''Xinpusaurus'' is an extinct

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

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 an ...

from the

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

Guanling Guanling Buyei and Miao Autonomous County (; Bouyei: ) is an autonomous county in Anshun City, in the southwest of Guizhou Province, China. Area: Population: 334,900 in 2008. Postal Code: 561300. Telephone Area Code: 0853 The county govern ...

in Guizhou, China. Several species have been named since 2000: the

type species In zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the species that contains the biological type specime ...

''X. suni'' along with the species ''X. bamaolinensis'' and ''X. kohi''. A 2013 study proposed that all three species are synonymous with each other, in which case ''X. suni'' would be the only valid species, although a 2014 study argued that ''X. kohi'' was also valid. A fourth species, ''X. xingyiensis'', was described in 2016.

Description

''Xinpusaurus'' is a thalattosaur, a group of triassic marine reptiles with long, paddle-like tails and short legs with independently movable digits. Specifically, it is a member of the group thalattosauroidea, which are characterized by their downturned

premaxilla The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammal has ...

e. ''Xinpusaurus'' had a short neck, a massive quadrate, and one of the few braincases preserved in thalattosaurs. The lower jaws of this genus show two different forms of

dentary In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable bone ...

surangular The suprangular or surangular is a jaw bone found in most land vertebrates, except mammals. Usually in the back of the jaw, on the upper edge, it is connected to all other jaw bones: dentary, angular, splenial and articular. It is often a mu ...

sutures, either a v-shaped suture with the surangular cutting into the dentary from the side (type 1) or an oblique suture with the surangular underlying the dentary (type 2). ''X. bamaolinensis'' preserves a type 2 suture while ''X. kohi'' has a type 1 suture. ''X. suni'''s suture differs between specimens while ''X. xingyiensis'' seems to have a type 2 suture. The dentary is also narrow and shorter than the snout, with robust, pointed teeth in the front of its jaws and broad crushing teeth in the back of its jaws, suggesting that it fed on hard-shelled prey, although these adaptations are not as extreme as those of '' Concavispina'', which had exclusively short and blunt teeth. This genus also has a uniquely upward-curving

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. T ...

, a wide proximal end of the humerus, and a

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 ...

with a convex lateral edge and concave medial edge. ''X. bamaolinensis'' and ''X. kohi'' had particularly elongated premaxillae, forming a narrow and pointed

rostrum Rostrum may refer to: * Any kind of a platform for a speaker: **dais **pulpit * Rostrum (anatomy), a beak, or anatomical structure resembling a beak, as in the mouthparts of many sucking insects * Rostrum (ship), a form of bow on naval ships * Ros ...

. However, the specimens with the longest snouts are also the smallest in other proportions, so this may be a juvenile trait and cannot be used to distinguish these species from ''X. suni''. ''X. xingyiensis'' is the largest species known and is also probably the least advanced, as it does not have a posterior process of the jugal unlike the other species and shares some similarities with the genus ''Concavispina''.

History

''Xinpusaurus suni'' was first described by Yin et al. in 2000 based on four specimens: Gmr 010, 011, 012, and 013, with only Gmr 010 (the holotype) receiving an informal description. Other specimens of this species have been found from 2001 to 2002 including GGSr 001 (a well-preserved skull and partial skeleton) IVPP V 11860 (an isolated skull), IVPP V 12673 (a partial skeleton), and IVPP V 14372 (an isolated skull). IVPP V 12673 is the only well described specimen. ''Xinpusaurus bamaolinensis'' was described in 2003 from SPCV 30015, a complete skeleton with a well-preserved skull and jaws found in two isolated slabs. These slabs may not represent the same individual, in which case only the front slab can be safely assigned to the genus. Inconsistencies within its description (such as reporting a

quadratojugal The quadratojugal is a skull bone present in many vertebrates, including some living reptiles and amphibians. Anatomy and function In animals with a quadratojugal bone, it is typically found connected to the jugal (cheek) bone from the front and ...

, a feature unknown in thalattosaurs) have led to some confusion over its diagnosis, and a redescription would be necessary to resolve these issues and determine if it is a valid species or not. ''Xinpusaurus kohi'' was described in 2004 from GMPKU 2000/05, an incomplete skeleton. This specimen was also found in two slabs, with the rear slab missing most of the hindlimbs and possibly not belonging to the same individual as the first. In addition, there is evidence that some of the bone structure near the break has been tampered with and that most of the pelvic material has been lost. A 2013 analysis showed that many of the traits initially believed to differentiate ''X. kohi'' from ''X. suni'' were inaccurate, either due to incomplete data, misidentification, or poor preservation. However, a redescription in 2014 resurrected the legitimacy of the species. This redescription provided additional diagnostic features and argued that the alterations were relatively minimal and that the posterior slab was almost certainly from the same individual as the anterior one. ''Xinpusaurus xingyiensis'' was described in 2016 from XNGM WS-53-R3, a nearly complete articulated skeleton from the Zhuganpo member of the Falang formation.

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 thalattosaurs published in 2013 found '' Concavispina'' to be the closest relative of ''Xinpusaurus''. Below is a

cladogram A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to ...

based on that analysis:

References

Thalattosaurs Late Triassic reptiles of Asia Prehistoric reptile genera {{Triassic-reptile-stub