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Origins of turtle shells

By Colin Ricketts - 01 Jun 2013 7:7:48 GMT
Origins of turtle shells

The unique hawksbill, Eretmochelys imbricata, will be a great loss if its critically-endangered status means that we no longer see it at the surface, nesting, or by its favoured coral reefs; Hawksbill sea turtle image; Credit: © Shutterstock

For 200 years, people have puzzled over why some reptiles have the shell that we otherwise know as a carapace. The so-called stem group of turtles was absent from the fossil record, but this has recently been corrected with new data from Euntosaurus africanus. The Permian period in geological time was a long time ago (260 million years ago). The period had reptiles but not a sign of a shell until the Euntosaurus. From those simple shell beginnings, fossil species such as Odontochelys and Proganochelys show a progression of forms that develop the carapace to perfection, similar to that protecting our modern species.

By the middle of the Permian, these turtles diverted from the reptile stem group to create "chelonian" lines of descendants. Molecular studies agree with the time-scale, leaving the authors of this ScienceDirect paper with an accomplished, "life-history of the shell," thanks to the many species of turtle that have lived.

Professor Tyler Lyson of Yale University and the Smithsonian who is lead author of the paper indicates that, "the turtle shell is a complex structure whose initial transformations started in the Permian period. Like other complex structures, the shell evolved over millions of years and was gradually modified into its present-day shape."

Here is the ontogeny of the shell as this paper presents it.

(1) The reptilian ribs broaden in a primitive stem-reptile.

(2) The ribs become broader still and T- shaped, with a coordinating reorganisation of locomotion and respiration muscle.

(3) The spines on top of the spine widen while the belly supports a lower shell (plastron) which also incorporates the collarbone.

(4) The breastbone becomes part of the shell (carapace), which becomes bonier.

Details are on the clear diagrams, even mentioning ancient species on the right! Look up the paper of course for any detail.

turtle evolution

Credit: © ScienceDirect

Above: There are 50 bones in the carapace of a turtle alone, made up of ribs and those vertebrae we mentioned having spines in (3.) Shells in other animals are formed at the surface; turtles form theirs from their endoskeleton. As we mention above, the turtle of course can't us its ribs for breathing. He has to use muscles to achieve inhalation and exhalation, just as we use our diaphragm to help us breathe.

215 mya, turtles all had perfect shells, but till now we couldn't see how they developed from animals with no shell at all. First the plastron on the belly formed; then the small carapace appeared, about 220 mya. Before that, we now have the 260 million year old star of the show, Eunotosaurus africanus. It has the reduced chelonian number of 9 broad ribs, and it can't breathe with them!

Modern sea turtles split off from tortoises, terrapins and freshwater turtles 110 mya. They have kept the large size of the early turtles, but not some of the gigantic heads! Dermochelys, the leatherback is very different and must have split off from the rest quite early. It has achieved the greatest size of current turtle species and also the greatest range, from the Arctic to the Antarctic. This is down to its ability to partially regulate its body temperature. And all this was achieved with the loss of that carefully-tuned bony carapace.

All sea turtles are facing a great problem with marine pollution. Apart from entanglement in all the fishing gear that finds them even at depth, the incredible amount of plastic waste suffocates them, especially in murkier waters where it can resemble jellyfish for those that prey on them. When you consider how long these species have survived, and how lengthy their life-span is, there is a great need to get out there and preserve the paltry total of only 7 living species, all endangered, many critically.

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Topics: Turtles