Prehistoric Mammal Discoveries of 2020

The largest known Oligocene whale was
moved into the new genus Ankylorhiza
this year

Well, that year was... different. Fortunately, it didn't stop me posting and, indeed, a cancelled summer break meant that you got one more post than anticipated you lucky people, you. But as 2020 finally passes into the history books where it can bloody well stay, it's time for the end-of-year roundup of paleontological discoveries that didn't make the regular blog. As always, no theme, just a random assortment of journal papers that may, for all I know, be thoroughly contradicted by this time next year. But this is a collection of things that some scientist, somewhere, thinks that they demonstrated in 2020:

Down, Down, Deeper and Down

Ictidosuchoides also survived the extinction

We've all heard of the extinction event that wiped out the non-avian dinosaurs, leaving the mammals free to diversify and take their current role (along with the birds) as the most visibly numerous land-dwelling vertebrates. But this was not the only major extinction event in the history of the Earth.

How many there actually were rather depends on how big one has to be before being considered 'major'. Part of the reason that we can divide geologic history into so many periods and other subdivisions is that many of them ended with an extinction event of some sort, so that the creatures living afterwards looked different from those living beforehand. But that's not always the case, and, anyway, not all extinctions were equally large and dramatic - at least so far as we can tell from the fossils left behind.

Hearing Through (Part of) Your Jaw

The skull of Diademodon, a 6 foot long cynodont

The most defining feature of mammals alive is that they give milk to their young. We can be reasonably sure that most fossil mammals that we know of did the same, but it's of no use for fossils at the very origin of the group. Neither are most of the other features that we might use to identify mammals today, such as the presence of sweat glands, or, given that the earliest mammals won't have been as water-adapted as dolphins, hair.

Instead, as I've discussed elsewhere, we have to pick a feature that survives in skeletons. Any decision we come to has to be arbitrary, and it's not going to line up with the very first point that mammals started giving milk to their young, since we don't know when that was. But one has to draw the line somewhere, and the one we've picked is the structure of the middle ear.

The mammalian middle ear consists of a cavity in the skull crossed by three tiny bones, connecting the outer ear (the visible bit, and the tube that runs in from it) to the inner ear, where sound is converted into neural signals that can be sent to the brain for interpretation. The first of these three bones is the malleus, or "hammer", which attaches to the inside of the eardrum at one end and to the second bone, the incus or "anvil", at the other. Finally, the incus connects to the innermost bone, the stapes, or "stirrup", which is attached, at its other end, to a particular part of the inner ear called the fenestra ovalis or "oval window".

Dog-toothed Beasts from the Time Before Dinosaurs

Endothiodon

What sort of animals did mammals evolve from? For birds, the answer is simple: they evolved from reptiles, and specifically, from dinosaurs. (Which is why we say, using the modern definitions of things, that birds are dinosaurs). With mammals, though, the situation isn't quite the same.

The first mammals appeared around 225 million years ago, not long after the first dinosaurs did. However, they were not descended from reptiles, but from an entirely separate evolutionary line - the Synapsida - that had lived alongside the reptiles for a very long time. Yes, if you go back far enough, reptiles and mammals do have a common ancestor, and that animal would have looked, to modern eyes, more like a reptile than it does a mammal. But it wasn't a reptile, not in the modern scientific sense, and it belonged to a group of animals that just aren't around any more.

But between the point when that animal lived, and the first mammals appeared, there were a whole host of other, non-mammalian, synapsids. We often divide these creatures into two broad "grades" of evolution: the early, reptile-like, pelycosaurs, and the later, more mammal-like, therapsids. (In fact, of course, the two lived together for quite some time - it's not that one simply turned into the other. Evolution is more complicated than that). A greatly simplified tree of the "true" therapsids is included below, and shows that early mammals had rather a lot of relatives.

Life of an Almost-Mammal

Scaloposaurus, a therocephalian.
This artist has chosen to show it hairy, which is plausible,
but uncertain.

When I was at school, I was taught that mammals evolved from reptiles. However, at least under the modern definition of 'reptile', that isn't really true. In fact, the mammals belong to their own, distinct, evolutionary lineage, one that has existed alongside the reptiles ever since the latter first came into being. This is a staggeringly long period of time - by definition, it's from long before there were any dinosaurs, for example.

However, this evolutionary group didn't just include the mammals themselves. Mammals may not have evolved from reptiles, but they haven't been around anything like as long in the fossil record. So, if we look back to before the first mammals evolved, we find that there were many other kinds of creature belonging to this group that are no longer around today. Because evolution isn't really a ladder, some of these animals survived alongside the early mammals. In fact, the last ones didn't die out until well after the split between placentals and marsupials.

Studying fossils of these creatures can tell us a lot about where mammals came from, and how they became so different from the reptiles.