In order to first explain why our favourite sail-backed reptile is not a dinosaur, let's bring us back to one of the very first Mesozoic Mondays post on What is a dinosaur? Fortunately, this explanation is shorter than why pterosaurs were not dinosaurs. Remember back in the beginning when I was talking about holes in the back of the skull? The temporal fenestrae as we call them? Well if you remember, dinosaurs belonged to the group called the diapsids, which have two holes, one above and one below. Dimetrodon, on the other hand,belongs to the group called the synapsids, which have only one hole in the back of their skull. This is a very fundamental difference, and something that developed quite early in the evolution of tetrapods.
|Skull of Dimetrodon. The first hole on the left is the temporal fenestra, which distinguishes it as a synapsid.|
Dinosaurs definitely weren't the only formidable predators around in the past. Long before the dinosaurs, Dimetrodon was terrorising the small animals with its long caniniform (canine-like) front teeth, and remarkably strong bite force. At up to 5 and half metres long and 250 kg, he could pack a punch. The sail-like structure on it's back also helped it be a faster predator. As Dimetrodon was an ectotherm, meaning its body temperature depended on its surroundings and could not be controlled internally, a way of helping to warm it up and cool it down is very handy. The sail on its back was actually formed by a highly vascularised (so it had lots of blood running through it) membrane stretched between elongated spines of its vertebrae. This membrane acted as a thermoregulator, increasing its temperature quickly when it went into the sun, and decreasing quickly in the shade. This allowed some control over its body temperature, which means it could be more active when it wanted to be. Reptiles are very slow and sluggish when it gets too cold, and this is a way of adapting to that.
|Dimetrodon. Image from Wikimedia Commons user DiBgd|