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''Chaohusaurus'' is an
extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...
genus of
basal Basal or basilar is a term meaning ''base'', ''bottom'', or ''minimum''. Science * Basal (anatomy), an anatomical term of location for features associated with the base of an organism or structure * Basal (medicine), a minimal level that is nec ...
ichthyopterygian, depending on definition possibly
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 ...
, from the Early Triassic of Chaohu and
Yuanan Yuan'an County () is a county in the west of Hubei province, People's Republic of China. It is under the administration of the prefecture-level city of Yichang. ''Yuanansuchus'', an extinct mastodonsauroid temnospondyl Temnospondyli (fro ...
,
China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's most populous country, with a population exceeding 1.4 billion, slightly ahead of India. China spans the equivalent of five time zones and ...
.


Discovery

The type species ''Chaohusaurus geishanensis'' was named and described by Yang Zhongjian and Dong Zhiming in 1972, based on a fossil found during the construction of a railway. The generic name refers to lake Chao Hu. The
specific name Specific name may refer to: * in Database management systems, a system-assigned name that is unique within a particular database In taxonomy, either of these two meanings, each with its own set of rules: * Specific name (botany), the two-part (bino ...
refers to the Geishan location. The holotype, IVPP V 4001, was uncovered in a layer of the Majianshan Limestone Formation dating from the Anisian. It consists of a partial skeleton, containing the skull and the front torso. In 1985
Chen Lizhu Chen may refer to: People *Chen (surname) (陳 / 陈), a common Chinese surname * Chen (singer) (born 1992), member of the South Korean-Chinese boy band EXO * Chen Chen (born 1989), Chinese-American poet * (), a Hebrew first name or surname: * ...
named two additional species based on fossils found in the same formation: ''Anhuisaurus chaoxianensis'' and ''Anhuisaurus faciles''. However, the generic name had already been preoccupied by the
lizard Lizards are a widespread group of 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 Amphisbaenia alt ...
''
Anhuisaurus ''Chaohusaurus'' is an extinct genus of basal ichthyopterygian, depending on definition possibly ichthyosaur, from the Early Triassic of Chaohu and Yuanan, China. Discovery The type species ''Chaohusaurus geishanensis'' was named and descri ...
'' Hou 1974. Therefore, ''Anhuisaurus'' Chen 1985 was in 1991 renamed into ''Chensaurus'' by
Jean-Michel Mazin Jean-Michel is a French masculine given name. It may refer to : * Jean-Michel Arnold, General Secretary of the Cinémathèque Française * Jean-Michel Atlan (1913–1960), French artist * Jean-Michel Aulas (born 1949), French businessman * Jean-Mic ...
e.a. In 1998, Ryosuke Motani and Hailu You established that the ''Chensaurus'' fossils represented remains of juveniles, with those of ''C. faciles'' being the youngest, and that these formed a growth series with ''Chaohusaurus''. This implied that the known material of ''Chaohusaurus'' was increased with the specimens AGM P45-H85-25, the holotype of ''Chensaurus chaoxianensis''; AGM P45-H85-25, the holotype of ''Chensaurus faciles''; and the front flippers IVPP V 11361 and IVPP V 11362. In 2001 a detailed description of the
osteology Osteology () is the scientific study of bones, practised by osteologists. A subdiscipline of anatomy, anthropology, and paleontology, osteology is the detailed study of the structure of bones, skeletal elements, teeth, microbone morphology, funct ...
of ''Chaohusaurus'' was published by
Michael Maisch Michael may refer to: People * Michael (given name), a given name * Michael (surname), including a list of people with the surname Michael Given name "Michael" * Michael (archangel), ''first'' of God's archangels in the Jewish, Christian and ...
. In 2014, three additional specimens were reported: AGM I-1, AGM CHS-5, and AGM CH-628-22. AGM I-1 also contains the remains of three embryo's. The new finds are part of recent discoveries by a Chinese-Italian paleontological project of eighty ''Chaohusaurus'' fossils. In 2013, a second species was named and described by Chen Xiaohong e.a.: ''Chaohusaurus zhangjiawanensis''. The specific name refers to its provenance near the village of Zhangjiawan. The holotype, WHGMR V26001, was uncovered in a layer of the Jialingjang Formation dating from the
Spathian In the geologic timescale, the Olenekian is an age in the Early Triassic epoch; in chronostratigraphy, it is a stage in the Lower Triassic series. It spans the time between Ma and Ma (million years ago). The Olenekian is sometimes divided int ...
. It consists of a relatively complete skeleton with skull. A second skeleton lacking the skull, WHGMR V26002, was referred. In 2015, Ryosuke Motani and his colleagues reported a large adult forelimb specimen that resembles the immature specimens of ''C. chaoxianensis'', and argued that ''C. chaoxianensis'' is not the juvenile of ''C. geishanensis'' and is indeed a valid species within the genus ''Chaohusaurus''.


