Hyphalosaurus
| Hyphalosaurus Temporal range: Early Cretaceous
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| Hyphalosaurus lingyuanensis | |
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| Genus: | Hyphalosaurus
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Hyphalosaurus is among the most interesting fossil animals found in the Early Cretaceous Yixian Formation in Liaoning Province, China, formed around 130-120 million years ago.
Somehow, the slab and counterslab of the holotype were given to different groups of researchers. Each described the taxon and published their results independently, giving the animal two different names. It was quickly recognized that Sinohydrosaurus and Hyphalosaurus were mirror images of one another and in fact represented the same specimen. Sinohydrosaurus is considered a junior synonym of Hyphalosaurus, because the latter name appeared in publication before the former.
Hyphalosaurus fossils are relatively widespread in the Yixian beds. Entire growth series are known, from embryos in eggs to fully-grown adults. This animal achieved an adult body size of about 0.8 meters.
This animal was clearly aquatic, a lifestyle reflected by its elongate neck and tail and relatively reduced limbs. Superficially, it resembles a small plesiosaur or nothosaur, which were large marine reptiles that existed contemporaneously. This resemblance arose convergently and does not reflect evolutionary relatedness.
Hyphalosaurus belongs to the order Choristodera, and is related to the large, crocodile-like Champsosaurus and the smaller, lizard-like Monjurosuchus. Its closest relative was the similarly built species, Shokawa ikoi, of early Cretaceous Japan. The choristoderes were an order of aquatic reptiles that survived the end-Cretaceous extinction, and were the only order of reptiles to have survived the KT Extinction event, only to become totally extinct by the Miocene.
A fossil dating 120 million years ago was discovered in 2007 with two heads, marking the oldest known case of polycephaly.[1][2]