What it is
A four-legged hunter, about three meters long, with a tall sail along its back. It was one of the biggest hunters on Pangaea, walking the river plains near the sea, where life crowded toward the wet edges.
Not a dinosaur
It turns up in almost every box of toy dinosaurs. But it lived on your branch of the family tree, the one that leads to mammals, and it was gone about forty million years before the first dinosaur. It is closer kin to you than to any dinosaur.
Its teeth
Its name means two sizes of teeth: it had big fangs and smaller teeth in the same jaw. A 2014 study found that some kinds of Dimetrodon also grew teeth with tiny saw edges, like a steak knife. It is the oldest land animal known with teeth like that.
Still a mystery
The sail was skin stretched over long spines growing up from the backbone. Nobody knows for sure what it was for. Soaking up the morning sun and showing off to rivals or mates are the leading ideas.