Research press release



Palaeontology: Turtle evolution comes out of its shell



今回Chun Li、Xiao-Chun Wuたちの論文では、完全に関節のつながった非常に大きな化石標本(全長2.5メートル)について記述されている。この化石標本には腹甲も背甲もないが、非常に幅広で平らな背側の肋骨が扇状に並んだ構造を有している。この化石標本の動物は、新種とされ、Eorhynchochelys sinensisと命名された。この化石標本は、Odontochelysが発見された堆積層より約7.5メートル深い層から出土し、Odontochelysより古い時代のものであることが示されている。最も際立っているのが頭蓋骨で、Odontochelysよりもカメ類にかなり近く、上側頭窓が閉じており、歯のないくちばしの外表面を覆う角鞘があったことを示す証拠も見つかった。

The fossil of a newly discovered Triassic turtle species, found in Guizhou Province, China, is described in a paper in this week’s Nature. This specimen, dated to around 228 million years ago, is an early relative of turtles and reveals their complex early history.

The origins and phylogenetic relationships of turtles have remained one of evolution’s most enduring puzzles. These animals have such a derived body plan that it is difficult to compare them with other groups to learn how they might have evolved. In recent years, however, some intermediate forms have been found. One was Odontochelys from around 220 million years ago, which had a plastron, a fully formed shell covering the under-surface of the animal, but no distinct carapace, or ‘lid’ of the shell. Then came Pappochelys, a creature much older than Odontochelys at 240 million years ago, and which had a cuirass, or strengthened dermal bones over the belly, instead of a plastron and carapace. In addition, analyses of a skull from South Africa of an early reptile called Eunotosaurus that lived around 260 million years ago, substantiated suggestions that this form was an early offshoot of the lineage that eventually led to turtles.

Chun Li, Xiao-Chun Wu, and colleagues describe a very large (2.5 metres long) completely articulated specimen that has neither plastron nor carapace, but does have very broadened and flattened dorsal ribs arranged to form a carapacial disc. This new species, named Eorhynchochelys sinensis, was collected from sediments approximately 7.5 metres below where Odontochelys was found, indicating its older age. Most striking is the skull, which is much more turtle-like than Odontochelys - with a closed upper temporal region and evidence of a rhamphotheca, an outer surface on the toothless beak.

doi: 10.1038/s41586-018-0419-1|英語の原文

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