The story
Odontochelys semitestacea, described by Li Chun and colleagues in 2008 from the Xiaowa Formation of Guizhou, China, is a turtle about 40 cm long from roughly 220 million years ago. It has a complete belly shell (plastron) and widened ribs, but no full domed carapace, and it still has teeth in its jaws, unlike later turtles with horny beaks. Its name means toothed turtle with a half shell.
Earlier fossils fill in the steps. Eunotosaurus from the Permian of South Africa has broad ribs, Pappochelys from the German Middle Triassic has belly ribs that began forming a plastron, and Proganochelys, about 210 million years old, has a full shell, though it still has teeth on its palate and cannot pull its head in. Where turtles sit among reptiles is still debated, but genetic data link them with the archosaurs, the lineage of crocodiles and birds. Turtles passed through every extinction since and survive today.
Why it mattered
- Odontochelys showed that the plastron formed before the carapace, settling a long debate about how the shell evolved.
- Turtles are among the oldest living reptile lineages and survived the end-Triassic and end-Cretaceous extinctions.
- Their fossil sequence shows how a drastically new body plan can build up from small changes in the ribs.
Sources
- New Fossil Shows How the Turtle Got Its Shell Scientific American
- Heroes in a half-shell show how turtles evolved National Geographic
- Key link in turtle evolution discovered Smithsonian
Contested history
Consensus: Odontochelys and Pappochelys show the shell built up in steps from broadened ribs and belly bones, but where turtles sit among reptiles is unsettled.
- Heroes in a half-shell show how turtles evolved National Geographic
Community notes
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