''Utatsusaurus hataii'' is the earliest-known

ichthyopterygia Ichthyopterygia ("fish flippers") 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 primitiv ...

n which lived in the

Early Triassic The Early Triassic is the first of three epochs of the Triassic Period of the geologic timescale. It spans the time between Ma and Ma (million years ago). Rocks from this epoch are collectively known as the Lower Triassic Series, which is a u ...

period (c. 245–250 million years ago). It was nearly long with a slender body. The first specimen was found in Utatsu-cho (now part of Minamisanriku-cho),

Miyagi Prefecture is a prefecture of Japan located in the Tōhoku region of Honshu. Miyagi Prefecture has a population of 2,305,596 (1 June 2019) and has a geographic area of . Miyagi Prefecture borders Iwate Prefecture to the north, Akita Prefecture to the nor ...

Japan Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...

. It is the only described species in the genus ''Utatsusaurus'' and the only member of the family Utatsusauridae. The name ''Utatsusaurus'' was given after the city. The fossils have been found from the Early Triassic Osawa Formation of

and British Columbia, Canada. ''Utatsusaurus'' is one of the most primitive grades of ichthyosaurs, a basal ichthyosaur.

Description

''Utatsusaurus'' was a relatively small ichthyopterygian, measuring long and weighing . Unlike the more advanced ichthyosaurs, ''Utatsusaurus'' has no

dorsal fin A dorsal fin is a fin located on the back of most marine and freshwater vertebrates within various taxa of the animal kingdom. Many species of animals possessing dorsal fins are not particularly closely related to each other, though through co ...

and has a broad

skull The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, th ...

. The snout gently tapers, compared to the more rounded one of more derived

ns. The postorbital underlaps the elongate posterior process of the postfrontal. This is an evident plesiomorphic condition for ichthyopterygians. 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 Ichthyosaurs (Ancient Greek for "fish lizard" – and ) are large extinct marine reptiles. Ichthyosaurs belong to the order known as Ichthyosauria or Ichthyopterygia ('fish flippers' – a designation introduced by Sir Richard Owen in 1842, altho ...

. But, after that, it was reported that they were rather bluntly pointed and robust by reexamining the

holotype A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of several ...

. ''Utatsusaurus'' had small fins, with five digits. In addition, those digits have up to five extra finger bones, which is referred to as hyperphalangy. The tail had a long low fin, suggesting that the animal swam by

undulation Undulation may refer to: * Lateral undulation, the most primitive of vertebrate locomotor patterns * Undulation of the geoid, the separation between the geoid and the reference ellipsoid of the Earth * Undulation point, a point on a curve where ...

, rather than using its paddles and tail. ''Utatsusaurus'' has transitional features between ancestral terrestrial

amniotes Amniotes are a clade of tetrapod vertebrates that comprises sauropsids (including all reptiles and birds, and extinct parareptiles and non-avian dinosaurs) and synapsids (including pelycosaurs and therapsids such as mammals). They are distingu ...

and the more derived

ichthyosaurs Ichthyosaurs (Ancient Greek for "fish lizard" – and ) are large extinct marine reptiles. Ichthyosaurs belong to the order known as Ichthyosauria or Ichthyopterygia ('fish flippers' – a designation introduced by Sir Richard Owen in 1842, altho ...

. 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. They also used phylogenetic analyses and concluded that ichthyosaurs were a member of the

Diapsida Diapsids ("two arches") are a clade of sauropsids, distinguished from more primitive eureptiles by the presence of two holes, known as temporal fenestrae, in each side of their skulls. The group first appeared about three hundred million years ag ...

and the sister group of the

Sauria Sauria is the clade containing the most recent common ancestor of archosaurs (such as crocodilians, dinosaurs, etc.) and lepidosaurs (lizards and kin), and all its descendants. Since most molecular phylogenies recover turtles as more closely r ...

Paleobiology

''Utatsusaurus'' fed on a diet of

fish Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% of li ...

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

Classification

Ryosuke Motani from the

University of California, Berkeley The University of California, Berkeley (UC Berkeley, Berkeley, Cal, or California) is a public land-grant research university in Berkeley, California. Established in 1868 as the University of California, it is the state's first land-grant univ ...

, and Nachio Minoura and Tatsuro Ando from

Hokkaido University , or , is a Japanese national university in Sapporo, Hokkaido. It was the fifth Imperial University in Japan, which were established to be the nation's finest institutions of higher education or research. Hokkaido University is considered ...

re-examined the fossils of ''Utatsusaurus'' in 1998 using computer imagery to reverse the

distortion In signal processing, distortion is the alteration of the original shape (or other characteristic) of a signal. In communications and electronics it means the alteration of the waveform of an information-bearing signal, such as an audio signal ...

of the original skeleton. They found that ''Utatsusaurus'' was closely related to the

lizard Lizards are a widespread group of Squamata, squamate reptiles, with over 7,000 species, ranging across all continents except Antarctica, as well as most oceanic island chains. The group is paraphyletic since it excludes the snakes and Amphisbae ...

-like

diapsid Diapsids ("two arches") are a clade of sauropsids, distinguished from more primitive eureptiles by the presence of two holes, known as temporal fenestrae, in each side of their skulls. The group first appeared about three hundred million years ag ...

reptiles such as ''

Petrolacosaurus ''Petrolacosaurus'' ("rock lake lizard") is an extinct genus of diapsid reptile from the late Carboniferous period. It was a small, long reptile, and the earliest known reptile with two temporal fenestrae (holes at the rear part of the skull). T ...

'', making ichthyopterygians a distant relative to

snake Snakes are elongated, limbless, carnivorous reptiles of the suborder Serpentes . Like all other squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales. Many species of snakes have skulls with several more joints ...

s and

crocodile Crocodiles (family Crocodylidae) or true crocodiles are large semiaquatic reptiles that live throughout the tropics in Africa, Asia, the Americas and Australia. The term crocodile is sometimes used even more loosely to include all extant mem ...

s. They also used

phylogenetic analyses In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...

and concluded that ichthyosaurs were a member of the

and the sister group of the

.Additionally, in 2013, Cuthbertson and colleagues from the University of Calgary, Canada, using phylogenetic analyses, reported that

Ichthyopterygia Ichthyopterygia ("fish flippers") 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 primitiv ...

is a

monophyletic In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic gro ...

group and ''Utatsusaurus'' and Parvinatator are a basal clade.

Destruction of the Gyoryū-kan

Minamisanriku-cho,

is a renowned place which has yielded a number of fossils of

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 The occurred at 14:46 JST (05:46 UTC) on 11 March. The magnitude 9.0–9.1 (M) undersea megathrust earthquake had an epicenter in the Pacific Ocean, east of the Oshika Peninsula of the Tōhoku region, and lasted approximately six minut ...

. 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.

References

Sources