| Xinminosaurus Temporal range: Late Triassic, | |
|---|---|
Scientific classification  | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Order: | † Ichthyosauria |
| Family: | † Cymbospondylidae |
| Genus: | † Xinminosaurus Jiang et al., 2008 |
| Type species | |
| †Xinminosaurus catactes Jiang et al., 2008 | |
Xinminosaurus is an extinct genus of cymbospondylid ichthyosaur known from the Middle Triassic (mid-late Anisian stage) of Guizhou Province, China.
The generic name is derived from Xinmin, the district where the fossil was found, and Greek sauros, "lizard". The specific name is derived from Greek kataktes, "crusher", in reference to one of the taxon's autapomorphies - the presence of bulbous and laterally compressed crushing teeth in maxilla and posterior dentary.
Xinminosaurus is known only from the holotype GMPKU-P-1071, a nearly complete skeleton deposited in Geological Museum, Peking University. It was collected from the conodont Nicoraella kockeli biozone (Pelsonian of the Anisian stage), from the upper member of the Guanling Formation. It was found near Yangjuan Village of the Xinmin District, Panxian County. [1] Maisch suggested that Xinminosaurus might be a subjective junior synonym of Tholodus . [2] Although Jiang et al. regarded Tholodus to be a possible nomen dubium , [1] 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. [2]
Mulder and Jagt (2019) demonstrated that the putative mosasaur "Globidens" timorensis is not a mosasaur and instead represents a Xinminosaurus-like ichthyosaur. [3]
The holotype specimen of Xinminosaurus has a total length of 2.32 metres (7.6 ft), 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 maxillae (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. [1]
Xinminosaurus has a total of 140 vertebrae. 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 ichthyopterygians, there is a prominent bend in the middle of the tail, where the neural spines slope forwards. In Xinminosaurus, the tail is bent downwards at a 35° angle, starting at caudal vertebra 38. [1]
Each scapula (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. [1] The coracoids (a pair of shoulder bones) are greater in width than length. [4] Xinminosaurus has short, wide collarbones. [1] The interclavicle (a shoulder bone between the collarbones) does not have a backwards projection. [4]
The long limb bones of Xinminosaurus are atypical for an ichthyosaur. The humeri (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 ulnae (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 of Xinminosaurus resemble those of the non-ichthyosaurian ichthyopterygian Utatsusaurus , with the first being the shortest and the third and fourth being the longest. [1] The first four are shaped like hourglasses [4] while the fifth is shaped like a kidney. 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 (digit bones) are long and at least the upper ones are hourglass-shaped. [1] [4]
The upper and lower hindlimb bones are shorter than those of the forelimb. The upper ends of the tibiae (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) larger than the front one (astragalus) 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 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. [1]
Ichthyosauria is an order of large extinct marine reptiles sometimes referred to as "ichthyosaurs," although the term is also used for wider clades that the order resides in.
Utatsusaurus hataii is the earliest-known ichthyopterygian which lived in the Early Triassic period. It was nearly 2.5–3 metres (8.2–9.8 ft) long with a slender body. The first specimen was found in Utatsu-cho, Miyagi Prefecture, Japan. It is the only described species in the genus Utatsusaurus and the only member of the family Utatsusauridae. The name Utatsusaurus was given after the city. The fossils have been found from the Early Triassic Osawa Formation of Miyagi Prefecture, Japan and British Columbia, Canada.
Aegirosaurus is an extinct genus of platypterygiine ophthalmosaurid ichthyosaurs known from the late Jurassic and early Cretaceous of Europe. It was originally named as a species of Ichthyosaurus.
Chaohusaurus is an extinct genus of basal ichthyopterygian, depending on definition possibly ichthyosaur, from the Early Triassic of Chaohu and Yuanan, China.
Omphalosaurus is an extinct genus of marine reptile from the Early Triassic to Middle Triassic, thought to be in the order of Ichthyosauria. Most of what is known about Omphalosaurus is based on multiple jaw fragments, ribs, and vertebrae. Specimens of Omphalosaurus have been described from the western United States, Poland, Austria and the island of Spitsbergen off the northern coast of Norway.
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 around 1–1.5 metres (3.3–4.9 ft) long. Fossil remains from Svalbard from the specimen SVT 203 were originally assigned to G. longirostris but are now thought to have belonged to a non-ichthyopterygian diapsid related to Helveticosaurus.
Chacaicosaurus is a genus of neoichthyosaurian ichthyosaur known from the Middle Jurassic of Argentina. The single known specimen of this genus was excavated from the Los Molles Formation in Neuquén Province, and is housed at the Museo Olsacher under the specimen number MOZ 5803. This specimen consists of a skull, forelimb, some vertebrae, and some additional postcranial elements. The genus was named by Marta Fernández in 1994, and contains a single species, Chacaicosaurus cayi, making it the first named distinctive ichthyosaur from the Bajocian stage. It is a medium-sized ichthyosaur with a very long snout, which bears a ridge running along each side. The forelimbs of Chacaicosaurus are small and contain four main digits.
Protoichthyosaurus is a genus of ichthyosaur from the early Jurassic of southern England. Two species are known, P. prostaxalis—the type species, named by Appleby in 1979—and P. applebyi. A third species, P. prosostealis, was named by Appleby, but it was removed from the genus in 2017 due to its similarity to Ichthyosaurus. The genus Protoichthyosaurus was synonymized with Ichthyosaurus by Maisch and Hungerbuhler in 1997, and again by Maisch and Matzke in 2000. However, it was found to be distinct in 2017 by Dean Lomax and colleagues, who separated it from Ichthyosaurus on account of differences in the arrangement and shape of the carpal ossifications, as well as the absence of the fifth digit. The species most likely lived during the Hettangian stage, but may have lived as early as the Rhaetian and as late as the Sinemurian.
