''Xinminosaurus'' is an

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

of cymbospondylid

ichthyosaur 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. Ichthyosaurians thrived during much of the Mesozoic era; based on fo ...

known from the

Middle Triassic In the geologic timescale, the Middle Triassic is the second of three epoch (geology), epochs of the Triassic period (geology), period or the middle of three series (stratigraphy), series in which the Triassic system (stratigraphy), system is di ...

(mid-late

Anisian In the geologic timescale, the Anisian is the lower stage (stratigraphy), stage or earliest geologic age, age of the Middle Triassic series (stratigraphy), series or geologic epoch, epoch and lasted from million years ago until million years ag ...

stage) of

Guizhou Province ) , image_skyline = , image_caption = , image_map = Guizhou in China (+all claims hatched).svg , mapsize = 275px , map_alt = Map showing the location of Guizhou Province , map_caption = Map s ...

China China, officially the People's Republic of China (PRC), is a country in East Asia. With population of China, a population exceeding 1.4 billion, it is the list of countries by population (United Nations), second-most populous country after ...

Etymology

The generic name is derived from ''Xinmin'', the district where the fossil was found, and

Greek Greek may refer to: Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group *Greek language, a branch of the Indo-European language family **Proto-Greek language, the assumed last common ancestor of all kno ...

''sauros'', "lizard". 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 ...

is derived from Greek ''kataktes'', "crusher", in reference to one of the taxon's

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 ...

- the presence of bulbous and laterally compressed crushing teeth in

maxilla In vertebrates, the maxilla (: maxillae ) 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 two maxil ...

and posterior

dentary In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla). The jawbone ...

Discovery

''Xinminosaurus'' is known only from the

holotype A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...

GMPKU-P-1071, a nearly complete

skeleton A skeleton is the structural frame that supports the body of most animals. There are several types of skeletons, including the exoskeleton, which is a rigid outer shell that holds up an organism's shape; the endoskeleton, a rigid internal fra ...

deposited in Geological Museum, Peking University. It was collected from the

conodont Conodonts, are an extinct group of marine jawless vertebrates belonging to the class Conodonta (from Ancient Greek κῶνος (''kōnos''), meaning " cone", and ὀδούς (''odoús''), meaning "tooth"). They are primarily known from their hard ...

''Nicoraella kockeli''

biozone In biostratigraphy, biostratigraphic units or biozones are intervals of geological strata that are defined on the basis of their characteristic fossil taxon, taxa, as opposed to a Lithostratigraphy, lithostratigraphic unit which is defined by the ...

(Pelsonian of the Anisian stage), from the upper member of the

Guanling Formation The Guanling Formation is a Middle Triassic (Anisian or Pelsonian in the regional chronostratigraphy) geologic Formation (geology), formation in southwestern China. Geology The formation encompasses two members. The first member is primarily c ...

. It was found near Yangjuan Village of the Xinmin District, Panxian County. Maisch suggested that ''Xinminosaurus'' might be a subjective

junior synonym In taxonomy, the scientific classification of living organisms, a synonym is an alternative scientific name for the accepted scientific name of a taxon. The botanical and zoological codes of nomenclature treat the concept of synonymy differently. ...

of ''

Tholodus ''Tholodus'' is an extinct genus of basal (phylogenetics), basal ichthyopterygian known from the Middle Triassic (mid-late Anisian to late Ladinian stage) of Germany, northeastern Italy and possibly China. It was first named by Christian Erich He ...

''. Although Jiang ''et al.'' regarded ''Tholodus'' to be a possible ''

nomen dubium In binomial nomenclature, a ''nomen dubium'' (Latin for "doubtful name", plural ''nomina dubia'') is a scientific name that is of unknown or doubtful application. Zoology In case of a ''nomen dubium,'' it may be impossible to determine whether a ...

'', Maisch rejected this opinion, stating that ''Tholodus'' is easily recognized and characterized by unequivocal dental autapomorphies, so that even jaw and tooth fragments are diagnostic, and it is thus a valid taxon. Furthermore, he noted that ''Tholodus'' is clearly distinguishable from all other known marine reptiles, except ''Xinminosaurus''. The only difference between the taxa, according to Maisch, is that ''Tholodus'' specimens are on average twice as large as the holotype of ''Xinminosaurus''. Mulder and Jagt (2019) demonstrated that the tooth crowns of the putative

mosasaur Mosasaurs (from Latin ''Mosa'' meaning the 'Meuse', and Ancient Greek, Greek ' meaning 'lizard') are an extinct group of large aquatic reptiles within the family Mosasauridae that lived during the Late Cretaceous. Their first fossil remains wer ...

''"Globidens" timorensis'' from West Timor reveal a striking resemblance to those of the Triassic durophagous ichthyosaur genera ''

'' and ''Xinminosaurus.'' There they tentatively reassign them to the latter genus as ''Xinminosaurus(?) timorensis'', because provenance of the Chinese taxon is geographically closer to the Timor.

Description

The holotype specimen of ''Xinminosaurus'' has a total length of , while its skull measures 29 centimeters long. The skull is poorly preserved, which obscures much of its anatomy. The teeth of ''Xinminosaurus'' lack constrictions. In the

e (back upper tooth-bearing bones) and back parts of the dentaries (lower tooth-bearing bones) they are bulbous but narrow from side to side. The replacement teeth of these bones were not located in the pulp cavities, instead being present as a second row of teeth. The very tips of its jaws, however, may have been toothless, while the teeth at the front of the denataries are cone-shaped.

