Turtles are unique among reptiles in that they have a large shell, composed of a carapace on their back and a plastron on their belly. This shell largely develops from fused ribs and bones derived from the skin. Since turtles presumably evolved from a lizard-like ancestor to become the distinctively shelled creatures we know of today, we might expect that there would be transitional fossils that illustrate this link.
Pappochelys 242-237 million years ago
At first glance, Pappochelys  might not look very turtle-like, as it had no shell. However, it did have what appears to be the beginnings of a shell. Specifically, it had very broad t-shaped ribs and little bones in its belly region known as gastralia. Beyond these, Pappochelys had various other features that it shares with other turtles, such as a process coming off of the pubis, a femur with an offset head, and a modified shoulder blade.
The overhead view of the reconstruction above gives the impression of a lizard-like animal with the beginnings of a shell.
Odontochelys 237-227 million years ago
Not too long after Pappochelys appears in the fossil record, we find Odontochelys . This extinct species apparently fused those belly bones (gastralia), making up a plastron that is very similar to that of modern turtles. In other words, this turtle only had half of a shell (the bottom half). In addition to this new feature, Odontochelys had a shorter tail than Pappochelys, a larger forelimb to hindlimb ratio, new bones called neurals, and it lost two holes in the sides of its skull that formerly allowed jaw muscles to bulge out during chewing.
Proganochelys 227-208.5 million years ago
Next to appear in the fossil record is Proganochelys, a species very close to modern turtles in overall form. It has a complete shell, with both the lower plastron and upper carapace. Additionally, whereas Pappochelys and Odontochelys had typical reptilian teeth on the margins of its mouth, Proganochelys lost these and probably replaced them with a keratinous beak. However, Proganochelys did retain teeth on its palate, as well as other traits related to the skull, shell, shoulder and pelvic girdles that are not found in turtles today.
Notably, this overall pattern of tooth reduction in turtle history (i.e., marginal teeth in Pappochelys -> marginal+palatine teeth in Odontochelys -> palatine teeth in Proganochelys -> no teeth in modern turtles) is paralleled by genetic data. Specifically, turtles retain remnants of tooth genes, suggesting that they formerly possessed teeth.
Questions for Creationists
Where did the turtles without shells and the turtles with half shells go? Or even turtles like Proganochelys, which had palatine teeth? Some of these species, such as Odontochelys, were likely aquatic, so should they not have survived Noah’s flood? Is it a coincidence that the species found in lower (=older) rocks are less like modern turtles than the species in higher (=younger) rocks? Is it coincidence that the reduction in teeth in the turtle fossil record mimics the loss of tooth genes in modern turtles?