''Concavispina'' 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 Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

thalattosaur Thalattosauria (Greek for "sea lizards") is an extinct order of prehistoric marine reptiles that lived in the middle to late Triassic period. Thalattosaurs were diverse in size and shape, and are divided into two superfamilies: Askeptosauroidea a ...

reptile from the early

Late Triassic The Late Triassic is the third and final epoch (geology), epoch of the Triassic geologic time scale, Period in the geologic time scale, spanning the time between annum, Ma and Ma (million years ago). It is preceded by the Middle Triassic Epoch ...

(

Carnian The Carnian (less commonly, Karnian) is the lowermost stage of the Upper Triassic Series (or earliest age of the Late Triassic Epoch). It lasted from 237 to 227 million years ago (Ma). The Carnian is preceded by the Ladinian and is followed by t ...

stage)

Xiaowa Formation The Xiaowa Formation is a Carnian-age geological formation found in southern China. It is a sequence of limestone and marls from the Carnian stage of the Triassic. Its lower section was previously known as the Wayao Formation or Wayao Member of th ...

of Guangling,

Guizhou Guizhou (; formerly Kweichow) is a landlocked province in the southwest region of the People's Republic of China. Its capital and largest city is Guiyang, in the center of the province. Guizhou borders the autonomous region of Guangxi to t ...

, southern

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 ...

. It contains a single species, ''Concavispina biseridens''. It is known only from 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 ...

ZMNH M8804, a nearly complete 364 cm long skeleton. ''Concavispina'' can be differentiated from other thalattosaurs by possessing two rows of blunt teeth on the anterior part of the

maxilla The maxilla (plural: ''maxillae'' ) in vertebrates is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The t ...

(upper jaw bone) and a V-shaped notch on the dorsal margin of each

neural spine The spinal column, a defining synapomorphy shared by nearly all vertebrates,Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristic i ...

in the

dorsal Dorsal (from Latin ''dorsum'' ‘back’) may refer to: * Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism * Dorsal, positioned on top of an aircraft's fuselage * Dorsal co ...

(back) vertebrae. Both its

generic Generic or generics may refer to: In business * Generic term, a common name used for a range or class of similar things not protected by trademark * Generic brand, a brand for a product that does not have an associated brand or trademark, other ...

and specific names refer to these

autapomorphies In phylogenetics, an autapomorphy is a distinctive feature, known as a derived trait, that is unique to a given taxon. That is, it is found only in one taxon, but not found in any others or outgroup taxa, not even those most closely related to t ...

(unique characteristics), as ''Concavispina'' means "concave spine" and ''biseridens'' means "two rows of teeth". It is thought to be most closely related to ''

Xinpusaurus ''Xinpusaurus'' is an extinct genus of thalattosaur from the Late Triassic of Guanling in Guizhou, China. Several species have been named since 2000: the type species ''X. suni'' along with the species ''X. bamaolinensis'' and ''X. kohi''. A 201 ...

'', as both taxa share three derived characters: a maxilla that is curved upward at its anterior end, a

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 ...

(upper arm bone) that is wider near the shoulder than near the elbow, and the presence of less than five cervicals (neck vertebrae).

Description

''Concavispina'' has a large skull, short neck, short limbs, and long tail. ZMNH M8804 is long, making ''Concavispina biseridens'' the second longest thalattosaur after '' Miodentosaurus brevis''. It has the largest skull of any thalattosaur. The skull is long and about half the length of the torso. ''Concavispina'' has less than five cervical vertebrae and its

pectoral girdle The shoulder girdle or pectoral girdle is the set of bones in the appendicular skeleton which connects to the arm on each side. In humans it consists of the clavicle and scapula; in those species with three bones in the shoulder, it consists of t ...

is very close to its skull. 114 caudal or tail vertebrae are present, although the tip of the tail is not preserved. The tail makes up most of the length of the body. It is laterally compressed due to the long neural spines and

chevrons Chevron (often relating to V-shaped patterns) may refer to: Science and technology * Chevron (aerospace), sawtooth patterns on some jet engines * Chevron (anatomy), a bone * ''Eulithis testata'', a moth * Chevron (geology), a fold in rock lay ...

that extend from each vertebra. The

radius In classical geometry, a radius ( : radii) of a circle or sphere is any of the line segments from its center to its perimeter, and in more modern usage, it is also their length. The name comes from the latin ''radius'', meaning ray but also the ...

ulna The ulna (''pl''. ulnae or ulnas) is a long bone found in the forearm that stretches from the elbow to the smallest finger, and when in anatomical position, is found on the medial side of the forearm. That is, the ulna is on the same side of t ...

fibula The fibula or calf bone is a leg bone on the lateral side of the tibia, to which it is connected above and below. It is the smaller of the two bones and, in proportion to its length, the most slender of all the long bones. Its upper extremity is ...

and

tibia The tibia (; ), also known as the shinbone or shankbone, is the larger, stronger, and anterior (frontal) of the two bones in the leg below the knee in vertebrates (the other being the fibula, behind and to the outside of the tibia); it connects ...

