New light is thrown on turtle evolution

Turtles have a bountiful fossil record filled with well-preserved specimens and dating back to around 200 million years ago: a biologist’s dream. Surprisingly, it hasn’t been given much attention.

A study by Joyce et al. published in BMC Evolutionary Biology in October describes a new fossil from China and revisits how modern day turtles came to be distributed around the world.

The fossil turtle Sichuanchelys palatodentata is peculiar in that it retains the ancestral character of teeth on the palate, although the authors doubt this was useful to the species. This species was found to be part of a now-extinct lineage of turtles which is now starting to come to light.

The seven specimens described were unearthed in Northwest China and date from around 160 million years ago, a time when the landscape was dominated by dinosaurs. They include a skull, shell, as well as bones of the legs and arms of the turtles.

Classifying fossils like that of Sichuanchelys p. allows scientists to reconstruct the geographical history of living groups, which tells us why species are where they are today.

Today, turtles occupy a variety of environments: land, freshwater and saltwater. The ability to tolerate saltwater seems to have evolved multiple times independently, and allows turtles to disperse freely across oceans. This makes it difficult to retrace their evolution.

There are two main groups of turtles: the hidden necked and side necked turtles. The former predominantly occupy the southern hemisphere, while the latter are found in the northern. So how did this come to be?

The old hypothesis was that the two groups diverged on the same landmass and each found themselves better adapted in one hemisphere, while the other went locally extinct.

However, by ignoring the saltwater turtles in their analysis, Joyce et al. saw a different pattern emerge. It turns out to be more likely that the two groups were separated during the break-up of Pangaea, and then evolved in isolation from one another. Furthermore, southern hemisphere deserts could have local barriers to freshwater turtles, driving the diversification of the group.

This study reinforced the presence of now-extinct turtle lineages, ignored in the past. It also cats a new light on extinction, diversification and distribution of the modern turtles. In our current biodiversity crisis, a understanding of these topics is more relevant than ever.

Image: Fossil of Sichuanchelys palatodentata shell dorsal and ventral view.

Article: A toothed turtle from the Late Jurassic of China and the global biogeographic history of turtles. Joyce et al. 2016 (DOI 10.1186/s12862-016-0762-5)