Guizhouichthyosaurus

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Guizhouichthyosaurus
Temporal range: Triassic, Ladinian–Carnian
Skeleton of Guizhouichthyosaurus tangae in Geological Museum of Guizhou
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: †Ichthyosauria
Genus: †Guizhouichthyosaurus
Species:
†G. tangae
Binomial name
†Guizhouichthyosaurus tangae
Cao & Luo, 2000 (type)
Synonyms

Guizhouichthyosaurus is an

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 5 metres (16 ft) long with some specimens exceeding 6 metres (20 ft). 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 (shoulder blades) 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

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

ichthyosaurs from the Xiaowa Formation (which was deposited in the lower[1] Carnian stage of the Late Triassic)[2] of China named by Y. Cao and Y. Luo in 2000.[1] 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.[3] 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.[4] In 2003, another ichthyosaur, Panjiangsaurus epicharis, was named by Chen Xiaohong and Cheng Long, based on the nearly perfectly-preserved specimen TR 00001,[5] the holotype, and the additional specimen SPCV30014, a three-dimensionally preserved skull.[6]

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.[1] 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.[7]

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.[3]

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

thalattosaur as stomach contents.[13]

Callawayia wolonggangense

In 2007, X. Chen and colleagues named a new species of the ichthyosaur Callawayia, C. wolonggangse, based on material from Guizhou, China.[15][11] 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.[8]

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.

sister taxon of the type species of either genus.[14][12][16][17]

Description

Guizhouichthyosaurus has been described as moderate to very large in size for an ichthyosaur.[3] XNGM-WS-50-R4, a nearly complete skeleton assigned to Guizhouichthyosaurus sp. measures 4.8 m (16 ft) long.[13] As for specimens of G. tangae, the total length of the nearly complete skeleton IVPP V 11853 exceeds 5.2 m (17 ft),[3] while TR 00001 (the holotype of Panjiangsaurus) measures 5.4 m (18 ft) long[5] and the complete skeleton GNP-d41 measures more than 6 m (20 ft) long.[7] These specimens of G. tangae are estimated to have weighed between 914–1,404 kg (2,015–3,095 lb).[18] Additionally, specimens of Guizhouichthyosaurus reaching around 7 m (23 ft) in total length have also been reported.[13]

Skull

Skull of Guizhouichthyosaurus sp., XNGM-WS-50-R4, with a close-up of the teeth

Guizhouichthyosaurus has a long, powerful snout.

postfrontals. The orbits of Guizhouichthyosaurus are distinctively shaped, having convex front, straight rear and lower, and weakly[7] concave upper margins. The front thirds of their upper borders are formed by the prefrontals, which are small in Guizhouichthyosaurus.[1]

The portion of the skull located behind the orbits is rather short, not much more than half of the orbital length in adults.

pineal foramen (an opening on top of the skull) is flanked by sharp ridges on the frontals and parietals (two pairs of skull roof bones).[1] The low, thin,[1] 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 (openings on top of the skull).[3] The frontals form part of these shelves, but do not reach the fenestrae,[1] with the rest of the shelves formed by the postfrontals and parietals.[7] 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.[1] These bones, in addition to the parietals, form the exterior rim of the temporal fenestrae.[7]

The

dentaries (lower tooth-bearing bones). 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.[1][7]

Postcranial skeleton

There are about 65 vertebrae in front of the hips in Guizhouichthyosaurus, followed by 2 hip vertebrae. The tail of Guizhouichthyosaurus is elongate, containing around 170 vertebrae, and is abruptly bent downwards near its end. The wide

ulnare of Guizhouichthyosaurus. Both the forefins and hindfins of Guizhouichthyosaurus have four primary digits, in both cases representing digits II to V,[3] but additional accessory digits may be present.[10] The phalanges (digit bones) towards the tip of digit II in the hindfins are much larger than those closer to the base of the digit in some specimens.[11][10]

Paleobiology

Life restoration of Guizhouichthyosaurus tangae

A 2013 study by Shang and Li found that all specimens of Guizhouichthyosaurus with sufficient good preservation could be divided into two morphotypes, though they were all sufficiently similar to belong to the same species. The morphotypes were named "type A" and "type B." Type A is characterized by long, narrow hindflippers, with the upper elements of digit II being reduced in size and no additional digit in front of it present. Sometimes there is an additional digit behind the main digits of the hindfin, although there usually is not. Type B is characterized by broader hindflippers without the size reduction of the upper digit II elements, as well an additional digit in front of and another behind the main ones. The skulls of the two types also differ, with type A having a more robust skull, a shorter snout, and a wider angle between the two halves of the

pachypleurosaurs.[10][19]