(the bones which make up the lower forelimb and lower hindlimb, respectively) are very short and very wide compared to the humerus and

femur The femur (; ), or thigh bone, is the proximal bone of the hindlimb in tetrapod vertebrates. The head of the femur articulates with the acetabulum in the pelvic bone forming the hip joint, while the distal part of the femur articulates with ...

(which make up the upper forelimb and upper hindlimb). The

carpal The carpal bones are the eight small bones that make up the wrist (or carpus) that connects the hand to the forearm. The term "carpus" is derived from the Latin carpus and the Greek καρπός (karpós), meaning "wrist". In human anatomy, th ...

(wrist) and tarsal (ankle) bones are weakly developed.

Paleobiology

Like all thalattosaurs, ''Concavispina'' was a

marine reptile Marine reptiles are reptiles which have become secondarily adapted for an aquatic or semiaquatic life in a marine environment. The earliest marine reptile mesosaurus (not to be confused with mosasaurus), arose in the Permian period during the ...

that probably swam by

lateral undulation Undulatory locomotion is the type of motion characterized by wave-like movement patterns that act to propel an animal forward. Examples of this type of gait include crawling in snakes, or swimming in the lamprey. Although this is typically the ...

or side-to-side movement of its elongated body. The limbs are too short to have provided much propulsive power. ''Concavispina'' was probably not as strong a swimmer as its closest relative ''Xinpusaurus'' because it has shorter neural spines with V-shaped notches, which would have reduced the surface area over which the ''M. semispinalis'' muscle (the main muscle along the top of the tail in most reptiles) could anchor. The tail is also less tall in ''Concavispina'' than in ''Xinpusaurus''. Although ''Concavispina'' and ''Xinpusaurus'' both had pointed teeth at the front of the jaw and blunter teeth towards the front, ''Xinpusaurus'''s teeth were larger and more robust in general. This may support the hypothesis that ''Concavispina'' fed on relatively soft-bodied animals such as fish and

jellyfish Jellyfish and sea jellies are the informal common names given to the medusa-phase of certain gelatinous members of the subphylum Medusozoa, a major part of the phylum Cnidaria. Jellyfish are mainly free-swimming marine animals with umbrella- ...

, while ''Xinpusaurus'' may have fed on harder prey with shells. However, ''Concavispina'''s maxillary teeth are wider than they are tall, proportionally similar to teeth of marine predators in the "crushing"

guild A guild ( ) is an association of artisans and merchants who oversee the practice of their craft/trade in a particular area. The earliest types of guild formed as organizations of tradesmen belonging to a professional association. They sometimes ...

(such as ''

Globidens ''Globidens'' ("Globe teeth") is an extinct genus of mosasaur lizard classified as part of the Globidensini tribe in the Mosasaurinae subfamily. ''Globidens alabamaensis'' was the first species of ''Globidens'' described, in a publication b ...

'' and modern

walruses The walrus (''Odobenus rosmarus'') is a large flippered marine mammal with a discontinuous distribution about the North Pole in the Arctic Ocean and subarctic seas of the Northern Hemisphere. The walrus is the only living species in the fami ...

), which specialize in thick-shelled animals like

molluscs Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant taxon, extant species of molluscs are recognized. The number of fossil sp ...

. ''Xinpusaurus'''s teeth are generally taller than wide, similar to "crunching" guild marine predators (such as

thalattosuchians Thalattosuchia is a clade of marine crocodylomorphs from the Early Jurassic to the Early Cretaceous that had a cosmopolitan distribution. They are colloquially referred to as marine crocodiles or sea crocodiles, though they are not members of Cro ...

and some large

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, alt ...

) which preferred armored

crustaceans Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean group ...

and fish but were not too specialized to avoid softer prey. The Xiaowa Formation is a

Lagerstätte A Lagerstätte (, from ''Lager'' 'storage, lair' '' Stätte'' 'place'; plural ''Lagerstätten'') is a sedimentary deposit that exhibits extraordinary fossils with exceptional preservation—sometimes including preserved soft tissues. These for ...

from which hundreds of fossils of thalattosaurs and

crinoid Crinoids are marine animals that make up the class Crinoidea. Crinoids that are attached to the sea bottom by a stalk in their adult form are commonly called sea lilies, while the unstalked forms are called feather stars or comatulids, which are ...

invertebrates. ''Concavispina biseridens'' lived alongside many other species of thalattosaurs, including '' Anshunsaurus huangguoshuensis'', '' Miodentosaurus brevis'', '' Xinpusaurus suni'', '' Xinpusaurus bamaolinensis'', and '' Xinpusaurus kohi'', making the thalattosaur fauna of the Xiaowa Formation the most diverse in the world.

Relationships

phylogenetic 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 ...

analysis of ''Concavispina'' in 2013 produced the following result, with ''Xinpusaurus'' as its closest relative within the

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...

Thalattosauroidea Thalattosauroidea is a superfamily of thalattosaurs, a Triassic group of marine reptiles. It was named in 1904 by paleontologist John Campbell Merriam to include the genus '' Thalattosaurus'' from California. Thalattosauroids are one of two gro ...

References

{{Taxonbar, from=Q5158271 Fossil taxa described in 2013 Late Triassic reptiles of Asia Thalattosaurs Paleontology in Guizhou Prehistoric reptile genera