Hyphalosaurus

Hyphalosaurus (meaning "submerged lizard") is a genus of freshwater aquatic reptiles, belonging to the extinct order Choristodera. They lived during the early Cretaceous period (late Barremian to early Aptian age), about 123-120 million years ago. The genus contains two species, H. lingyuanensis from the Yixian Formation and H. baitaigouensis from both the Yixian and Jiufotang Formation of Liaoning Province, China. They are among the best-known animals from the Jehol Biota, with thousands of fossil specimens representing all growth stages in scientific and private collections.

==Description and biology== thumb|left|Size compared to a human Hyphalosaurus fossils are relatively widespread in the Jehol beds, which represent a series of freshwater lakes. Several specimens of H. lingyuanensis and thousands of H. baitaigouensis specimens are known from the Yixian Formation, including entire growth series from embryos in eggs to fully grown adults. H. baitaigouensis was originally reported from the younger Jiufotang Formation, though subsequent study showed that the fossil beds it was found in might also belong to the Yixian Formation, specifically from younger rocks and a different region than H. lingyuanensis. However, a 2024 study confirmed the presence of H. baitaigouensis from the Jiufotang Formation. H. lingyuanensis and H. baitaigouensis were largely similar in anatomy, both achieved a maximum adult body size of about . They had small heads with numerous needle-like teeth, and extremely long tails with more than 55 vertebrae. The primary difference between the two species is the number of vertebrae in the neck. H. lingyuanensis had 19 neck vertebrae, while H. baitaigouensis had 26.