Just been reading a fascinating article in the Daily Mail on the latest discoveries regarding the evolution of the turtles shell. New information has been revealed since the discovery last year of the world's oldest turtle fossils in China dating back 220 million years with only a half formed shell on their backs. The previous earliest fossils had fully formed shells and were found in Germany dating from 210 million years ago.
This new discovery provides the missing lnk and indicates that the shells were an outgrowth of the ribs and backbone, and that the undershell was the first to develop, indicating they were aquatic rather than land based.
is the link to the full article.