On The Origin Of Turtles

One of the specimens of the 240 million year old turtle Pappochelys rosinae

Turtles and tortoises comprise the group of reptiles known as chelonians. Their evolutionary history records a seemingly improbable anatomical change: the movement of the ribs to outside of the body to form a thick, protective shell.

Over the past few decades the fossil record has yielded a chain of organisms linking weird, wide-bodied reptiles to modern day chelonians. Early species were small, but as time progressed certain groups became veritable giants and dominant predators in their respective ecosystems.

Previously the oldest known turtle was Odontochelys, a 220 million year old reptile known from Chinese rocks. Now a new fossil discovery has pushed their evolutionary history back by 20 million years. 18,240 million year old specimens were discovered in a quarry in southern Germany. Unlike Odontochelys, which had a rudimentary shell, this new species displayed just a hint of its biological affiliations. Its ribs were wide, flat and were beginning to fuse along the length. Modern turtles and indeed tortoises form their shells in this way covered in a layer of keratin - the material comprising fingernails, horns and hair.

An artist's impression of the 240 million year old turtle Pappochelys

Yet it was still much closer morphologically to other reptiles than to chelonians, with a long tail and a skull lined with peg-like teeth rather than a beak. The skull itself possessed two openings known as temporal fenestrae. This shows that it was a diapsid reptile. Turtles and tortoises by contrast have no temporal fenestrae and are referred to as anapsids. It also possessed legs rather than fins, showing that it was a true transition form suited to life in water and on land.

The researchers responsible for analysing the fossils ultimately gave it the name of Pappochelys rosinae, meaning grandfather turtle. 'Pappochelys indeed forms a missing link for two reasons. It is far older than all so far known turtles. And its anatomy is more primitive in many features, showing the ancestral condition of various body regions,' said Rainer Schoch from Germany's State Museum of Natural History, Stuttgart.

On a wider note not only does Pappochelys show us the transition from lizard-like ancestor to chelonian, it is also an excellent example of a transition fossil. 'Transitional creatures are the most important contribution that palaeontology can make to the study of evolution. They are often unexpected and show surprising features,' said Schoch. 'They also give evidence on the sequence of evolutionary steps.' The next stage will be to determine the approximate series of genetic changes which will inform us as to how the turtle got its shell.