Guizhouichthyosaurus
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.
Guizhouichthyosaurus is a large ichthyosaur, typically measuring approximately long with some specimens exceeding. The snout is long and powerful, and there is a low sagittal crest on the rear part of its skull. The tail is bent downwards near its end, the scapulae are shaped like sickles, and an opening is enclosed between each set of lower leg bones. Each limb contains a minimum of four digits. Two distinct morphotypes of Guizhouichthyosaurus are known, differentiated by skull and limb morphology, which likely represent males and females.
Guizhouichthyosaurus, like other early ichthyosaurs, would have used anguilliform locomotion, unlike the more efficient cod- and tuna-like swimming of later ichthyosaurs. While one specimen of Guizhouichthyosaurus contained gastroliths as stomach contents, their ingestion may have been accidental, as stomach stones are very uncommon in ichthyosaurs. Despite its small, smooth-edged teeth, Guizhouichthyosaurus is known to have been able to consume other large marine reptiles, as evidenced from a thalattosaur skeleton in the stomach region of one specimen, indicating that Guizhouichthyosaurus was an apex predator.
History of research
Guizhouichthyosaurus tangae was one of three new genera and species of ichthyosaurs from the Xiaowa Formation of China named by Y. Cao and Y. Luo in 2000. G. tangae was named based on Gmr 009, a well-preserved specimen, although it is missing the hips, a hindflipper, and much of its tail. Li Chun and You Hai-Lu named a new species of Cymbospondylus, C. asiaticus, from the same region in 2002 based on two nearly complete skulls. They noted that this was the first time that Cymbospondylus had ever been reported from outside North America and Europe, in addition to the first occurrence outside the Middle Triassic. In 2003, another ichthyosaur, Panjiangsaurus epicharis, was named by Chen Xiaohong and Cheng Long, based on the nearly perfectly-preserved specimen TR 00001, the holotype, and the additional specimen SPCV30014, a three-dimensionally preserved skull.A study of the cranial anatomy of Guizhouichthyosaurus was conducted by Michael Maisch and colleagues in 2006. They described multiple specimens; GNG dq-46, a good skull discovered by farmers at Wolonggang in 1998, then sent to the Guanling National Geopark of Fossil Biota where it was prepared by Jin-ZhaoDing and Da-Peng Zhang; GNG dp-22, a partial skull mixed up with the remains of other ichthyosaurs; and GNG D-41, a complete, though incompletely prepared, skeleton. Pan Xinru and colleagues restudied Guizhouichthyosaurus and published their results in another 2006 paper, in which they described GNG D-41 in more detail. They found Panjiangsaurus epicharis and Cymbospondylus asiaticus to be junior synonyms of G. tangae.
In 2009, Shang Qing-Hua and Li Chun described a new specimen of Guizhouichthyosaurus, IVPP V 11853, a nearly complete articulated skeleton, missing only some parts of the flippers. They further described the anatomy of Guizhouichthyosaurus, using this new specimen to provide information on the shoulders, hips, and tail, which until then had been poorly known. However, they considered Guizhouichthyosaurus to be so similar to Shastasaurus that they synonymized it with that genus, although they kept S. tangae as a distinct species. They concurred with previous studies that Panjiangsaurus epicharis and Cymbospondylus asiaticus were junior synonyms of this species.
However, in 2010, Maisch provisionally accepted Guizhouichthyosaurus as a distinct genus. While he considered the two genera to be quite similar, he noted that the majority of the traits that had been used to synonymize it with Shastasaurus were ancestral characteristics of that group of Triassic ichthyosaurs instead of novel evolutionary changes, and stated that further research was still needed before the two could be synonymized. A 2011 paper by P. Martin Sander and colleagues considered Guizhouichthyosaurus to be quite distinct from Shastasaurus, and while Shang and Li again used the name Shastasaurus tangae in a 2013 paper, Guizhouichthyosaurus has since been accepted as valid and different from Shastasaurus. In 2020, Da-Yong Jiang and colleagues described XNGM-WS-50-R4, a specimen that they assigned to Guizhouichthyosaurus sp., which came from the Zhuganpo Formation, dating to the Ladinian stage of the Middle Triassic. It was dug up in 2010 and contained a thalattosaur as stomach contents.
