Abstract
Although the long-necked choristodere Hyphalosaurus is the most abundant tetrapod fossil in the renowned Yixian Formation fossil beds of Liaoning Province, China, the genus has only been briefly described from largely unprepared specimens. This paper provides a thorough osteological description of the type species Hyphalosaurus lingyuanensis and the con-generic species Hyphalosaurus baitaigouensis based on the study of fossils from several research institutions in China. The diagnoses for these two species are revised based on comparison of a large sample of specimens from the type area and horizon of each of the two species. The skull, better known in H. baitaigouensis, exhibits key choristodere synapomorphies including an elongate contact between the prefrontals and posteriorly expanded supratemporal fenestrae that strongly support the placement of the highly derived hyphalosaurids within Choristodera. Both species of Hyphalosaurus share a proportionally small head, an elongate neck, a relatively unspecialized appendicular skeleton and a long, dorsoventrally heightened tail. Soft tissue preservation in several specimens provides rare insight into the integument of an extinct group. The integument of Hyphalosaurus is made up of small polygonal scales with several parasagittal rows of large, keeled, ovoid scutes. These possibly ornamental scutes bear a strong resemblance to the rows of large scutes in the monjurosuchid choristodere Monjurosuchus splendens. Observations from a variety of growth stages reveal that significant ontogenetic change in the proportions of the body and limb bones occurred in both species of Hyphalosaurus.