| Utatsusaurus | |
|---|---|
 | |
| Fossil | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Superorder: | †Ichthyopterygia |
| Family: | †Utatsusauridae McGowan & Motani, 2003 |
| Genus: | †Utatsusaurus Shikamaet al., 1978 |
| Type species | |
| †Utatsusaurus hataii Shikamaet al., 1978 | |
Utatsusaurus hataii is the earliest-knownichthyopterygian which lived in theEarly Triassic period (c. 245–250 million years ago). It was nearly 2.5–3 metres (8.2–9.8 ft) long with a slender body.[1][2] The first specimen was found inUtatsu-cho (now part ofMinamisanriku-cho),Miyagi Prefecture,Japan. It is the only described species in the genusUtatsusaurus and the only member of the familyUtatsusauridae.[3] The nameUtatsusaurus was given after the city.[4] The fossils have been found from the Early TriassicOsawa Formation ofMiyagi Prefecture,Japan andBritish Columbia,Canada.[5]
Utatsusaurus is one of the most primitive grades of ichthyosaurs, a basal ichthyosaur.[6][7]
Utatsusaurus was a relatively small ichthyopterygian, measuring 2.5–3 m (8.2–9.8 ft) long and weighing 57.8 kg (127 lb).[1][2] Unlike the more advanced ichthyosaurs,Utatsusaurus has nodorsal fin and has a broadskull. The snout gently tapers, compared to the more rounded one of more derivedichthyopterygians.[8] The postorbital underlaps the elongate posterior process of the postfrontal. This is an evident plesiomorphic condition for ichthyopterygians.[8] 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 thanGrippia, which is a closely relatedichthyosaur.[4] But, after that, it was reported that they were rather bluntly pointed and robust by reexamining theholotype.[9]Utatsusaurus had small fins, with five digits.[6] In addition, those digits have up to five extra finger bones, which is referred to as hyperphalangy.[6][7] The tail had a long low fin, suggesting that the animal swam byundulation, rather than using its paddles and tail.
Utatsusaurus has transitional features between ancestral terrestrialamniotes and the more derivedichthyosaurs. 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 ofUtatsusaurus has the equal length. While all other ichthyosaurs have the longer humerus, terrestrial amniotes have the longer femur. Furthermore, the hindlimb ofUtatsusaurus seems to be larger than the forelimb.[2]They also used phylogenetic analyses and concluded that ichthyosaurs were a member of theDiapsida and the sister group of theSauria.
Utatsusaurus fed on a diet offish.
It has approximately 40 presacral vertebrae which are cylindrical, suggesting that it probably swam with an eel-like motion.[2]
Ryosuke Motani from theUniversity of California, Berkeley, andNachio Minoura andTatsuro Ando fromHokkaido University re-examined the fossils ofUtatsusaurus in 1998 usingcomputer imagery to reverse thedistortion of the original skeleton. They found thatUtatsusaurus was closely related to thelizard-likediapsid reptiles such asPetrolacosaurus, making ichthyopterygians a distant relative tolizards,snakes andcrocodiles. They also usedphylogenetic analyses and concluded that ichthyosaurs were a member of theDiapsida and the sister group of theSauria.Additionally, in 2013, Cuthbertson and colleagues from theUniversity of Calgary,Canada, using phylogenetic analyses, reported thatIchthyopterygia is amonophyletic group andUtatsusaurus andParvinatator are a basal clade.[8]
Utatsusaurus in acladogram after Huang et. al., 2019:[10]
Minamisanriku-cho,Japan is a renowned place which has yielded a number of fossils ofichthyosaurs and the holotype specimen ofUtatsusaurus. A museum (called Gyoryū-kan (魚竜館), literally translating as "a house of fish-dragons") was built to keep and display those fossils, and over sixty thousand people had visited there a year. However, on Friday 11 March 2011, the museum was destroyed during the2011 Tōhoku earthquake and tsunami. At the time of the earthquake, the fossils ofUtatsusaurus were kept at another place, and the majority of the other fossils displayed at the museum were salvaged, but the museum, itself has not yet been restored and reopened.[11]