Besanosaurus is a genus of Middle Triassic ichthyosaur from Monte San Giorgio of Italy and Switzerland, containing the single species B. leptorhynchus. Besanosaurus was named by Cristiano Dal Sasso and Giovanni Pinna in 1996, based on the nearly complete flattened skeleton BES SC 999, the holotype specimen. This skeleton is preserved across multiple thin rock slabs spanning 3.5 by 4 metres when assembled and took thousands of hours to prepare. Additional specimens from Monte San Giorgio that have previously been considered separate genera, including a partial skull named Mikadocephalus and a well-preserved, largely complete skeleton, have been reinterpreted as additional specimens of Besanosaurus.
Hupehsuchia is an order of diapsid reptiles closely related to ichthyosaurs. The group was short-lasting, with a temporal range restricted to the late Olenekian age, spanning only a few million years of the Early Triassic. The order gets its name from Hubei Province, China, from which many specimens have been found. They are probable members of the clade Ichthyosauromorpha.
Callawayia is an extinct genus of ichthyosaur. It contains the species Callawayia neoscapularis.
Guizhouichthyosaurus is an extinct genus of ichthyosaur which is known primarily from the Xiaowa Formation of the lower Carnian stage of the Late Triassic in southwest China. The type species of this genus is Guizhouichthyosaurus tangae, of which multiple skeletons are known. It has been reassigned as a species of the genus Shastasaurus in the past, though it has since been considered distinct. The ichthyosaurs Cymbospondylus asiaticus, named in 2002, and Panjiangsaurus epicharis, named in 2003, are junior synonyms of G. tangae. The genus is also known from the Ladinian-aged Middle Triassic Zhuganpo Formation; additionally, the species "Callawayia" wollongangense may belong to Guizhouichthyosaurus.
Thaisaurus is an extinct genus of ichthyopterygian marine reptile that lived during the Spathian. Fossils have been found in Thailand.
Toretocnemus is an extinct genus of ichthyosaur. Its remains have been found in California, United States, in Triassic layers of the Carnian Hosselkus Limestone.
Wimanius is a genus of ichthyosaur from the Middle Triassic of Switzerland, containing a single species, Wimanius odontopalatus. It was described by Michael Maisch and Andreas Matzke in 1998 based on an incomplete skull from Monte San Giorgio, a mountain on the Swiss-Italian border. Wimanius possesses teeth on its palate, though whether they were located on the palatine or pterygoid is disputed. Other features of Wimanius include a large orbit and jugals with two rami of similar lengths. Different phylogenetic placements of Wimanius have been recovered by different studies, including it being a mixosaurid relative or a merriamosaur, and a monotypic family, Wimaniidae has been named for it. However, its validity has also been questioned, and synonymy with various other genera has been proposed.
Mixosauridae was an early group of ichthyosaurs, living between 247.2 and 235 million years ago, during the Triassic period. Fossils of mixosaurs have been found all over the world: China, Timor, Indonesia, Italy, Germany, Spitsbergen, Switzerland, Svalbard, Canada, Alaska, and Nevada.
Dianopachysaurus is an extinct genus of pachypleurosaur known from the lower Middle Triassic of Yunnan Province, southwestern China. It was found in the Middle Triassic Lagerstätte of the Guanling Formation. It was first named by Jun Liu, Olivier Rieppel, Da-Yong Jiang, Jonathan C. Aitchison, Ryosuke Motani, Qi-Yue Zhang, Chang-Yong Zhou and Yuan-Yuan Sun in 2011 and the type species is Dianopachysaurus dingi, thanking a Professor Ding for his help.
Concavispina is an extinct genus of thalattosaur reptile from the early Late Triassic Xiaowa Formation of Guangling, Guizhou, southern China. It contains a single species, Concavispina biseridens. It is known only from the holotype 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 and a V-shaped notch on the dorsal margin of each neural spine in the dorsal (back) vertebrae. Both its generic and specific names refer to these autapomorphies, as Concavispina means "concave spine" and biseridens means "two rows of teeth". It is thought to be most closely related to Xinpusaurus, as both taxa share three derived characters: a maxilla that is curved upward at its anterior end, a humerus that is wider near the shoulder than near the elbow, and the presence of less than five cervicals.
Tholodus is an extinct genus of basal ichthyopterygian known from the Middle Triassic of Germany, northeastern Italy and possibly China. It was first named by Christian Erich Hermann von Meyer in 1851 and the type species is Tholodus schmidi. It is known from many disarticulated and fragmentary remains, mainly teeth and jaw fragments. Most specimens were collected from various localities across the Ladinian-aged Muschelkalk, Germany, mainly from the Jena Formation of the upper Lower Muschelkalk, where the holotype was found. Dalla Vecchia (2004) recently described two additional specimens, a mandibular ramus and a maxilla, both bearing teeth and nearly uncrushed, and some postcranial remains, from a single late Anisian outcrop, from the southern Alps of Italy. The humerus resembled that of immature individuals of the Asian genus Chaohusaurus, suggesting possible affinities to Grippidia.
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