220 million-year-old Fossel of turtle ‘on the half shell’ explains how species evolved

November 26, 2008

China (Chattahbox) — A newly discovered fossil of a half-shell turtle species that swam in China’s coastal waters 220 million years ago is the oldest turtle known to date. The fossil China has shed light on how the turtle’s shell evolved. Last week, a team of scientists had reported the discovery of the oldest aquatic turtle, dating back 164 million years. That was a short-lived title. The new half-shell aquatic turtle, studied by Chun Li of the Chinese Academy of Sciences in Beijing and his colleagues and dubbed Odontochelys semistestacea, swam around even longer ago. TheĀ  fossil described in Nature journal, shows that the turtle had a belly shell, but its back was basically bare of armor. The breast plate of this fossil was an extension of its ribs, but only hardened skin covered its back.

Researchers say the breast plate may have protected it while swimming. The turtle fossil, found near Guangling in south-west China, is thought to be the ancestor of all modern turtles, although it differs markedly; it has teeth rather than a bony plate, the shell only covers its underside and it has a long tail. The fossil find helps to answer key questions about the evolution of turtles, Dr Xiao-Chun Wu from the Canadian Museum of Nature was one of the first to examine the fossil.

The fossilised turtle ancestor, which has been named Odontochelys semitestacea, meaning half-shelled turtle with teeth, probably inhabited the river deltas or coastal shallows of China’s Nanpanjiang trough basin – the area where the fossil was unearthed. Before the discovery of Odontochelys, the oldest known turtle species (aquatic or land-based) was the terrestrial turtle Proganochelys, which lived about 210 million years ago. But this turtle had a fully formed shell, providing little evidence as to how the shell evolved. Researchers say the development of the shell to first protect the underside points to a mainly aquatic lifestyle. The researchers say further evidence to support the idea that this species lived mainly in water comes from the structure and proportions of the fossil’s forelimbs, which closely resemble those of modern turtles that live in similar conditions.


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