origins as previously thought.

The squamates are one of the most diverse groups of land vertebrates, and include lizards and snakes. Understanding of the genesis of this group has been confused, however, by the 70-million-year gap in the fossil record between their oldest-known fossils and estimated time of origin, the limited incorporation of squamates in studies of the reptile ‘family tree’, and conflicting recent evolutionary histories offered by anatomical and DNA-based studies.

Tiago Simoes and colleagues re-examined a fossil of Megachirella wachtleri that was found previously in the Italian Alps and assigned to the more-inclusive group lepidosauria, which includes squamates. They used a high-resolution CAT scanner to reveal previously unnoticed features in the fossil skeleton including, a small lower-jaw bone that is found uniquely in squamates. Alongside this, the authors assembled the largest ever dataset of fossil and living reptiles to assess where Megachirella lies in the history of squamates.

The findings suggest that Megachirella is the oldest-known member of the squamate lineage, being about 72 million years older than the earliest-known true squamate, from the Jurassic period. This finding helps to fill a gap in our understanding of the origin of squamates and other reptiles, showing that they began to diversify not long after the Permian/Triassic mass extinction.

doi: 10.1038/s41586-018-0093-3

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