Postcranial skeleton

''Xinminosaurus'' has a total of 140

vertebrae Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spinal ...

. Of these, about 60 are located in front of the hips, a very high number, similar to some other ichthyosaurs. These vertebrae are followed by 3 sacral (hip) vertebrae and 77 caudal (tail) vertebrae. As in other

ichthyopterygia Ichthyopterygia ("fish flippers") was a designation introduced by Richard Owen, 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 ...

ns, there is a prominent bend in the middle of the tail, where the

neural spine Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spinal ...

s slope forwards. In ''Xinminosaurus'', the tail is bent downwards at a 35° angle, starting at caudal vertebra 38. Each

scapula The scapula (: scapulae or scapulas), also known as the shoulder blade, is the bone that connects the humerus (upper arm bone) with the clavicle (collar bone). Like their connected bones, the scapulae are paired, with each scapula on either side ...

(shoulder blade) is composed of a broad, fan-like blade and a process that forms the shoulder joint, separated by a shaft. The latter of these structures has an outwards-bowed lower end with a notch and extends further forwards than backwards. The coracoids (a pair of shoulder bones) are greater in width than length. ''Xinminosaurus'' has short, wide collarbones. The

interclavicle An interclavicle is a bone which, in most tetrapods, is located between the clavicles. Therian mammals ( marsupials and placentals) are the only tetrapods which never have an interclavicle, although some members of other groups also lack one. In ...

(a shoulder bone between the collarbones) does not have a backwards projection. The long limb bones of ''Xinminosaurus'' are atypical for an ichthyosaur. The

humeri The humerus (; : humeri) 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 ...

(upper arm bones) of ''Xinminosaurus'' bear bladelike projections on their front edges, and Ichthyopterygian characteristic. The middle part of the humeri is not narrower than the ends of the bones. The lower ends of the

ulna The ulna or ulnar bone (: ulnae or ulnas) is a long bone in the forearm stretching from the elbow to the wrist. It is on the same side of the forearm as the little finger, running parallel to the Radius (bone), radius, the forearm's other long ...

e (rear lower arm bones) are very large, curving upwards and extending more than halfway up along the shafts. ''Xinminosaurus'' has three upper wrist bones, which are rectangular in shape. The ulnares (bones below the ulnae) are the largest of these. Distinctive traits in the wrist of this genus include the coalescence of two of the lower wrist bones, distal carpals 3 and 4, and the absence of a bony distal carpal 1. The other distal carpal, distal carpal 2, is roughly elliptical in shape. While narrower, the

metacarpals In human anatomy, the metacarpal bones or metacarpus, also known as the "palm bones", are the appendicular skeleton, appendicular bones that form the intermediate part of the hand between the phalanges (fingers) and the carpal bones (wrist, wris ...

of Xinminosaurus resemble those of the non-ichthyosaurian ichthyopterygian ''

Utatsusaurus ''Utatsusaurus hataii'' is the earliest-known ichthyopterygian which lived in the Early Triassic 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 Minamisa ...

'', with the first being the shortest and the third and fourth being the longest. The first four are shaped like

hourglass An hourglass (or sandglass, sand timer, or sand clock) is a device used to measure the passage of time. It comprises two glass bulbs connected vertically by a narrow neck that allows a regulated flow of a substance (historically sand) from the ...

es while the fifth is shaped like a

kidney In humans, the kidneys are two reddish-brown bean-shaped blood-filtering organ (anatomy), organs that are a multilobar, multipapillary form of mammalian kidneys, usually without signs of external lobulation. They are located on the left and rig ...

. The number of bones in each digit in the foreflipper is 3, 5, 5, 5, and 2 respectively, small counts for an ichthyosaur. The

phalanges The phalanges (: phalanx ) are digit (anatomy), digital bones in the hands and foot, feet of most vertebrates. In primates, the Thumb, thumbs and Hallux, big toes have two phalanges while the other Digit (anatomy), digits have three phalanges. ...

(digit bones) are long and at least the upper ones are hourglass-shaped. The upper and lower hindlimb bones are shorter than those of the forelimb. The upper ends of the

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

e (front lower hindlimb bones) are nearly twice as wide as their lower ends. There are two upper ankle bones in ''Xinminosaurus'', the rear one (

calcaneum In humans and many other primates, the calcaneus (; from the Latin ''calcaneus'' or ''calcaneum'', meaning heel; : calcanei or calcanea) or heel bone is a bone of the tarsus of the foot which constitutes the heel. In some other animals, it is t ...

) larger than the front one (

astragalus Astragalus may refer to: * ''Astragalus'' (plant), a large genus of herbs and small shrubs *Astragalus (bone) The talus (; Latin for ankle or ankle bone; : tali), talus bone, astragalus (), or ankle bone is one of the group of foot bones known ...

) and both being vaguely round. The lower ankle bones resemble the lower wrist bones, with an absent distal tarsal 1 and combined distal tarsals 3 and 4. The

metatarsals The metatarsal bones or metatarsus (: metatarsi) are a group of five long bones in the midfoot, located between the tarsal bones (which form the heel and the ankle) and the phalanges (toes). Lacking individual names, the metatarsal bones are nu ...

and phalanges are similar to the metacarpals and the phalanges in the foreflippers, although the exact number of phalanges in each digit in the hindflippers is unknown.

References

Fossil taxa described in 2008 Late Triassic ichthyosaurs Late Triassic reptiles of Asia Extinct animals of Indonesia Anisian life Anisian genus first appearances Anisian genus extinctions Guanling Formation Ichthyosauromorph genera {{ichthyosaur-stub