【古生物学】カメ類の初期の進化過程を解明する手掛かりとなる化石

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.