''Callawayia wolonggangense''
In 2007, X. Chen and colleagues named a new species of the ichthyosaur Callawayia, C. wolonggangse, based on material from Guizhou, China. In 2010, Maisch moved this species to Guizhouichthyosaurus, as G. wolonggangense, noting that it was distinctly different from Callawayia. He considered the characteristics used to differentiate G. wolonggangense unconvincing, however, and that this species was probably just a junior synonym of G. tangae. He still maintained it as provisionally valid though, as detailed investigation had not yet been done.In 2016, Ji and colleagues found no characteristics uniting "C." wolonggangense and the type species of Callawayia, C. neoscapularis, and thus rejected the assignment of the former species to this genus. However, they found no traits unambiguously linking it to Guizhouichthyosaurus either, thus also rejected its assignment to that genus. Further studies have variably referred to the species as "C." wolonggangense or G. wolonggangense, but phylogenetic analyses have generally found results similar to those of Ji and colleagues, where the species is not found to be the sister taxon of the type species of either genus.
Description
Guizhouichthyosaurus has been described as moderate to very large in size for an ichthyosaur. XNGM-WS-50-R4, a nearly complete skeleton assigned to Guizhouichthyosaurus sp. measures long. As for specimens of G. tangae, the total length of the nearly complete skeleton IVPP V 11853 exceeds, while TR 00001 measures long and the complete skeleton GNP-d41 measures more than long. These specimens of G. tangae are estimated to have weighed between. Additionally, specimens of Guizhouichthyosaurus reaching around in total length have also been reported.Skull
Guizhouichthyosaurus has a long, powerful snout. An extensive groove is present in front of each external naris. The premaxillae do not extend beneath the external nares. However, a spear-shaped projection of each premaxilla forms most of the upper rim of each external naris. The external nares are oval-shaped and small. The maxillae extend quite far forwards. They also bear long, rearwards extensions that almost touch the prefrontals. The nasals extend as far back as to be level with the middle of the orbits, where they contact another pair of skull roof bones, the postfrontals. The orbits of Guizhouichthyosaurus are distinctively shaped, having convex front, straight rear and lower, and weakly concave upper margins. The front thirds of their upper borders are formed by the prefrontals, which are small in Guizhouichthyosaurus.The portion of the skull located behind the orbits is rather short, not much more than half of the orbital length in adults. There is a large, triangular indentation behind the jugals, the bones that form the lower rims of the orbits. The contact between the jugals and quadratojugals is hidden on the outside of the skull by two other pairs of bones, the postorbitals and squamosals. The pineal foramen is flanked by sharp ridges on the frontals and parietals. The low, thin, well-developed sagittal crest on the parietals splits in two at its front and rear ends. A reasonably prominent flat shelf is present in the front regions of the depressions surrounding each temporal fenestra. The frontals form part of these shelves, but do not reach the fenestrae, with the rest of the shelves formed by the postfrontals and parietals. The front half of the outer edges of the fenestrae are formed by the postorbitals internally, but these bones are excluded from the exterior surface by the contacts between two pairs of skull roof bones, the postfrontals and supratemporals. These bones, in addition to the parietals, form the exterior rim of the temporal fenestrae.
The pterygoids lack prominent outwardly-directed projections. The space between the pterygoids is large. The surangulars bear deep depressions that are confluent with depressions on the dentaries. The teeth of Guizhouichthyosaurus are conical, have somewhat blunted tips, and are set into sockets. They have 3 to 5 very fine vertical ridges, but lack cutting edges.