Numerous embryonic and/or newborn specimens of Hyphalosaurus have been recovered from the Yixian Formation, dating 122 million years ago.
In 2007, a specimen was found with two heads, the oldest known case of polycephaly.

In the holotype specimen of H. baitaigouensis, several eggs containing embryos were preserved in and around the body.
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.
Hou and colleagues (2010) also described several eggs with flexible shells, containing H. baitaigouensis embryos.
These shells were soft, and more similar to the eggs of lizards than to those of crocodiles, but nonetheless possessed a thin mineralized shell.

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.
Numerous embryonic and/or newborn specimens of Hyphalosaurus have been recovered from the Yixian Formation, dating 122 million years ago. In 2007, a specimen was found with two heads, the oldest known case of polycephaly.
In the holotype specimen of H. baitaigouensis, several eggs containing embryos were preserved in and around the body. 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. Hou and colleagues (2010) also described several eggs with flexible shells, containing H. baitaigouensis embryos. These shells were soft, and more similar to the eggs of lizards than to those of crocodiles, but nonetheless possessed a thin mineralized shell.
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.
Hyphalosaurus