Researchers of the Institut Català de Paleontologia Miquel Crusafont (ICP), the Museum of Conca Dellà (MCD) and the Universitat Auṭnoma de Barcelona (UAB) published an article describing the discovery of a turtle which lived at the end of the dinosaur era in the online version of the journal Cretaceous Research. Josep Marmi, Àngel Luján and Àngel Galobart from ICP, Rodrigo Gaete from MCD, and Violeta Riera and Oriol Oms from UAB have named the new species Polysternon isonae, in recognition of the municipalities of Isona and Conca Dellà, where the fossil remains of the specimen were found.
The abundance of fossil remains of dinosaurs which lived some 65 to 70 million years ago in the area currently known as the Pyrenees is widely known. The mountain range is home to dozens of sites with skeletal remains, dinosaur tracks and eggs of the last dinosaurs inhabiting the Earth. The Tremp Basin is one of the areas of the Pyrenees with the highest concentration of fossils.
Less known however are all the other organisms making up that particular ecosystem at the end of the Cretaceous Period and consisting of other vertebrates, invertebrates, plants, fungi, etc. A common element found in those ecosystems were turtles. In the sites found in the Pyrenees, a large number of turtle fossils can be found. These in general consist of isolated shell plates or small sets of plates which help form a general idea of the morphology and size of the animal. In contrast, finding a complete turtle shell is rarely frequent and even more exceptional is the discovery of skeletal remains within the shell.
In the past few years, several turtle fossil remains have been discovered in different sites of the municipalities of Isona and Conca Dellà. At one of these sites, the Barranc de Torrebilles, archaeologists have been able to gather several fairly complete shells and this has helped them describe a new species, the Polysternon isonae. The remains found at Barranc de Torrebilles consist of dozens of isolated plates produced by the fragmentation of shell through their sutures, and what is most important: a fragment of the plastron, or bottom part of one shell and another almost fully preserved shell which displays morphological traits of great interest to palaeontologists and which have allowed them to describe this new species. These remains were recovered during two excavation campaigns conducted during the summers of 2008 and 2009.
So far, two species of the genus Polysternon were known: P. provinciale and P. atlanticum (plus a possible third P. mechinorum), distributed only in what is now the south of France and the Iberian Peninsula. They were animals adapted to swimming and living in fresh waters, in the deeper areas of rivers and lakes. Specifically, the shell of the new species P. isonae was oval, measuring about 50 cm long and 40 cm wide. The remains were found preserved in a very hard sandstone strata now exposed in the Barranc de Torrebilles. Just over 65 million years ago, when the animal died, this sandstone was not lithified and instead consisted of fine sand which was washed away by river streams and deposited, along with the remains of other turtles of the Barranc de Torrebilles, at the bottom of one of these rivers.
Unlike other types of turtles, it seems that Polysternon did not survive the end of the Cretaceous Period and became extinct with the dinosaurs. The close proximity of the site Barranc de Torrebilles to the geological level that marks the end of the Cretaceous extinction, indicates that Polysternon isonae was possibly one of the last species of the genus Polysternon.
Image: Reconstruction of the Polysternon isonae turtle by illustrator Oscar Sanisidro.
Reference article: Marmi J, Luján AH, Riera V, Gaete R, Oms O, Galobart A (2012) "The youngest species of Polysternon: a new bothremydid turtle from the uppermost Maastrichtian of the southern Pyrenees". Cretaceous Research, 35: 133-142.