Utatsusaurus

Utatsusaurus is the earliest-known form of an ichthyopterygian (the group of marine reptiles that includes the ichthyosaurs), which lived in the early Triassic period (c. 245–250 millions years ago).It is nearly 3m (about 9 feet) long with a slender body.[1] The first specimen was found in Utatsu-cho (now part of Minamisanriku-cho), Miyagi Prefecture, Japan. The name Utatsusaurus was given after the city.[2] The fossils have been found from the Lower Triassic of Miyagi Prefecture, Japan and British Columbia, Canada.[3]

Utatsusaurus is one of the most primitive grades of ichthyosaurs, a basal ichthyosaur.

Description
Unlike the more advanced ichthyosaurs, Utatsusaurus has no dorsal fin and has a broad skull. The snout gently tapers, compared to the more rounded one of more derived ichthyopterygians.[6]The postorbital underlaps the elongate posterior process of the postfrontal. This is an evident plesiomorphic condition for ichthyopterygians.[6] For the size of the skull, the teeth are rather small, and arranged in a primitive groove. They have longitudinal grooves and were first thought to be longer and more acute than Grippia, which is a closely related ichthyosaur.[2] But, after that, it was reported that they were rather bluntly pointed and robust by reexamining the holotype.[7] Utatsusaurus had small fins, with five digits.[4] In addition, those digits have up to five extra finger bones, which is referred to as hyperphalangy.[4] [5]The tail had a long low fin, suggesting that the animal swam by undulation, rather than using its paddles and tail.

Utatsusaurus has transitional features between ancestral terrestrial amniotes and the more derived ichthyosaurs. First, the attachment of the pelvic girdle to the vertebral column was probably not robust enough to support the body on land unlike terrestrial amniotes. The pelvic girdle is attached to the vertebral column by the sacral ribs probably articulating with the ilium, but the ribs are not fused to the sacral vertebrae. Second, the humerus and femur of Utatsusaurus has the equal length. While all other ichthyosaurs have the longer humerus, terrestrial amniotes have the longer femur. Furthermore, the hindlimb of Utatsusaurus seems to be larger than the forelimb.[1] They also used phylogenetic analyses and concluded that ichthyosaurs were a member of the Diapsida and the sister group of the Sauria.

Paleobiology
Utatsusaurus fed on a diet of fish.

It has approximately 40 presacral vertebrae which are cylindrical, suggesting that it probably swam with an eel-like motion.