Hyphalosaurus

Hyphalosaurus Temporal range: Early Cretaceous
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Sauropsida
Order:	Choristodera
Family:	Hyphalosauridae Gao and Fox 2005
Genus:	Hyphalosaurus Gao, et al. 1999
Species
H. lingyuanensis (type) H. baitaigouensis

Hyphalosaurus is an extinct genus from the Order Choristodera, found in the Early Cretaceous Jehol group in China (around 130-120 million years ago). It contains two species, H. lingyuanensis from the Yixian Formation, and H. baitaigouensis from the Jiufotang Formation.

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 a junior synonym of Hyphalosaurus, as 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 aquatic, a lifestyle reflected by its elongate neck and tail and relatively reduced limbs. Superficially, it resembles a small nothosaur. This resemblance arose convergently and does not reflect evolutionary relatedness.

Hyphalosaurus is related to the large, crocodile-like Champsosaurus and the smaller, lizard-like Monjurosuchus. Its closest relative was the similarly built species, Shokawa ikoi, from the Early Cretaceous of Japan. The choristoderes were a clade of aquatic reptiles that survived the end-Cretaceous extinction along with crocodilians, turtles, lizards and snakes. The choristoderes became extinct by the Miocene.

A fossil of Hyphalosaurus dating 120 million years ago was found in 2007 with two heads, marking the oldest known case of polycephaly.

Gallery

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  Hyphalosaurus baitaigouensis fossils displayed in Hong Kong Science Museum.
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  Hyphalosaurus baitaigouensis fossil displayed in Hong Kong Science Museum.
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  Sinohydrosaurus, the junior synonym of Hyphalosaurus, displayed in Hong Kong Science Museum.
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  A specimen of Hyphalosaurus lingyuanensis, on display at the Paleozoological Museum of China.