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 genus is also known from the Ladinian-aged Middle Triassic Zhuganpo Formation; additionally, the species "Callawayia" wollongangense may belong to Guizhouichthyosaurus.

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.

Guizhouichthyosaurus tangae was one of three new genera and species of 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.

[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.

[11][12][13][14] 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.

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

He considered the characteristics used to differentiate G. wolonggangense unconvincing, however, and that this species was probably just a junior synonym of G. tangae.

The orbits of Guizhouichthyosaurus are distinctively shaped, having convex front, straight rear and lower, and weakly[7] concave upper margins.

A reasonably prominent flat shelf is present in the front regions of the depressions surrounding each temporal fenestra (openings on top of the skull).

[7] The interclavicle (a shoulder bone between the collarbones) is shaped like the letter T. The central, lower projection of it is very small and narrow.

While as an ichthyopterygian, Guizhouichthyosaurus would have been a live-bearer, a similar trend in trunk-tail proportions is also seen in another group of vivaporous marine reptiles, the pachypleurosaurs.

[10][19] In 2019, Susana Gutarra and colleagues studied the energy demands of swimming in ichthyosauriforms using computational fluid dynamics and models of nine taxa, one of which was Guizhouichthyosaurus.

They found that the body shapes of the different ichthyosauriforms, when scaled to the same size and the effects of the limbs disregarded, all resulted in similar net energy costs and drag coefficients.

Through their simulations, the researchers found that the water would have stagnated around the snouts of the ichthyosauriforms, then flowed more quickly across their bodies before slowing down in the animal's wake.

They are preserved in two clusters, although Cheng and colleagues doubted that Guizhouichthyosaurus would have been stored them in two different regions, a configuration seen in no other vertebrate.

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] 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.

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.

[13] However, Guizhouichthyosaurus ate other prey items as well, as evidenced by a G. tangae specimen preserving fish bones as well as some bivalves in its stomach region.