Chaohusaurus is an extinct genus of basal ichthyopterygian, depending on definition possibly ichthyosaur, from the Early Triassic of Chaohu and Yuanan, China.
Discovery
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 generic name refers to lake Chao Hu. The specific name refers to the Geishan location. The holotype, IVPP V 4001, was uncovered in a layer of the Majianshan Limestone Formation dating from the Anisian. It consists of a partial skeleton, containing the skull and the front torso. In 1985 Chen Lizhu named two additional species based on fossils found in the same formation: Anhuisaurus chaoxianensis and Anhuisaurus faciles. However, the generic name had already been preoccupied by the lizard Anhuisaurus Hou 1974. Therefore, Anhuisaurus Chen 1985 was in 1991 renamed into Chensaurus by Jean-Michel Mazin e.a. In 1998, Ryosuke Motani and Hailu You established that the Chensaurus fossils represented remains of juveniles, with those of C. faciles being the youngest, and that these formed a growth series with Chaohusaurus. This implied that the known material of Chaohusaurus was increased with the specimens AGM P45-H85-25, the holotype of Chensaurus chaoxianensis; AGM P45-H85-20, the holotype of Chensaurus faciles; and the front flippers IVPP V 11361 and IVPP V 11362. In 2001 a detailed description of the osteology of Chaohusaurus was published by Michael Maisch.
In 2014, three additional specimens were reported: AGM I-1, AGM CHS-5, and AGM CH-628-22. AGM I-1 also contains the remains of three embryo's. The new finds are part of recent discoveries by a Chinese-Italian paleontological project of eighty Chaohusaurus fossils.
In 2013, a second species was named and described by Chen Xiaohong e.a.: Chaohusaurus zhangjiawanensis. The specific name refers to its provenance near the village of Zhangjiawan. The holotype, WHGMR V26001, was uncovered in a layer of the Jialingjang Formation dating from the Spathian. It consists of a relatively complete skeleton with skull. A second skeleton lacking the skull, WHGMR V26002, was referred.
In 2015, Ryosuke Motani and his colleagues reported a large adult forelimb specimen that resembles the immature specimens of C. chaoxianensis, and argued that C. chaoxianensis is not the juvenile of C. geishanensis and is indeed a valid species within the genus Chaohusaurus.
Description
Chaohusaurus is a basal ichthyopterygian. It thus shows traits that are typical for the direct ancestors of ichthyosaurs. 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. These proportions were not caused by a large number of vertebrae, the total of presacrals being about forty, but by an elongated build of each individual vertebra. The head is short, in adults having about the third the length of the trunk, with a narrow pointed beak and large eye-sockets. Its teeth are conspicuously heterodont, differing in shape: those of the upper jaws are pointed though blunt; those of the rear lower jaws are bulbous with a convex profile, a possible adaptation to a durophagous diet, crushing shellfish. The neck is relatively long. Chaohusaurus did have flippers, rather than webbed feet. The tail fin is wide-based in side-view and short.
Chaohusaurus is one of the smallest known ichthyopterygians, measuring about long and weighing .
Being a basal ichthyopterygian, Chaohusaurus provides important information about the early evolution of the group. It shows that some typical ichthyosaurian traits were probably already part of the original ichthyopterygian Bauplan. It furthermore has a mix of basal traits that were subsequently lost, early derived traits indicating the genus is not the basalmost ichthyopterygian known and some autapomorphies of its own.
An original ichthyopterygian trait is the fact that the skull roof is short and wide. The basisphenoid at the rear lower end of the braincase is not fused with the basioccipital, the lower bone of the rear skull. The parasphenoid at the front of the braincase has a long cultriform process. There is no separate ectopterygoid present. The lower jaw has a retroarticular process at its rear.