Description

''Chaohusaurus'' is a basal ichthyopterygian. It thus shows traits that are typical for the direct ancestors of ichthyosaurs. As a result, basal ichthyosaurs like ''
Cymbospondylus ''Cymbospondylus'' (a Greek word meaning "boat vertebrae") was a basal early ichthyosaur that lived between the early and middle Triassic period (249-237 million years ago). Previously, the genus was classified as a shastasaurid, but more rec ...
'' and '' Mixosaurus'' are closer in build to ''Chaohusaurus'' while later genera like '' Ichthyosaurus'' have a more derived morphology. ''Chaohusaurus'' did not have the dolphin-like form of later ichthyosaurs; it had a more lizard-like appearance with an elongated body. These proportions were not caused by a large number of vertebrae, the total of presacrals being about forty, but by an elongated build of each individual vertebra. The head is short, in adults having about the third the length of the trunk, with a narrow pointed beak and large eye-sockets. Its teeth are conspicuously
heterodont In anatomy, a heterodont (from Greek, meaning 'different teeth') is an animal which possesses more than a single tooth morphology. In vertebrates, heterodont pertains to animals where teeth are differentiated into different forms. For example, ...
, differing in shape: those of the upper jaws are pointed though blunt; those of the rear lower jaws are bulbous with a convex profile, a possible adaptation to a durophagous diet, crushing
shellfish Shellfish is a colloquial and fisheries term for exoskeleton-bearing aquatic invertebrates used as food, including various species of molluscs, crustaceans, and echinoderms. Although most kinds of shellfish are harvested from saltwater envir ...
. The neck is relatively long. ''Chaohusaurus'' did have flippers, rather than webbed feet. The tail fin is wide-based in side-view and short. ''Chaohusaurus'' is one of the smallest known ichthyopterygians, measuring about long and weighing . Being a basal ichthyopterygian, ''Chaohusaurus'' provides important information about the early evolution of the group. It shows that some typical ichthyosaurian traits were probably already part of the original ichthyoptergyian '' Bauplan''. It furthermore has a mix of basal traits that were subsequently lost, early derived traits indicating the genus is not the basalmost ichthyopterygian known and some autapomorphies of its own. An original ichthyopterygian trait is the fact that the skull roof is short and wide. The basisphenoid at the rear lower end of the braincase is not fused with the basioccipital, the lower bone of the rear skull. The parasphenoid at the front of the braincase has a long cultriform process. There is no separate ectopterygoid present. The lower jaw has a retroarticular process at its rear. A conspicuous basal trait is the shortness of the head; in later forms it would be longer relative to the trunk. Also the snout is relatively short, only about twice as long as the part behind the eye-socket. The nasals do not reach further backwards than the eye-sockets. The suture between the
nasal bone The nasal bones are two small oblong bones, varying in size and form in different individuals; they are placed side by side at the middle and upper part of the face and by their junction, form the bridge of the upper one third of the nose. Eac ...
and the frontal is transversely oriented. From the prefrontal a flange overhangs the upper front edge of the eye-socket, perhaps to protect the eyes. The frontal is part of the top edge of the eye-socket and thus lacks a lateral buttress. The frontal is about as large as the parietal. In top view the rear edge of the skull roof is notched. The parietal eye (or at least the ''foramen parietale'') is located between the parietals, not (partly) between the frontals as with later forms. The squamosal is a large element, as large as the quadratojugal with which it is firmly fused. The basisphenoid is narrow. The cultriform process of the parasphenoid at the rear gradually merges with the main body, not via a narrow waist as with later forms. The palate was not firmly attached to the basipterygoid, allowing the snout some vertical movement relative to the remainder of the skull. In most ichthyosaurs no such movement was possible. The opening between the pterygoids was narrow and slit-like, not wide. The pterygoids do not cover the rear underside of the braincase. The neck is relatively long. The tail is elongated, about as long as the head, neck and trunk combined. The vertebrae of the tail base are elongated also, about as long as they are tall. The longest tail spines are located rather to the front indicating that a possible tail fin must have been more horizontal than with later forms. The clavicles have a wide inner flange. The scapula is short, wider than long. The
humerus The humerus (; ) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius and ulna, and consists of three sections. The humeral upper extremity consists of a roun ...
still has a distinct head but not yet a dorsal trochanter. At its lower end, the humerus has a larger facet contacting the radius than contacting the ulna. The upper end of the ulna is narrower than the lower end, not equal in size to it. In general the bones of the lower arm, including the hand bones, are rather elongated, not transformed into discs. The ulna and radius still have a shaft as with land animals. In the wrist the pisiform is about as large as the ulnar carpal, not much smaller or absent. The fifth metacarpal has a convex rear edge and is longer than the fourth distal carpal. There are still five fingers present, without a reduction of the first finger. Polydactyly, an extra number of fingers, is thus lacking.
Hyperphalangy The phalanges (singular: ''phalanx'' ) are digital bones in the hands and feet of most vertebrates. In primates, the thumbs and big toes have two phalanges while the other digits have three phalanges. The phalanges are classed as long bones. ...
, supernumerary phalanges, is likewise absent. The phalanx formula is 2-3-4-4-2. The upper phalanges are relatively elongated, longer than wide. The lower phalanges still show ossification below the cartilage perichondrium and have notches in their edges. The pubic bone is perforated by a ''
foramen obturatum The obturator foramen (Latin foramen obturatum) is the large opening created by the ischium and pubis bones of the pelvis through which nerves and blood vessels pass. Structure It is bounded by a thin, uneven margin, to which a strong membrane ...
'', which closed in side view and located some distance from the rear edge of the body. The hindfin is about as large as the forefin, not smaller. In the thighbone, the facet contacting the tibia is as large and as far reaching downwards as the facet contacting the fibula. Between the tibia and fibula still a space is present, and both bones, though flattened are relatively elongated with a clear shaft. The same is true for the metatarsals, that are cylinder-shaped. There are still five toes present, again rather elongated and hourglass-shaped. An early derived trait is the relative shortness of the spines of the tail. The humerus has a flange at the front edge but it is not secondarily reduced. The flange has a small notch. The fifth metatarsal is shortened and the first is even shorter. The fifth toe is shorter than the first toe. In ''Chaohusaurus'' the width of basisphenoid is about 63% of its length. ''Chaohusaurus'' has a combination of more pointed and bulbous teeth, that was probably separately evolved. The second species, ''Chaohusaurus zhangjiawanensis'', has some distinguishing traits. The skull is rather flat. The trunk vertebrae have well-developed transversal processes. The prefrontal touches the postfrontal, excluding the frontal from the rim of the eye-socket, a derived trait. The calcaneum is larger. The first sacral rib has an expanded outer end. The second sacral rib has a tapered end.


