The dimetrodons were the dominant carnivores of the Permian period. They were not giants: 2 to 3 m long, resembling a Komodo dragon more than a dinosaur, a big, slow, thickset reptile with a face full of teeth. And a huge metre-high sail of skin sticking straight up from their back. The dimetrodons are also called sailbacks, and it is this extraordinary feature that makes them so memorable.

Almost everything went extinct at the end of the Permian, about 250 million years ago, and their place was eventually taken by their distant cousins the dinosaurs. But nearer to the dimetrodons are the mammals. Together they form a major branch of the reptiles*, called the synapsids. The non-synapsid branch contains almost all the others (dinosaurs, snakes, turtles, and so on). This division is based on the arrangement of fenestrations, holes in the side of the skull, which might have been for lightening the load and giving more space for muscle attachment. The primitive reptilian condition is anapsid, with no such holes, and the Chelonia (turtles and tortoises) are still anapsid. The more familiar reptiles are diapsid, having two fenestrations. Members of the clade Synapsida have one fenestration, at least in the ancient forms: modern mammals, though clearly synapsids by descent, have now occluded it by moving skull bones around, so there is no longer a visible hole in the side of the skull.

Dimetrodons belong to the Pelycosauria, a branch of the Synapsida, and are close to the direct ancestor of Therapsida, the line that gave rise to Mammalia. Pelycosaurs emerged in the late Carboniferous, and account for 70% of known reptilian genera from before the great Permian-Triassic extinction. Not all of them had sails; in fact, it appears that sails evolved more than once independently in the pelycosaur lineage. Another sailbacked pelycosaur was the herbivorous Edaphosaurus.

Each vertebra sported an upright spike, short at one end, much much taller in the middle, then short again at the other end, rather like a bell curve. The spikes are grooved at both front and back, suggesting they carried blood vessels, and they would have been joined by skin. So this makes thermoregulation a good candidate for their purpose; though attracting mates and looking big and scary are other possibilities.

The name means "two measures (i.e. shapes) of tooth", these being sharp canines and shearing teeth. It seems to be the first land creature to make the Big Fierce Savage Monster niche its own.

Colin Tudge, The Variety of Life (Oxford, 2000)
http://tolweb.org/tree?group=Synapsida&contgroup=Amniota
http://www.prehistory.com/dimetrod.htm
http://www.amnh.org/exhibitions/expeditions/treasure_fossil/Fossils/Specimens/dimetrodon.html
http://www.suite101.com/article.cfm/paleontology/29045

* The traditional taxon Reptilia included Synapsida as well as typical diapsid reptiles like lizards. In modern cladistic terms this would have to include all its descendants, including mammals, so now we call this whole group Amniota. Mammals are amniotes; so we don't need to say they're reptiles. Given this, some authorities use the term Reptilia to mean just the non-synapsid branch of the amniotes, and in this case dimetrodons aren't reptiles either.

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