Hyphalosaurus (meaning "submerged lizard") is a genus of freshwater aquatic reptiles, belonging to the extinct order Choristodera.

[1] 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.

In addition to the small scales, two rows of large, round scutes with shallow keels ran along the animals sides.

[5] Numerous embryonic and/or newborn specimens of Hyphalosaurus have been recovered from the Yixian Formation, dating 122 million years ago.

[7] 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.

[8] Both Hyphalosaurus species were aquatic, a lifestyle reflected by their long necks and tails and relatively small limbs.

[7] Hyphalosaurus was among the most aquatically adapted choristoderans, with smoother, flatter scales than its relatives, a tall and flattened tail for swimming, a long neck and webbed feet.

Because the torso was fairly inflexible and the limbs were not particularly adapted for aquatic life, Hyphalosaurus probably swam using mainly its tall, flattened tail.

Hyphalosaurus is also conspicuously absent from the aquatic sediments of the Jiufotang Formation, which preserved a more swampy, shallow-water ecosystem.

[3] Hyphalosaurus is the most abundant tetrapod (four-limbed vertebrate) in the Yixian Formation, and probably played an important role in the aquatic food chain.

However, in June 2001 paleontologists Joshua Smith and Jerry Harris noted that since both were published at almost exactly the same time, a third party needed to select which name would better serve as the objective senior synonym.

Smith and Harris took the opportunity to do so, selecting Hyphalosaurus as the senior synonym because the manuscript for its description had apparently been submitted (though not published) earlier.

The choristoderes were a clade of aquatic reptiles that survived the end-Cretaceous extinction along with crocodilians, turtles, lizards and snakes.