Turtles are one of the most beloved and recognisable groups of modern reptiles because of their most unique feature: their shell. Shells and body armour in vertebrates are surprisingly rare. Some mammals, like pangolins, have hard scales made of keratin that form a protective barrier, and other vertebrates, like crocodilians, ankylosaurs, and glyptodonts, have osteoderms (bones embedded in the skin) for protection. Turtles, though, take this one step further. Unlike the aforementioned vertebrates, the bony shells of turtles are completely fused to the rest of their skeleton and cover the entire trunk of their body, with the top half of their shell being called the carapace and the bottom half called the plastron. So how and why did turtles evolve such high defence?
First, let’s clear up some terms. Turtles belong to the group Testudinata, which contains two main groups, Cryptodira and Pleurodira. The difference between the groups is the way they retract their heads into their shells. Tortoises, which are exclusively land-based, are a sub-group of Cryptodira. So, tortoises are in fact a type of turtle, not the other way around!
Historically, palaeontologists thought that a group of parareptiles called Pareiasauridae, from the Permian, were the ancestors of turtles because they share many similar traits, such as flat ribs and dermal armour. However, new evidence suggests that this may not be true. Researchers looked at the arrangement of the rib bones of turtle embryos throughout their development and were surprised to find that their ribs widen and fuse together to create the turtle carapace instead of developing osteoderms in the skin first like other vertebrates. This suggests that the ancestors of turtles evolved their shells in a similar way and, therefore, Pareiasauridae are not the ancestors of today’s turtles.
This new hypothesis is supported by evidence from the fossil record. Eunotosaurus africanus is the earliest stem-turtle that has been discovered so far, dating back to the Permian Period, 260 million years ago. These reptiles had evolved broadened, t-shaped ribs that touched against each other, unlike the ribs of most other vertebrates. These adaptations would have helped give Eunotosaurus extra protection, but it lacks the ossification between the ribs that is seen in turtles today.
These broad, flat ribs are seen in stem-turtles throughout the Permian and into the Triassic. Pappochelys, dating back approximately 240 million years ago, features them, but they also feature something else: large, reinforced, paired gastralia – abdominal bones common in reptiles. This suggests that the plastron evolved from the ossification of these gastralia. Indeed, a fully ossified plastron has been observed in Odontochelys, which dates back 220 million years, whilst retaining the broad, flat ribs of the other stem-turtles. In essence, it only had one half of its shell!
The first stem-turtle we know about with a fully ossified set of ribs is Proganochelys, dating back to the Early Triassic, about 214 million years ago. Stem-turtles and true turtles have been observed in the fossil record throughout the Mesozoic and Cenozoic, and have given rise to giants like Archelon, which could be up to four and a half metres long.
But why did turtles evolve a shell like this? The most obvious answer is that it was for defence, making them harder for predators to prey upon and thus boosting their chances of survival. Other hypotheses have been suggested, such as an adaptation for digging or aiding in long periods of walking, but the fossil record of turtles is very poor and quite biased, so it’s hard to know for certain. Hopefully, new discoveries will one day help us better understand the evolution of these amazing creatures.
 Chelonia mydas taken from Australian Institute of Marine Science.
 Eunotosaurus africanus taken from Lyson et al. (2013).
 Odontochelys semitestaca taken from Li et al. (2008).
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