Chaohusaurus is an extinct genus of basal ichthyosauriform, depending on definition possibly ichthyosaur, from the Early Triassic of Chaohu and Yuanan, China.

The type species Chaohusaurus geishanensis was named and described by Yang Zhongjian and Dong Zhiming in 1972, based on a fossil found during the construction of a railway.

The holotype, IVPP V 4001, was uncovered in a layer of the Majianshan Limestone Formation dating from the Anisian.

[5] In 1998, Ryosuke Motani and Hailu You established that the Chensaurus fossils represented remains of juveniles, with those of C. faciles being the youngest,[6] and that these formed a growth series with Chaohusaurus.

As a result, basal ichthyosaurs like Cymbospondylus and Mixosaurus are closer in build to Chaohusaurus while later genera like Ichthyosaurus have a more derived morphology.

Chaohusaurus did not have the dolphin-like form of later ichthyosaurs; it had a more lizard-like appearance with an elongated body.

The head is short, in adults having about the third the length of the trunk, with a narrow pointed beak and large eye-sockets.

[citation needed] Chaohusaurus is one of the smallest known ichthyopterygians, measuring about 0.7–1 m (2.3–3.3 ft) long and weighing 1.3–3.1 kg (2.9–6.8 lb).

[11][12][13] Being a basal ichthyopterygian, Chaohusaurus provides important information about the early evolution of the group.

[citation needed] An original ichthyopterygian trait is the fact that the skull roof is short and wide.

[citation needed] A conspicuous basal trait is the shortness of the head; in later forms it would be longer relative to the trunk.

From the prefrontal a flange overhangs the upper front edge of the eye-socket, perhaps to protect the eyes.

The cultriform process of the parasphenoid at the rear gradually merges with the main body, not via a narrow waist as with later forms.

The palate was not firmly attached to the basipterygoid, allowing the snout some vertical movement relative to the remainder of the skull.

The pubic bone is perforated by a foramen obturatum, which closed in side view and located some distance from the rear edge of the body.

Between the tibia and fibula still a space is present, and both bones, though flattened are relatively elongated with a clear shaft.

In Motani's study, Chaohusaurus was classified as a non-ichthyosaurian ichthyopterygian, while Maisch and Matzke, who used a more inclusive definition of Ichthyosauria, considered it to be a basal ichthyosaur.

Even though this would make the genus paraphyletic, C. zhangjiawanensis was nevertheless placed in Chaohusaurus because of the morphological similarity to the type species.

[18][19][20] Ji and colleagues attributed this change in phylogenetic position to a greater understanding of Chaohusaurus anatomy and additional specimens.

[19] While sometimes still classified within Ichthyopterygia, Moon noted that as this group was not redefined to account for this newer topology, Chaohusaurus technically falls outside of it.

This differs from the method used by most extant marine viviparous Amniota, which expel the young tail-first to prevent them from suffocating.

Many younger ichthyosaur specimens had earlier been found showing embryos in both positions, leaving it undecided which was the normal one.

Motani et alii (2014) concluded that, because Chaohusaurus is a very basal form, this provided strong evidence that, at least originally, ichthyopterygian young were born with the head first.