Hyphalosaurus

Genus of extinct freshwater aquatic reptiles

Hyphalosaurus (meaning "submerged lizard") is a genus of freshwater aquatic reptiles, belonging to the extinct order Choristodera. They lived during the early Cretaceous period (Aptian age), about 122 million years ago. The genus contains two species, H. lingyuanensis and H. baitaigouensis, both from the Yixian 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.

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, though from younger rocks and a different region than H. lingyuanensis.

Several specimens of Hyphalosaurus have been described with clear impressions of the skin. One specimen represents H. lingyuanensis, and another (with clearer impressions) cannot be assigned to a species because part of the neck (the length of which is a key indicator of species) was destroyed when fossil dealers grafted a skull from a different specimen onto the slab. However, both specimens show nearly identical patterns of scales. Hyphalosaurus was covered mostly in small, irregularly patterned polygonal scales, though these varied across the body. The scales of the hind legs were smaller, finer and more irregular than those of the torso, while the scales of the tail were nearly square and arranged in more regular rows. In addition to the small scales, two rows of large, round scutes with shallow keels ran along the animals sides. One row ran directly along the flank, with the other either slightly higher or lower and composed of scutes only 1/4 the size of the flank scutes. The flank row of larger scutes extended all the way to the base of the tail, and remained uniform in size across the entire row.

Numerous embryonic and/or newborn specimens of Hyphalosaurus have been recovered from the Yixian Formation, dating 122 million years ago. These eggs appeared to lack mineralized shells, which Ji and colleagues later interpreted as evidence that Hyphalosaurus gave birth to live young and that egg shells never fully developed inside the body of the mother. However, in 2006, Ji and colleagues re examined the holotype specimen and noted clearly defined, though thin and leathery, shells. They agreed, however, that these eggs must have developed inside the mother which would later have given live birth (a reproductive method sometimes called ovoviviparity), but were likely expelled from the body when the mother died. Another fossil specimen of H. baitaigouensis, described by Ji and colleagues in 2010, appeared to be pregnant, containing 18 fully developed embryos arranged in pairs. One of the rear most embryos was positioned in a reverse, head first position, a complication which may have killed the mother. This confirmed that Hyphalosaurus and other choristoderans were viviparous, the only known Mesozoic fresh water reptiles to give birth to live young.