New Link in the Chain of Turtle Evolution
Turtles are iconic and unusual animals and for a long time their evolutionary history has been mysterious. Last year a new fossil was recovered which was widely believed to represent a very early turtle ancestor (see How the Turtle Got its Shell). It was called Eunotosaurus and lived around 260 million years ago but this still left a gap of 40 million years between that and the first real ‘turtle’ – Odontochelys from the Triassic. Now a new fossil from 240 million years old deposits in Germany has filled this gap and answered some important questions about the evolution of turtles.
The new species has been named Pappochelys rosinae which means ‘grandfather turtle’ and this immediately shows how important its discoverers believe it to be. Superficially it would have resembled a lizard rather then a modern turtle with a long tail and terrestrially adapted limbs. In fact if you imagine a modern marine iguana then you wouldn’t be far off although Pappochelys was only 20cm long. In fact its discoverers think it may have led a very similar life to these modern diving reptiles. The rocks in which Pappochelys was found are from an ancient lake bed suggesting that the animal spent a great deal of its time near the water’s edge. If so its dense bones many have been used to help it stay under the water whilst feeding; this a common adaptation seen in diving birds.
One particularly significant feature of the new find is its skull. Pappochelys has two small holes in the skull (called temporal fenestra) which are very important because the number of these holes forms the basis for classifying the three major lineages of land-living vertebrate life. Modern turtles don’t have any holes which put them in the ‘anapsid’ lineage but it has long been unclear whether this was because they evolved from anapsid ancestor of whether they secondarily lost these holes and were instead descended from a two-holed (diapsid) ancestor. The diapsid lineage includes all modern lizards and many ancient groups such as the dinosaurs. The discovery that Pappochelys was a diapsid proves that the turtles actually evolved from this lineage rather then the anapsids.
Reference: Schoch, R. R. et al. 2015. A Middle Triassic stem-turtle and the evolution of the turtle body plan. Nature. doi:10.1038/nature14472
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