How did the turtle get its shell?

Turtles are a beautiful mystery in the animal kingdom. Anyone who has ever been lucky enough to see one swimming in the wild will know just how much of a heart-stopping sight they are to behold.

For scientists, one of the great and wonderful questions to ask is also one of the simplest: How did turtles get their shells? One of the most remarkable features about these graceful weirdos, we actually know very little about the evolutionary origins.

It has long been thought that the tough, bony shells of turtles and their ancestors are formed from their ribs. Becoming heavily modified over millions of years, they joined together to form the tough, protective armour that we know and love so much about modern turtles. This is distinct from say, crocodiles, where their bony carapace is formed from growths within their skin known as osteoderms. In turtles, the ribs and vertebrae grow into the outer skin layers and connect together like tectonic plates, forming a solid shell. This means that in turtles, unlike crocodiles, the outer shell is actually integrated into the rest of the skeleton.

But just how did this strange evolutionary feature come about? How did it start? To answer this, we have to consult that ever-giving encyclopaedia of the history of life, the fossil record.

The fossil record of turtles is quite incredible, covering dozens of now extinct species from a number of different. They have a history of repeatedly adapting to live on land or out at sea. The earliest known turtles stretch back in time to the Triassic period, around 240 million years ago, and perhaps even further.

A new study has now tried to shed some light on this evolutionary conundrum. Researchers looked at three of the oldest species known from the Triassic, Pappochelys, Eorhynchochelys and Odontochelys, to try and see if they could trace the early evolution of the first shelled-turtles. For this, they used a technique called paleohistology, which is the study of the internal microstructure of bones. Histology works by carefully creating thin slices of bone and then looking at the texture and features under a microscope.

In particular, these three species are important as together they illustrate the earliest evolutionary stages of the development of the plastron. The carapaces for each species were not complete yet, but have different arrangements of their ribs and gastralia (underside body armour) that show different stages in the evolutionary sequence.

What the researchers found were two main things. In Pappochelys, the bone microstructure in the ribs was very close to the ‘turtle condition’, found in Odontochelys. This includes the presence of strain-resisting fibres within the bone structure itself, an important step in the evolution of the turtle shell. They may have been interconnected into a form of ‘protocarapace’.

The researchers also found information on how the animals might have lived. It seems that Pappochelys was most likely amphibious, and might have even dug burrows – a ‘fossorial’ mode of life. Its bone microstructure was distinct from any other aquatic reptiles, ruling out a free-swimming lifestyle. This is consistent with the fact that fossils of Pappochelys were found in rocks interpreted as being formed in a small, freshwater lake. Of course, having a sort of ‘proto-shell’ might have been useful in defense against some of the fearsome predators around during the Triassic. Furthermore, Pappochelys had short, robust arms, with long claws – similar to an armadillo – that would have been useful for digging.

However, not everyone agrees the evidence is so strong. Kai Casper, a PhD student at Duisburg-Essen University in Germany said on Twitter “I agree with the general conclusions but the histological evidence is actually rather weak. Comparisons with modern fossorial reptiles could be informative.”

However, it seems that the early stages of turtle shell evolution took place in a terrestrial environment. This means that it might have been a combination of environmental setting, as well as physical protection, that provided evolutionary pressure on the evolution of the turtle shell. The shell might have been critical in helping to provide support for the rest of the turtle body while digging, but ultimately have been secondarily useful in providing protection too.

Reference

Schoch, R. R., Klein, N., Scheyer, T. M., & Sues, H. D. (2019). Microanatomy of the stem-turtle Pappochelys rosinae indicates a predominantly fossorial mode of life and clarifies early steps in the evolution of the shell. Scientific reports, 9(1), 10430. https://doi.org/10.1038/s41598-019-46762-z