choristodere

A 120 million-year-old example of polycephaly, found in China in 2006. This is a bizarrely two-headed fossil of a newborn Hyphalosaurus, a member of a group of extinct semi-aquatic reptiles known as choristoderes.

Hyphalosaurus lived in freshwater lakes, and their remains are so numerous that their entire lifespan is represented from embryos to fully grown adults. Females have even been found with developed embryos still inside their bodies, showing that they gave birth to live young.