A New Species Of Ancient Turtle

The fossils of Polysternon isonae

The Pyrenees contain some of the most spectacular mountain ranges in Europe. Vast pillars of snow-capped rock rise up into a blue sky while the bases of the mountains drop down into vast lakes or green forests. However its geological history is just as spectacular. The sedimentary rocks which form the Pyrenees were first layed down during the Palaeozoic era some 500 million years ago. Since that remote point in time, miles of shales, carbonate rocks and sandstones were also layed down up until 100 million years ago.

A reconstruction of the 65 million year old Polysternon isonae turtle

At this point, the accretion of Europe from small volcanic islands squeezed the underwater plateaus of rock in a tight grip, pushing a whole new continent out of the sea and with it the Pyrenees. During the late Cretaceous some 65 to 70 million years ago, the whole area was a vast tidal plain inhabited by a vast range of organisms from invertebrates and fish to dinosaurs and mammals. Common residents of many of the fossil-bearing rocks are turtles.

Their hard shells and scaly skin mean that they are reasonably well represented in the fossil record from the Jurassic onwards, when the reptile family underwent a wave of diversification. Palaeontologists excavating near Barranc de Torrebilles in the district of Isona I Conca Dellà  in the Spanish Pyrenees, in 2008 - 2009, discovered a new species. The extinct turtle genus Polysternon contains three species, which are very similar in shape. The problem with naming new species of turtle is that most fossils are fragments of shell.

This new specimen, while incomplete, showed detail of the ventral (underside) side of the shell which is very useful for indentifying different species of turtle. This creature, while superficially similar to other Polysternon species so different that it prompted the palaeontologists to make it a new species. Named Polysternon isonae, its shell was oval in shape being 50 centimetres long and 40 centimetres wide. So far it is the species closest to the Cretaceous extinction event, possibly making it the last of the Polysternon turtles on Earth.