Creature of the Week #21: Utatsusaurus

Galvasean

Utatsusaurus is widely considered to be the first of the true ichthyosaurs. It lived some 250 million years ago during the Olenekian stage of the Triassic period. It's remains have been found in both Canada and Japan (the name comes from a place in Japan called Utatsugyoryu where it was first discovered). Only one species is known thus far, U. hataii, named in 1978.



A small to medium sized ichthyosaur, Utatsusaurus was under ten feet in lenght. It displays many features that distinguish it from more advanced ichthyosaurs. Perhaps most obviously, it lacks the dolphin like dorsal fin which is associated with later and more famous ichthyosaurs such as Ichthyosaurus and Ophthalmosaurus (of Walking With Dinosaurs fame) which lived in the Jurassic.
While most ichthyosaurs had long thin heads, Utatsusaurus' was fairly broad, tapering ever so slightly towards the front. It also had back paddles which were larger than the front ones. In later ichthyosaurs this is reversed. Curiously, it's paddles consisted of four fingers rather than five, suggesting that Utatsusaurus could have been an offshoot of what would be the main ichthyosaur line.



Utatsusaurus is considered important as it povides clues to the origin of the ichthyosaurs, something which puzzled scientists since the 1800s. There were many theories as to where this group came from. Many a class of land based reptile had been suggested as being ancestral to the ichthyosaurs, while some even guessed that their ancestors never walked on land at all, instead evolving directly from aquatic amphibians. It was not until 1998 when a team of palaeontologists led by Ryosuke Motani and Nachio Minoura re-examined the remains of Utatsusaurus that the mystery was solved. They concluded that Utatsusaurus swam in an undulating manner and had a very flexible, if not particularly fast, method of locomotion.
They also figured out that Utatsusaurus was closely related to the diapsid branch of reptiles (such as Petrolacosaurus as seen in the second episode of Walking With Monsters). This means that ichthyosaurs were distant relatives of the ancestors of both lizards and snakes and very far removed from the other types of sea going reptiles of the Mesozoic.