Utatsusaurus Temporal range: Early Triassic, | |
---|---|
Fossil | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Class: | Reptilia |
Superorder: | † Ichthyopterygia |
Family: | † Utatsusauridae McGowan & Motani, 2003 |
Genus: | † Utatsusaurus Shikama et al., 1978 |
Type species | |
†Utatsusaurus hataii Shikama et al., 1978 |
Utatsusaurus hataii is the earliest-known ichthyopterygian which lived in the Early 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 in Utatsu-cho (now part of Minamisanriku-cho), Miyagi Prefecture, Japan. It is the only described species in the genus Utatsusaurus and the only member of the family Utatsusauridae. [3] The name Utatsusaurus was given after the city. [4] The fossils have been found from the Early Triassic Osawa Formation of Miyagi Prefecture, Japan and British 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 no dorsal fin and has a broad skull. The snout gently tapers, compared to the more rounded one of more derived ichthyopterygians. [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 than Grippia , which is a closely related ichthyosaur. [4] But, after that, it was reported that they were rather bluntly pointed and robust by reexamining the holotype. [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 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. [2] They also used phylogenetic analyses and concluded that ichthyosaurs were a member of the Diapsida and the sister group of the Sauria.
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. [2]
Ryosuke Motani from the University of California, Berkeley, and Nachio Minoura and Tatsuro Ando from Hokkaido University re-examined the fossils of Utatsusaurus in 1998 using computer imagery to reverse the distortion of the original skeleton. They found that Utatsusaurus was closely related to the lizard-like diapsid reptiles such as Petrolacosaurus , making ichthyopterygians a distant relative to lizards, snakes and crocodiles. They also used phylogenetic analyses and concluded that ichthyosaurs were a member of the Diapsida and the sister group of the Sauria.Additionally, in 2013, Cuthbertson and colleagues from the University of Calgary, Canada, using phylogenetic analyses, reported that Ichthyopterygia is a monophyletic group and Utatsusaurus and Parvinatator are a basal clade. [8]
Minamisanriku-cho, Japan is a renowned place which has yielded a number of fossils of ichthyosaurs and the holotype specimen of Utatsusaurus. 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 the 2011 Tōhoku earthquake and tsunami. At the time of the earthquake, the fossils of Utatsusaurus 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. [10]
Ichthyosauria is an order of large extinct marine reptiles sometimes referred to as "ichthyosaurs", although the term is also used for wider clades in which the order resides.
Ichthyopterygia was a designation introduced by Sir Richard Owen in 1840 to designate the Jurassic ichthyosaurs that were known at the time, but the term is now used more often for both true Ichthyosauria and their more primitive early and middle Triassic ancestors.
Shonisaurus is a genus of very large ichthyosaurs. At least 37 incomplete fossil specimens of the marine reptile have been found in the Luning Formation of Nevada, USA. This formation dates to the late Carnian age of the late Triassic period, about 237–227 million years ago.
Mixosaurus is an extinct genus of Middle Triassic ichthyosaur. Its fossils have been found near the Italy–Switzerland border and in South China.
Chaohusaurus is an extinct genus of basal ichthyopterygian, depending on definition possibly ichthyosaur, from the Early Triassic of Chaohu and Yuanan, China.
Grippia is a genus of early ichthyopterygian, an extinct group of reptiles that resembled dolphins. Its only species is Grippia longirostris. It was a relatively small ichthyopterygian, measuring around 1–1.5 metres (3.3–4.9 ft) long. Fossil remains from Svalbard from the specimen SVT 203 were originally assigned to G. longirostris but are now thought to have belonged to a non-ichthyopterygian diapsid related to Helveticosaurus.
Isfjordosaurus is an extinct genus of ichthyopterygian marine reptile that lived during the Early Triassic. Fossils have been found on the island of Spitsbergen, part of the Svalbard archipelago off the northern coast of Norway. It was formally described by Ryosuke Motani in 1999 and contains the species Isfjordosaurus minor.
Parvinatator, from Latin, “parvus” little and “natator” swimmer, is an extinct genus of small ichthyopterygian marine reptile that lived during the Early to Middle Triassic. Its fossils have been found in British Columbia, Canada.
Thaisaurus is an extinct genus of ichthyopterygian marine reptile that lived during the Spathian. Fossils have been found in Thailand.
Toretocnemus is an extinct genus of ichthyosaur. Its remains have been found in California, United States, in Triassic layers of the Carnian Hosselkus Limestone.
The Guanling Formation is a Middle Triassic geologic formation in southwestern China.
Mixosauridae was an early group of ichthyosaurs, living between 247.2 and 235 million years ago, during the Triassic period. Fossils of mixosaurs have been found all over the world: China, Timor, Indonesia, Italy, Germany, Spitsbergen, Switzerland, Svalbard, Canada, Alaska, and Nevada.
Macgowania is an extinct genus of parvipelvian ichthyosaur known from British Columbia of Canada.
Parvipelvia is an extinct clade of euichthyosaur ichthyosaurs that existed from the Late Triassic to the early Late Cretaceous of Asia, Australia, Europe, North America and South America. Named by Ryosuke Motani, in 1999, it contains the basal taxa like Macgowania and Hudsonelpidia. Maisch and Matzke (2000) found in their analysis seven synapomorphies that support Parvipelvia. They also found 10 synapomorphies that support the existence of post-Triassic clade of ichthyosaurs, for which the name Neoichthyosauria was found to be available. Parvipelvians were the only ichthyosaurs to survive the Triassic-Jurassic extinction event.
Merriamosauria is an extinct clade of ichthyosaurs. It was named by Ryosuke Motani in his 1999 analysis of the relationships of ichthyopterygian marine reptiles and was defined in phylogenetic terms as a stem-based taxon including "the last common ancestor of Shastasaurus pacificus and Ichthyosaurus communis, and all of its descendants." The name honours John Campbell Merriam. Based on this definition, Merriamosauria includes most ichthyosaurs except for several Triassic groups such as the clade Mixosauria, the family Cymbospondylidae, and perhaps the family Toretocnemidae. Merriamosaurs are characterized by features in their pectoral girdles and limb bones, including an extensive connection between the scapula and the coracoid bone, the absence of the first metacarpal and the absence of a pisiform bone.
Xinminosaurus is an extinct genus of cymbospondylid ichthyosaur known from the Middle Triassic of Guizhou Province, China.
The Ichthyosauromorpha are an extinct clade of marine reptiles consisting of the Ichthyosauriformes and the Hupehsuchia, living during the Mesozoic.
This timeline of ichthyosaur research is a chronological listing of events in the history of paleontology focused on the ichthyosauromorphs, a group of secondarily aquatic marine reptiles whose later members superficially resembled dolphins, sharks, or swordfish. Scientists have documented ichthyosaur fossils at least as far back as the late 17th century. At that time, a scholar named Edward Lhuyd published a book on British fossils that misattributed some ichthyosaur vertebrae to actual fishes; their true nature was not recognized until the 19th century. In 1811, a boy named Joseph Anning discovered the first ichthyosaur fossils that would come to be scientifically recognized as such. His sister Mary would later find the rest of its skeleton and would go on to become a respected fossil collector and paleontologist in her own right.