If you answered fur/hair, three middle ear bones, a neocortex and milk, good for you. Most of you probably only knew two out of those and maybe had an inkling about the brain. The least commonly known feature about the three ear bones is actually really, really important.
That's ALL it takes to be considered a mammal. Live births are done by every vertebrate except birds; some mammals lay eggs. Even being warm-blooded is barely a qualifier, seeing as naked mole rats and a few other underground mammals have had to turn off their inner furnaces.
O hai.
Now, suppose that you were given an unknown skull. How would you know if it was a mammal or a reptile? You do not have fur, nipples or brain tissue to work with, so what is left?
Yep. Those darn ear bones.
During mammalian evolution, something weird happened to the mammalian jaw. It lost two components that evolved into two tiny little ear bones that gave them a greater range of hearing and a more solid jaw. Looking for these is a pretty good indicator that you have a mammal on your hands and not another lizard.
An even better indicator is the teeth. Reptiles rarely have differentiated teeth; they do not chew their food, they gulp it down. When they do have differentiated teeth, science usually cares enough to say it in Greek.
Skull of Heterodon ('different tooth') nasicus, the Western hognose snake. This snake will get its own entry at some point- probably whenever Kuro gets a legal permit to handle them.
Thus, Dimetrodon - 'two-measure tooth.'
Mammals had epic ancestors.
Dimetrodon was not a dinosaur. It lived during approximately the right age (but earlier than the Triassic) and had a fin like Spinosaurus, but it was not a dinosaur. Look at the legs and how very lizardlike they are; that should be a sign. More importantly, look at the teeth.
Dimetrodon was a synapsid - a distant relative of mammals. It was probably an apex predator. There are also several other sail-finned synapsids, but similar to how T-Rex is representative of an age when dinosaurs ruled the Earth (even though it was only around in the Cretaceous Period- the LAST period of the Mesozoic), Dimetrodon represents all synapsids. Rarely is it acknowledged as such, although The Simpsons did a pretty good job:
(Tyrannosaurus and Stegosaurus never lived at the same time, BTW.)
In many plastic Dimetrodon specimens, the teeth are homogenous. One look at the jaw on a real specimen tells you that they are not. There's a ridge on each side with three teeth that are clearly larger than the rest, indicating that Dimetrodon could shear smaller chunks of meat off of its kill for easier digestion. This is more characteristic of modern mammals than of lizards (although some dinosaurs did sport different teeth as well; T-Rex had a far more regular formation of two different sizes of teeth).
Compared to avian evolution, mammalian evolution gets very little press. Gee, I wonder why...
When the Good Book goes bad.
Modern mammals are still classified and distinguished by their dentition. Even though some mammals lack teeth entirely (anteaters and pangolins come to mind), this stuff still matters. Mammals are not as homogenous as most people think they are; they have just as obscure methods of classification as birds and reptiles.
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