''Xinminosaurus'' 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 ...
of
cymbospondylid 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 ...
known from the
Middle Triassic
In the geologic timescale, the Middle Triassic is the second of three epochs of the Triassic period or the middle of three series in which the Triassic system is divided in chronostratigraphy. The Middle Triassic spans the time between Ma and ...
(mid-late
Anisian
In the geologic timescale, the Anisian is the lower stage or earliest age of the Middle Triassic series or epoch and lasted from million years ago until million years ago. The Anisian Age succeeds the Olenekian Age (part of the Lower Triassic Ep ...
stage) of
Guizhou Province
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 the ...
,
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 ...
.
Etymology
The
generic name is derived from ''Xinmin'', the district where the fossil was found, and
Greek
Greek may refer to:
Greece
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 ...
''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 t ...
- the presence of bulbous and laterally compressed crushing teeth in
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 ...
and posterior
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 tooth, teeth in place. The mandible sits beneath the maxilla. It is the only movabl ...
.
Discovery
''Xinminosaurus'' 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 ...
GMPKU-P-1071, a nearly complete
skeleton
A skeleton is the structural frame that supports the body of an animal. There are several types of skeletons, including the exoskeleton, which is the stable outer shell of an organism, the endoskeleton, which forms the support structure inside ...
deposited in Geological Museum, Peking University. It was collected from the
conodont
Conodonts (Greek ''kōnos'', "cone", + ''odont'', "tooth") are an extinct group of agnathan (jawless) vertebrates resembling eels, classified in the class Conodonta. For many years, they were known only from their tooth-like oral elements, which ...
''Nicoraella kockeli''
biozone
In biostratigraphy, biostratigraphic units or biozones are intervals of geological strata that are defined on the basis of their characteristic fossil taxa, as opposed to a lithostratigraphic unit which is defined by the lithological properties of ...
(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 in southwestern China.
Description
The formation encompasses two members. The first member is primarily calcareous mudston ...
. It was found near Yangjuan Village of the Xinmin District,
Panxian County.
Maisch suggested that ''Xinminosaurus'' might be a subjective
junior synonym
The Botanical and Zoological Codes of nomenclature treat the concept of synonymy differently.
* In botanical nomenclature, a synonym is a scientific name that applies to a taxon that (now) goes by a different scientific name. For example, Linna ...
of ''
Tholodus
''Tholodus'' is an extinct genus of 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 Hermann von Meyer in 185 ...
''.
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 s ...
'',
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 putative
mosasaur
Mosasaurs (from Latin ''Mosa'' meaning the 'Meuse', and Greek ' meaning 'lizard') comprise a group of extinct, large marine reptiles from the Late Cretaceous. Their first fossil remains were discovered in a limestone quarry at Maastricht on th ...
''"Globidens" timorensis'' is not a mosasaur and instead represents a ''Xinminosaurus''-like ichthyosaur.
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
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 ...
e (back upper tooth-bearing bones) and back parts of the
dentaries
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 ...
(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
Pulp may refer to:
* Pulp (fruit), the inner flesh of fruit
Engineering
* Dissolving pulp, highly purified cellulose used in fibre and film manufacture
* Pulp (paper), the fibrous material used to make paper
* Molded pulp, a packaging material
* ...
, 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
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 ...
. 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 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 ...
ns, there is a prominent bend in the middle of the tail, where the
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 ...
s slope forwards. In ''Xinminosaurus'', the tail is bent downwards at a 35° angle, starting at caudal vertebra 38.
Each
scapula
The scapula (plural 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 eithe ...
(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 th ...
(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 (; ) 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 round ...
(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 (''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 ...
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
ulnare
The triquetral bone (; also called triquetrum, pyramidal, three-faced, and formerly cuneiform bone) is located in the wrist on the medial side of the proximal row of the carpus between the lunate and pisiform bones. It is on the ulnar side of the ...
s (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 form the intermediate part of the skeleton, skeletal hand located between the phalanges of the fingers and the carpal bones of the wrist, which forms the connection to the forearm. The metacarpa ...
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 Minamisan ...
'', 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, sand clock or egg timer) 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) ...
es
while the fifth is shaped like a
kidney
The kidneys are two reddish-brown bean-shaped organs found in vertebrates. They are located on the left and right in the retroperitoneal space, and in adult humans are about in length. They receive blood from the paired renal arteries; blood ...
. 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 (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.
...
(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 (; ), 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 ...
e (front lower hindlimb bones) are nearly twice as wide as their lower ends. There are two upper
ankle bones
In the human body, the tarsus is a cluster of seven articulating bones in each foot situated between the lower end of the tibia and the fibula of the lower leg and the metatarsus. It is made up of the midfoot (cuboid, medial, intermediate, and la ...
in ''Xinminosaurus'', the rear one (
calcaneum
In humans and many other primates, the calcaneus (; from the Latin ''calcaneus'' or ''calcaneum'', meaning heel) or heel bone is a bone of the tarsus of the foot which constitutes the heel. In some other animals, it is the point of the hock.
St ...
) larger than the front one (
astragalus
''Astragalus'' is a large genus of over 3,000 species of herbs and small shrubs, belonging to the legume family Fabaceae and the subfamily Faboideae. It is the largest genus of plants in terms of described species. The genus is native to tempe ...
) 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, are a group of five long bones in the foot, located between the tarsal bones of the hind- and mid-foot and the phalanges of the toes. Lacking individual names, the metatarsal bones are numbered from the medi ...
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
Anisian life
Anisian genus first appearances
Anisian genus extinctions
Guanling Formation
Ichthyosauromorph genera
{{ichthyosaur-stub