15 May 2013
Title:A basal thunnosaurian from Iraq reveals disparate phylogenetic origins for Cretaceous ichthyosaurs
Authors: Valentin Fischer, Robert M. Appleby, Darren Naish, Jeff Liston, James B. Riding, Stephen Brindley and Pascal Godefroit
Journal: Biology Letters
A paper published in Biology Letters today reveals a new species of ichthyosaur (a dolphin-like marine reptile from the age of dinosaurs) which revolutionises our understanding of their evolution and extinction.
The results, produced by a collaboration of researchers from universities in Belgium and the UK contradict previous theories that viewed the ichthyosaurs of the Cretaceous period (the span of time between 145 and 66 million years ago) as the last survivors of a group on the decline.
Ichthyosaurs are marine reptiles from the time of the dinosaurs ranging in size from less than one to over 20 metres in length. They were fast-swimming, deep-diving animals with enormous eyeballs.
Until recently, it was thought that ichthyosaurs declined gradually through multiple extinction events during the Jurassic period. These successive events were thought to have killed off all ichthyosaurs except those strongly adapted for fast-swimming life in the open ocean. Due to this pattern, it has been assumed that ichthyosaurs were constantly and rapidly evolving to be ever-faster open-water swimmers.
However, an entirely new ichthyosaur from the Kurdistan region of Iraq substantially alters this view of the group. In the new study researchers name it Malawania anachronus, which means ‘out of time swimmer’. Despite being Cretaceous in age, Malawania represents the last-known member of a kind of ichthyosaur long believed to have gone extinct during the Early Jurassic, more than 66 million years earlier. Remarkably, this kind of archaic ichthyosaur appears characterised by an evolutionary ‘stasis’: they seem not to have changed much between the Early Jurassic and the Cretaceous, a very rare feat in the evolution of marine reptiles.
“Malawania’s discovery is similar to that of the coelacanth in the 1930s: it represents an animal that seems ‘out of time’ for its age. This ‘living fossil’ of its time demonstrates the existence of a lineage that we had never even imagined,” says lead author Dr Valentin Fischer of the University of Liege in Belgium.
The specimen concerned was found during the 1950s by British petroleum geologists. “The fossil – a well-preserved partial skeleton that consists of much of the front half of the animal – wasn’t exactly being treated with the respect it deserves. Preserved within a large, flat slab of rock, it was being used as a stepping stone on a mule track,” says co-author Dr Naish of the University of Southampton. “Luckily, the geologists realized its potential importance and took it back to the UK, where it remains today.”