Phylogeny

''Chaohusaurus'' was in the original description of 1972 assigned to the
Omphalosauridae Omphalosauridae is an extinct family of Ichthyopterygians known from the Early to Late Triassic of Europe, North America, and Asia Asia (, ) is one of the world's most notable geographical regions, which is either considered a contine ...
. Modern researchers either abstain from placing it in a family, or assign it to the
Grippiidae 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 ...
. ''Chaohusaurus'' is a possible sister species of ''
Grippia ''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 long and weighing . Fossil remains from ...
'' in the
Grippidia 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 primiti ...
. The exact placement of ''Chaohusaurus'' depends on a choice of definitions. According to the phylogeny by Motani it is a basal ichthyopterygian but not an ichthyosaur: According to the definitions and analyses by Maisch & Matzke however, it is a basal ichthyosaur: A cladistic analysis published with the description of ''Chaohusaurus zhangjiawanensis'' did not find this taxon to be the sister species of ''Chaohusaurus geishanensis'' which was more closely related to ''Grippia''. Even though this would make the genus
paraphyletic In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...
, ''C. zhangjiawanensis'' was nevertheless placed in ''Chaohusaurus'' because of the morphological similarity to the type species.


Reproduction

One specimen among those reported in 2014, AGM I-1, also contained the remains of two
embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
s and one neonate. ''Chaohusaurus'' thus birthed its young viviparously in the water, like later ichthyosaurs. However, from the orientation of both the embryos inside the maternal body and the neonate that had already been giving birth to, it was clear that the young exited the birth canal head-first. This differs from the method used by most extant marine viviparous Amniota, which expel the young tail-first to prevent them from suffocating. Many younger ichthyosaur specimens had earlier been found showing embryos in both positions, leaving it undecided which was the normal one. Motani ''et alii'' (2014) concluded that, because ''Chaohusaurus'' is a very basal form, this provided strong evidence that, at least originally, ichthyopterygian young were born with the head first. This early method might have later been changed because it resulted in a too high mortality. They also cited this as evidence for a terrestrial evolution of viviparity in the land-dwelling ancestors of ichthyosaurs. AGM I-1 in 2014 represented the oldest reptile viviparous birth known.


See also

*
List of ichthyosaurs This list of ichthyosaurs is a comprehensive listing of all genera that have ever been included in the order Ichthyosauria or the parent clade Ichthyopterygia, excluding purely vernacular terms. The list includes all commonly accepted genera, but a ...
*
Timeline of ichthyosaur research 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, shar ...


References

{{Taxonbar, from=Q2715522 Fossil taxa described in 1972 Fossil taxa described in 2013 Early Triassic reptiles of Asia Triassic ichthyopterygians Taxa named by Dong Zhiming Paleontology in Hubei Anisian life Ichthyosauromorph genera