The sail-backed synapsid that predates dinosaurs by millions
Dimetrodon prowled Permian landscapes 294 to 271 million years ago with a tall neural-spine sail and heterodont jaws. Popular culture treats it as a dinosaur, yet it vanished forty million years before dinosaurs arose and belongs on the mammal side of the family tree.
Dimetrodon, whose name means two measures of teeth, lived from about 294 to 271 million years ago. Most species ran 1.7 to 4.6 metres long and 28 to 250 kilograms, ranging from the 60-centimetre D. teutonis up to D. grandis at 3.2 metres. It walked only on all fours, probably in a semi-sprawling stance somewhere between a lizard's and a mammal's, and its tall, narrow skull held teeth of several sizes, with a raised step and a gap in the tooth row near the snout tip. As a top predator it probably hunted fish, amphibians and reptiles.
For all its reptilian look, it belongs to the family closest to the therapsids, the lineage that produced mammals. A single opening behind each eye socket ties it to mammals, and small ridges inside the nose may have supported scent-detecting tissue, early forerunners of the structures later synapsids used to warm incoming air. Old labels such as pelycosaur and mammal-like reptile have given way to basal synapsid. The sail of lengthened vertebral spines may have braced the back or managed body heat, but big species with small sails and small species with big ones undermine the heating idea, and display to rivals or mates is now the favoured explanation.
The earliest bones, a jaw fragment found on Prince Edward Island in 1845, were named Bathygnathus borealis by Joseph Leidy in 1854 and moved into Dimetrodon in 2015; zoological naming authorities agreed in 2019 to keep the better-known name. Edward Drinker Cope described material from the Texas Red Beds collected by Jacob Boll, W. F. Cummins and Charles Hazelius Sternberg, coining the genus in 1878 with three species. He likened the sail to the crest of a basilisk lizard and confessed he could not picture what it was for.
E. C. Case, backed by the Carnegie Institution, later worked through many of Cope's thinly described fossils, and in 1940 Alfred Romer and Llewellyn Ivor Price's Review of the Pelycosauria sorted the species, most of which remain valid. Finds in Utah in 1966, Arizona in 1969, Ohio in 1975 and New Mexico in 1977 showed that a supposed inland seaway had not trapped it in Texas and Oklahoma, and in 2001 D. teutonis from Germany's Thuringian Forest carried the genus beyond North America. It vanished about 40 million years before the first dinosaurs.
Source: Dimetrodon
