Life › Seas, forests & the move to land › Synapsids: the mammal line
Dimetrodon
c. 295–272 million years ago· Fossil or site· Animals
Well establishedStrong fossil, genetic or geological evidence.
A sail-backed early Permian predator, a synapsid closer to mammals than to dinosaurs.
See it on the Life map →What happened
Dimetrodon ("two measures of teeth", for its different tooth sizes) was a top predator in what is now Texas and Oklahoma, with one species from Germany. Species ranged from about 1.7 to 4.6 m long. A tall sail of skin stretched over long spines rising from its backbone. It lived about 295–272 million years ago and died out roughly 40 million years before the first dinosaurs. Its skull had the single opening behind each eye that marks synapsids, our own line.
Why it matters
It shows that early mammal relatives, not dinosaurs, were the first big land predators, and that popular "dinosaur" images often mix up very distant branches.
The evidence (fossils, genes, rocks)
Many skeletons from the Red Beds of Texas, first named by Edward Drinker Cope in 1878. Its skull and teeth place it firmly among synapsids.
Still debated
What the sail was for, temperature control or display to rivals and mates, is still debated; many recent studies doubt the heating idea.
Connections (2)
▸Synapsids: the mammal line include DimetrodonWell established
The connection
Dimetrodon belongs to the early synapsids once lumped as "pelycosaurs". Its skull shows the single opening behind each eye that defines synapsids. Its family, the sphenacodontids, is the closest known relative of the therapsids, the later synapsids that led to mammals, so it sits near our own ancestral line.
When and how
Early Permian, about 295–272 million years ago, in the Red Beds of Texas and Oklahoma and in Germany; named by Edward Drinker Cope in 1878.
The evidence
Skull, jaw and tooth anatomy, studied since the 1800s and tested in modern family-tree analyses, place Dimetrodon firmly among synapsids.
Debates and open questions
Sails evolved separately in Dimetrodon and the plant-eating Edaphosaurus; why sails arose more than once is still studied.
▸Dimetrodon is often mistaken for, but predates, DinosaursWell established
The connection
Dimetrodon often turns up in dinosaur toy sets, but it was a synapsid, closer to mammals than to any dinosaur. It died out around 272 million years ago, about 40 million years before the first dinosaurs appeared and even before the Great Dying.
When and how
Dimetrodon lived about 295–272 million years ago in North America and Europe; the first dinosaurs date to about 233 million years ago.
The evidence
Rock dating separates the two by tens of millions of years, and skull anatomy places Dimetrodon on the mammal side of the amniote family tree.
Debates and open questions
Little is disputed about this link; the open question is why sail-backed forms evolved and what the sail did.