Turtles are strange reptiles. They’re familiar – their shelled ranks are readily-recognizable – but we’re really only just starting to understand what they are and how they came to be as they are today.
For a long time the earliest turtles looked, well, just like turtles, not terribly different from their modern counterparts. It’s only been in the last decade that paleontologists and anatomists have started to get a handle on how their carapace-encased forms originated and the order in which the key turtle innovations evolved. Not that everything is settled, of course. For paleo, as everything else, context is key, and researchers are still trying to piece together the environments and lifestyles that led to the origin of turtles.
The two main contenders in this debate are the water and the underground. The key traits that set turtles apart could have evolved in the shallows just as easily as burrows, it seems. This is no small matter. Where and how a prehistoric creature lived is essential for investigating major evolutionary shifts. (If you don’t believe me, just ask early whales or feathered dinosaurs.) So paleontologist Stephan Lautenschlager and colleagues have now looked inside the head of a prototurtle for additional evidence about this critical phase of reptilian history.
The focus of the new study is Proganochelys quenstedti, a 210 million-year-old turtle found in Germany. Until it was recently overshadowed by the likes of Odontochelys, this fully-shelled saurian was often used as a stand-in for what the earliest turtles were like. It takes up that role again now, or at least the void that once housed its brain does.
CT scans of two Proganochelys skulls revealed that this Triassic turtle “retained a simple tube-like brain”, Lautenschlager and coauthors write, indicating that this reptile had “mediocre hearing and vision, but a well-developed olfactory sense.” So Proganochelyswas more likely to smell you than see you. And when compared to other turtles, Proganochelys is an outlier – the shape of its brain isn’t quite like any other turtle whose ecology is well-understood.
The results, Lautenschlager and colleagues acknowledge, don’t resolve the long-running debate about where the earliest turtles evolved. In some analyses, the brain shape of Proganochelys looks like that of digging turtles. In others, it resembles the brain shape of distantly-related marine reptiles. Brain shape alone doesn’t answer the question, and, in fact only highlights the debate. Some of the characteristics related to digging burrows may convergently evolve in reptile lineages that became adapted to water, or traits that evolved in one venue – like large claws for digging – could have been held over into other environments.
As far as Proganochelys goes, Lautenschlager and coauthors conclude, the geological and skeletal details of this Triassic turtle suggest that it was mostly terrestrial but wasn’t digging underground like a gopher tortoise. The brain scan data don’t contradict that view. But the question of where and how the first turtles evolved will only intensify with more analysis and discovery, especially as additional species are excavated from the Triassic. Looked at one way, this could be a story of frustration. There are still far more questions than answers. But turned another way, the same uncertainty represents the fuel for discovery and debate that paleontology thrives on.