Non-mammalian mammals

So, what is a mammal exactly?

You might have heard the old adage: “it’s an animal with fur, warm-blood and milk”. This is extremely fortunate, because every group of non-therian mammal I’m discussing in this series has all three of these things… because they evolved long before mammals did!

Wait, what?

You see, what I described to you is what is often called Linnaean Taxonomy, after Carl Linnaeus. It's the classical “class, order, family, genus, species” droning you’ve learned in high school or perhaps in a nifty textbook. It bases itself on classifying organisms by common traits.

But, the thing is, Linnaean Taxonomy is generally disregarded by modern biologists. This is because, unlike Linnaeus, who was a firm creationist, modern biologists generally regard evolution as fact. Now, evolution runs contrary to Linnaean biology, dismantling away common traits and making the line between “classes”, “orders”, “families” and even species extremely blurry. This is why some authorities are seriously considering doing away with Linnaean Taxonomy altogether.

Instead, most biologists prefer Cladistics. Cladistics prioritizes genetic relationships over organisation, and while groups of organisms, now called clades, duh - obviously share common traits, losing these traits does not eliminate the genetic background. A dove doesn’t look at all like an iguana, but both evolved from a common reptilian ancestor, and as such belong to clade Sauropsida, which includes most animals traditionally classified as “reptiles”.

Now, how does this relate to mammals?

In Linnaean Taxonomy, mammals are defined by the above traits. But we now know that warm-blood (also known as an endothermic metabolism), fur and even milk evolved in animals traditionally not considered mammals by paleontologists. We could extend the definition of mammals to suit them… but this would raise even more questions, because we don’t know how far back these traits go!

So, most scientists define Mammalia via its crown group. In other words, the groups that are still alive today AND everything extinct that evolved from the same common ancestor. As discussed before, living mammals are composed of therians and monotremes, so extinct animals within this monotreme-therian clade are mammals.

That said, many extinct groups discussed here are of uncertain affinities. They could be within the monotreme-therian clade… or outside, being “more basal”. Furthermore, the immediate outgroups to these animals are so mammal-like that they might as well be true mammals.

Now, dinosaur fans out there might have heard of the term “non-avian bird”. Much like mammals, birds face a similar problem in which what exactly defines a bird is still controversial. So paleontologists and ornithologists have decided to restrict Aves, the traditional bird “class”, to crown-group birds. This excludes extinct species outside of the crown-group, such as Ichthyornis, but such animals are still so similar to birds that even professionals describe them as “birds” in a vernacular sense, just not in a scientific one.

That’s what I’m going to use here.

So, I’m going to talk about mammalian mammals, which are groups within Mammalia, but also about non-mammalian mammals, which are synapsids closely related to true mammals and so similar that they might as well be mammals. Perhaps fittingly, all these groups belong to Mammaliaformes, a clade already defined by its similarity to true mammals.

I sincerely hope this hasn’t been too confusing, except not because I’m a dick.