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.[1] 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, though from younger rocks and a different region than H. lingyuanensis.[2] H. lingyuanensis and H. baitaigouensis were largely similar in anatomy, both achieved a maximum adult body size of about 0.8 meters. They had small heads with numerous needle-like teeth, and extremely long tails with more than 55 ver