Computer simulation of flow velocity from Gutarra and colleagues' study; warmer colors indicate faster flow

In 2019, Susana Gutarra and colleagues studied the energy demands of swimming in

thunniform (tuna-like) swimming styles. This switch likely explains why these later ichthyosaurs had deeper bodies.[20]

Diet and feeding

Unusually for an ichthyosaur, TR 00001 preserves more than 100

intestines. As the gastroliths did not resemble the surrounding rock, the scientists concluded that Guizhouichthyosaurus must have obtained them somewhere else, perhaps near a beach. While the function of gastroliths has been interpreted by some as buoyancy control, this is controversial, and Cheng and colleagues found the roughly 1 kilogram of stones to be insufficient to usefully serve as ballast for an animal potentially weighing as much as a ton. Since no other specimen of Guizhouichthyosaurus has gastroliths, they considered it most likely that the stones in TR 00001 were swallowed accidentally when attacking prey near the seafloor.[5]

XNGM-WS-50-R4 (topmost), with stomach contents shown below

Unlike a typical predator of large animals, the teeth of Guizhouichthyosaurus are rather small and do not have cutting edges, suitable for holding on to prey items like

cephalopods. Nevertheless, the Guizhouichthyosaurus sp. specimen XNGM-WS-50-R4 contains the torso and limbs of the thalattosaur Xinpusaurus, another marine reptile. The thalattosaur was estimated by Jiang and colleagues to have been approximately 4 metres (13 ft) long when complete compared to the 4.8 metres (16 ft) long ichthyosaur, and about one seventh of the predator's mass. The authors noted that Guizhouichthyosaurus appeared to be an apex predator, and predation upon large animals was likely more common among marine reptiles than previously thought. Jiang and colleagues considered it most likely that the thalattosaur was killed by the ichthyosaur, due to its completeness and the scarcity of marine carrion. 23 metres (75 ft) away from the skeletons, a tail potentially belonging to the consumed thalattosaur was found. As the thalattosaur skeleton shows little evidence of digestion, and the neck of the ichthyosaur appears to have been broken, Jiang and colleagues hypothesized that soon after eating its prey, the ichthyosaur died.[13]

Jiang and colleagues noted that while teeth suited for grasping are useful in procuring cephalopods, they can also be used to hold prey underwater, thereby causing it to drown, as done by

generalist feeder.[21][22]

See also

References

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  3. ^ a b c d e f g h i j Shang, Q. H.; Li, C. (2009). "On the occurrence of the ichthyosaur Shastasaurus in the Guanling biota (Late Triassic), Guizhou, China" (PDF). Vertebrata PalAsiatica. 47 (3): 178–193.
  4. ^ Li, C.; You, H. L. (2002). "Cymbospondylus from the Upper Triassic of Guizhou, China" (PDF). Vertebrata PalAsiatica. 40: 9–16.
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  6. ^ Chen, X.; Cheng, L. (2003). "A new species of large-sized and long-body ichthyosaur from the Late Triassic Guanling biota,Guizhou,China". Geological Bulletin of China. 22 (4): 228–235.
  7. ^ a b c d e f g h i j Pan, X.; Jiang, D.; Sun, Z.; Cai, T.; Zhang, D.; Xie, J. (2006). "Discussion on Guizhouichthyosaurus tangae Cao and Luo in Yin et al., 2000 (Reptilia, Ichthyosauria) from the late triassic of Guanling County, Guizhou". Acta Scientiarum Naturalium Universitatis Pekinensis. 42: 697–703.
  8. ^ a b Maisch, M. W. (2010). "Phylogeny, systematics, and origin of the Ichthyosauria – the state of the art" (PDF). Palaeodiversity. 3: 151–214.
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  10. ^ a b c d Shang, Q. H.; Li, C. (2013). "On the sexual dimorphism of Shastasaurus tangae (Reptilia: Ichthyosauria) from the Triassic Guanling Biota, China" (PDF). Vertebrata PalAsiatica. 51 (4): 253–264.
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  15. ^ Chen, X.H.; Cheng, L.; Sander, P.M. (2007). "A new species of Callawayia (Reptilia: Ichthyosauria) from the Late Triassic in Guanling, Guizhou" (PDF). Geology in China. 34 (6): 974–982.
  16. ^ Maxwell, E. E.; Cortés, D. (2020). "A revision of the Early Jurassic ichthyosaur Hauffiopteryx (Reptilia: Ichthyosauria), and description of a new species from Southwestern Germany". Palaeontologia Electronica. 23: 1–43.
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  21. ^ Cheng, L.; Chen, X. H. (2007). "Gut contents in the Triassic ichthyosaur Panjiangsaurus from the Guanling biota in Guizhou". Geology in China. 34 (1): 61–65.
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