You might have read about this over the last day or two, I was going to add this to the "Common Ancestry" thread, I considered it somewhat off topic as it is more to do with evolutionary transition.
The shells of turtles and tortoises have always been a bit mystery for evolutionery biologists. A turtle shell is not a single structure but a complex one made up from the fused bones analogous to the ribs and vertebrae of other vertebrates. These bones in effect form a pseudo exoskeleton. All other shelled animals are made up of scales and not modified bones. This modification the skeletal structure means that the turtle has had to find new way to breath as the usual method is to use the ribs to ventilate the lungs. Turtles have overcome this by evolving a muscular sling
Until the discovery in China of a 220 milion year old fossil of a reptile Odontochelys semitestacea in 2008 all other fossilised turtles had fully formed shells, making the evolutionary steps that formed shells extremely difficult to ascertain. O. semitestacea however had a fully develop plastron (shell on its belly) made from flattened ribs, but only a partial shell on its back. Now another fossil has been studied, Eunotosaurus which is 30 - 55 million older than O. semitestacea.
The study, published in the journal "Current Biology", found that Eunotosaurus has nine broadened ribs as well as elongated vertebrae, similar to those seen in turtles and by examining marks left by fibres, the scientists have also deduced that it lacked intercostal muscles which are used to move the rib cage and aid breathing as turtles do. However Eunotosaurus didn't have features seen in other extinct fully shelled turtles like the Odontochelys and modern turtles.
What I found interesting is we didn't really need to find the fossils to see how this evolution occured. This evolutionary history is there for all to see by looking at the stages of embronic development of turtles. These newly found fossils are refelected by the developing embryos of modern turtles.
Of course this is one more nail in the already well sealed coffin of creationism. Nest time someone shouts "where are your btransitional fossils?" you calmly respond with Eunotosaurus and Odontochelys semitestacea along with an encylopaedia worth of others.