Turtles (a group which includes within it tortoises) are most famous for their shells, which are made of their fused ribs. No other animal has a shell constructed in this manner, and it is a feature that all turtles share. The turtle evolution question, therefore, is somewhat synonymous with how turtle shells evolved. It’s also a case where creationists might reasonably ask “where are the transitional forms” – how do you get to shell from no shell? Unfortunately for them, fossils that fit the bill do exist. In 2009, for example, fossils of Odontochelys were discovered in China. This turtle had a complete plastron, the bottom of the shell, but instead of the upper part (the carapace) it merely had broadened rib bones. That sounds like a transitional form to me.