What Was Dimetrodon? The Sail-Backed Animal That Was Not a Dinosaur

The amniotes, the vertebrates that lay eggs with a membrane and so escaped the water entirely, split early into two great lineages. One, the sauropsids, leads to lizards, snakes, turtles, crocodiles, dinosaurs and birds. The other, the synapsids, leads to mammals, and this animal is an early member of it. The distinguishing feature is a hole in the skull behind the eye, a single opening on each side that anchors the jaw muscles, and it is visible on your own skull if you put your fingers on your temple and clench, since the muscle you feel passes through it. Sauropsids have two such openings; synapsids have one, which is what the name records. Calling the sail-backed animal a mammal-like reptile is the old textbook phrase and is now avoided, since it was never a reptile at all but a member of the lineage reptiles are not part of.

It lived through the early Permian, roughly 295 to 272 million years ago, mostly in what is now Texas and Oklahoma, where the red beds of the region have produced it in quantity. Adults of the larger species reached three to four and a half metres and perhaps 250 kilograms, sprawling on legs held out to the sides rather than underneath, with a heavy tail and a large head. The name means two measures of tooth and refers to the feature that mattered: unlike the uniform pegs of most early reptiles, this jaw had teeth of more than one kind, with enlarged stabbing canines behind the front of the snout and smaller slicing teeth behind, some of them serrated. Differentiated teeth are a mammalian trait appearing very early, and they indicate a predator that could kill with a bite rather than simply grip. It hunted sharks, amphibians and other reptiles in a swampy delta landscape, and it was the largest predator of its world.

The structure everyone recognises is built from enormously elongated spines rising from the vertebrae, up to a metre and a half tall, which in life carried a web of skin with blood vessels running through it, judging by grooves left on the bone. What it was for is still argued, and the candidates are the usual ones:

• •Temperature regulation, the traditional answer: a large surface presented to the morning sun warms the animal quickly, and turned edge-on it sheds heat. Calculations suggest the effect is real but works better for large individuals than small ones
• •Display, now the favoured explanation: the sail grows faster than the body, appearing fully only in large adults, which is the pattern typical of a structure used in courtship or threat rather than one solving a physiological problem
• •Species recognition, since several sail-backed animals shared the same landscape and the spines differ in shape between them
• •Stiffening or muscle attachment, proposed occasionally and not well supported, since the spines are round in cross-section rather than blade-like

The early Permian of Texas was a low coastal plain of rivers, swamps and lakes, with forests of primitive conifers and seed ferns, giant amphibians in the water and the first substantial land ecosystems in which large plant eaters fed large predators, a structure that had not existed before. Its usual prey included Eryops, a heavy amphibian, and Diadectes, a bulky plant eater, and the other sail-backed animal of the same beds, Edaphosaurus, was a herbivore with a similar structure built from spines carrying crossbars, evolved separately. The wider group, the pelycosaurs, was the dominant land fauna of the period and was replaced during the middle Permian by the therapsids, a more advanced synapsid group with legs beneath the body, better-differentiated teeth and probably some degree of internal temperature control. From within the therapsids came the cynodonts, and from the cynodonts, in the Triassic, the first mammals. The line runs directly.

The mistake is understandable and almost universal. It is large, extinct, reptilian in build, found in the same red rocks that yield fossils to the same museums, and was named in 1878 by Edward Drinker Cope during the same American bone rush that produced the famous dinosaurs. It then entered popular culture through toy sets and film, most conspicuously in the prehistoric sequence of Fantasia, fighting a Tyrannosaurus in a scene separated in reality by more than 200 million years. Palaeontologists have been correcting it for a century with limited success, and the correction matters for a reason beyond pedantry: the animal is evidence that the mammal lineage was already large, differentiated and ecologically dominant long before the dinosaurs existed, and that the story of mammals as small creatures waiting under the feet of reptiles describes only the middle chapter.

The sail-backed predator of the early Permian belongs to the synapsids, the branch of amniotes that leads to mammals, identified by the single opening behind the eye through which the jaw muscle passes, and it died out around 40 million years before the first dinosaurs. It grew to four and a half metres, had teeth of more than one kind, which is an early mammalian trait, and carried a skin sail on elongated spines that most likely served display, since it develops fully only in large adults. Its relatives led through the therapsids and cynodonts to the first mammals.