Monday, 01 Dec, 2008 Environment

Evolution of Turtle's Shell Retraced


The recent discovery of turtle fossils in southwestern China gave scientists the possibility to determine the origin of their shells. As they found out, ancestors of modern turtles had hardened skin on their backs instead of shells. The finding proved the hypothesis that the shell developed as an expansion of ribs and the backbone.

It should be mentioned that there was an alternative theory claiming that shells formed from bony plates contained in the skin of animals which are referred to as osteoderms. However, fossils of prehistoric turtles proved that they didn't have such plates.

"So far there is no direct evidence for the osteoderm theory," said Chun Li from the Institute of Vertebrate Paleontology and Paleoanthropology. "On the contrary, here, in our hands, there is an ideal missing link for turtle evolution. It has no osteoderms on its back, but only ossified neural plates and expanded ribs."

The 220-million-year-old turtle was called Odontochelys semitestacea which means half-shelled turtle with teeth. Its description was presented in the journal Nature. As it turned out, the oldest species of turtles differed greatly from modern ones. It's surprising that they had teeth instead of a bony plate as well as a long tail. Besides, the shell covered only their underside.

Robert Reisz of the University of Toronto noted that "turtles come from reptile ancestors with teeth so we expected this, but it's still a great thing to find."

The development of modern turtle embryos also supports the theory that breast plates or plastrons developed earlier than the shell on the back. As researchers explain, the presence of breast plates was conditioned by an aquatic lifestyle of turtles and served for their protection.

Posted by sharaeff

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