Dimetrodon
NOT a reptile
If there was one vertebrate animal that was iconic for the Permian, about 295 to 272 million years ago, it would be the fin-backed Dimetrodon, discovered in 1878. When I was in college, dimetrodon was considered to be a reptile, and my historical geology textbook said, “the reason for such extraordinary specialization is entirely problematical,” meaning of course, we didn’t know what the fin was for. But even then they were often called mammal-like reptiles, for some skeletal characteristics that appeared to put them closer to mammals than reptiles.
Today dimetrodons and their relatives, pelecosaurs, a term that isn’t used much anymore, are considered to be synapsids, the group that includes mammals and mammalian ancestors that we would probably not call mammals today. I’ve seen synapsids given as a class of the tetrapods or four-limbed animals. We’re familiar with the other classes of tetrapods – amphibians, reptiles, birds, and mammals. Synapsids might be better considered to be a group that includes primitive ancestral forms that probably would not be classed as mammals today, together with modern mammals themselves, and they are sometimes called stem mammals or proto-mammals.
As synapsids, dimetrodons are not reptiles, but not quite mammals either. In early Permian time they became the largest land vertebrates, up to 15 feet long. Their fossils have come almost entirely from Texas and Oklahoma, where they lived in lowland deltaic wetlands, but there are dimetrodons from Germany as well. There are at least 12 species of fin-backed dimetrodons. The spines on dimetrodons’ backs extend from their spinal vertebrae, making a sail-like fin as much as 10 feet high. The speculation that they used the sail for temperature regulation – warming the blood circulating through it, and radiating heat to cool the animal – dates back to 1940, but the question isn’t settled yet. Plenty of work has been done, but with little other than bones to go by, such a characteristic is really difficult to prove. An alternative explanation is that dimetrodons exhibited sexual dimorphism – males and females had consistently different body features – and the fin might have been related to some kind of mating display.
Dimetrodons are often thought of as early dinosaurs, but they were extinct at least 40 million years before the dinosaurs appeared. They didn’t even make it to the Great Dying at the end of the Permian; dimetrodons were extinct by the middle Permian, about 272 million years ago. All the other synapsids except two lines were extinct by the Permian-Triassic extinction event. One of those two lines, the cynodonts, ultimately led to all modern mammals.
The classic drawing above, by Alfred Romer (1894-1973), was the long-standing traditional reconstruction of dimetrodon. For a discussion of modern takes on this, see this blog post by Scott Harmon.
Dimetrodon means “two measures of teeth.” Synapsid is from Greek words meaning “together arch,” reflecting a feature of their skulls that produces a single opening and a fused arch behind each eye, versus two such openings in their earlier relatives.
The 2015 conception of the skeletal reconstruction also makes the bearing of the fin a dynamic part of the creature rather than a clumsy adjunct to essential parts.
It's fun to consider that although the synapsids didn't survive the Permian Extinction, we modem humans carry a little of Dimetrodon in our ear bones.. the three bone anvil, stirrup, hammer structure. Thanks, cynodonts!