Another excellent fossil at the Denver Museum of Nature and Science — it’s Diplocaulus:
Diplocaulus (“double caul”) was an amphibian in early Permian North America. Living some 275 million years ago, this stocky creature with a distinctive boomerang-shaped skull may have used his head as a hydrofoil to swim better and faster. Given his small limbs and relatively short tail, he definitely wasn’t going to be a fast mover any other way.
This particular specimen came from the Vale Formation in Baylor County, Texas.
