Research press release



Palaeontology: Fossils provide clues to fishy evolutionary timeline



今回、Sam Gilesたちの研究グループは、約2~2.5億年前(原始恐竜が陸上で進化した頃)と年代決定され、広範囲に分布していた化石魚類スカニレピス目に属するFukangichthysの化石の高分解能コンピュータ断層撮影(CTスキャン)の結果を調べた。これまでスカニレピス目は、ビチャーに似ているとされていたが、これまでの研究報告で指摘されていた類似点は外見上のものだった。この新たなCTスキャンでは、保存されていた頭蓋骨を立体的に観察することが可能になった。そして、さまざまな物理的性質と12個の遺伝子のDNA塩基配列の解析と近縁種との比較が行われた結果、ビチャーがスカニレピス目に属することが実証された。その結果、三畳紀を起源とするスカニレピス目が、ビチャーに最も近縁な化石魚類となった。ところが、もしビチャーが本当に原始魚類であれば、現生条鰭類の起源は、これまで考えられていたよりもかなり遅かった可能性が生じる。今回の研究結果は、デボン紀から三畳紀に生息していた条鰭類の外観を有する化石の再検討をもたらす可能性がある。


A new analysis of fossil fishes, which shows that a primitive and enigmatic fish called the bichir, or ropefish, is a true ray-finned fish, is published online in Nature this week. The findings represent a significant advance in our understanding of fish evolution.

Modern ray-finned fishes (actinopterygii) comprise half of existing vertebrate species, and include forms as diverse as guppies and cod. The group is thought to have originated just before the Devonian ‘Age of Fishes’, around 385 million years ago. The bichir (Polypterus) belongs to a small, relic group of fishes that also includes sturgeon. These scaly, primitive, eel-like lobe-finned fishes, now found in Africa, have long defied classification but are now accepted to be the living sister group of all other ray-finned fishes. However, their fossil history extends back only 100 million years. If they are more primitive than other ray-finned fishes, they are expected to have originated earlier than other Actinopterygii, leaving a quarter-of-a-billion-year gap in the fossil record.

Sam Giles and colleagues examined a high-definition computed tomography (CT) scan of fossils of Fukangichthys, a member of a widespread group of fossil fish known as scanilepiformes, dating from around 200 to 250 million years ago ― when the first dinosaurs were evolving on land. Scanilepiformes have been compared to bichirs, although the resemblances described previously have been superficial. The new CT scan enabled a view of three-dimensionally preserved skulls. The analysis of a variety of physical characteristics and the DNA sequences of 12 genes, along with comparisons to related forms, demonstrates that bichirs belong to the scanilepiform group. This places scanilepiformes, which originated in the Triassic, as the closest fossil relatives of the bichir. However, if bichirs really are primitive, modern ray-finned fishes may have originated much later than previously thought. This finding could prompt a re-think of the fossils of what look like ray-finned fishes between the Devonian and Triassic.

The CT data that support the findings of this study, as well as 3D surface files of described material, will be available here once the paper has published:

doi: 10.1038/nature23654 | 英語の原文

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