While the scientific name of this animal, Dimetrodon, means "two shapes of tooth," its most striking characteristic is the great sail on its back. Dimetrodon's sail was formed by a row of long spines, each of which grew out of a separate vertebra. Like many bizarre anatomical features of ancient animals, we can only guess about the appearance and purpose of this sail. Was it brightly colored? Did it attract mates or frighten off other animals? Did it act as a temperature control device, soaking up heat from the sun when the animal was cold, or radiating heat to help it cool off? Questions such as these are likely to remain unanswered.

Dimetrodon was a member of a group of animals we call synapsids. Just behind the eye socket in this skull is an opening, called the synapsid opening. Its purpose is not known for certain, but it may have been a passage for jaw muscles that helped Dimetrodon and other synapsids chew.

One other group of animals has this synapsid opening -- mammals. So this 280-million-year-old Dimetrodon fossil is actually one of your early relatives, and another step on the path to your own evolutionary branching point.