But there were once others, many of them existing before placentals and marsupials (and presumably monotremes, although we don't have much in the way of fossils for them) came into existence. Technically speaking, this includes a few side-branches from the two man lines that arose before the last common ancestor of the living forms - for example, creatures close to the line that eventually gave rise to the placentals, but that arose early enough that we can't be sure that they literally had a placenta. But even once we trim out those, there are still quite a few left.
Some of these are very early forms that, depending on your definition, you might argue aren't really mammals at all. Others are represented only by a tiny number of specimens that we just can't place anywhere else, or that didn't last very long in the grand scheme of things. Certainly, by one way of looking at it, they were all ultimately failed experiments in evolution, given that they aren't around today. But, in fact, some of them lived for a vast period of time; the longest-lived group, the multituberculates, first appeared about 50 million year before the first mammals on the placental and marsupial lines did, and only disappeared around 35 million years or so ago. Which means that they survived for longer than any living group so far has, even defining the latter at their broadest.
Quite what multituberculates actually were, or which fossils they might have included, isn't entirely settled. For what it's worth, we can say that their pelvis doesn't look to have been wide enough for the females to have laid eggs, so they presumably gave birth to live young, most likely in a manner similar to living marsupials. On this basis, and backed by plenty of other skeletal and dental evidence, it is generally agreed that they are more closely related to placentals and marsupials than they are to monotremes. Which means that they are descended from the last common ancestor of living mammals (in technical parlance, they belong to the "crown group") but diverged from the "live birth" line before the ancestors of placentals and marsupials diverged from one another.
They are not the only fossil group of which this is thought to be true.
Probably the most successful of these other groups were the eutriconodonts. They first appeared around 190 million years ago, long before the earliest known fossils that can be ascribed to the living groups, and seem to have died out shortly before a killer asteroid heralded the dawn of the Age of Mammals. This places them entirely within the Jurassic and Cretaceous periods, making them clear contemporaries of the non-avian dinosaurs.
The classic image of mammals before the K/Pg extinction (see here for an explanation of why it is no longer the "K/T extinction") is that of small shrew-like animals scampering in the undergrowth beneath the feet of giant reptilian behemoths, or perhaps hiding up trees. This is by no means entirely false, but it's also far from being the complete picture. Eutriconodonts are perhaps the best embodiment of that principle.
The term "eutriconodont" translates as "true three-coned teeth", and indicates that, as with many of these early groups, they are defined on the basis of the shape of their cheek teeth. In this case, that's a relatively primitive pattern of three shearing triangles separated by grooves that interlock with the teeth in the other jaw. In fact, we currently know of only 34 species of eutriconodont, from all their millions of years of history, and all but nine of those are known only from their teeth or jaws. Well-preserved fossils of small animals from this far back are hard to come by.
|I have simplified this by deliberately leaving out a|
number of other extinct groups
Yanoconodon is one such animal, described in 2007 on the basis of an almost perfectly complete fossil. In life, the animal would have been about 12 cm (5 inches) long, plus tail, and would have looked somewhat like a large shrew. It lived about 125 million years ago in what is now China, but was then the eastern end of a great northern landmass that was only just beginning to split away from the southern half of the Pangaea supercontinent. The original paper focussed primarily on its middle-ear bones - a clear diagnostic feature often used to separate mammals from their immediate ancestors - which retained a small cartilage that is lost well before birth in living mammals.
This wasn't the only primitive feature in the fossil, though. In most living mammals, the backbone is divided into five segments. Starting with the front end (or top, if you're human), first come the cervical vertebrae, which form the neck, then the dorsal vertebrae, to which the ribs are attached. Next are the lumbar vertebrae, behind the ribs, then the fused sacrum, to which the pelvis is attached, and then the caudal bones of the tail. But, in Yonoconodon, the lumbar vertebrae also had their own ribs, all free-floating at the tips, and not attached to the breastbone, and becoming increasingly small the further they got back, until they were no more than tiny nubs either side of the backbone.
This is not unprecedented; monotremes do, in fact, have lumbar ribs, as did a number of other fossil mammals alive at the time of the dinosaurs, and it is not at all unusual in living reptiles. So we can assume that this is the ancestral pattern, which virtually all living mammals have lost in favour of a more well-defined ribcage, but that was still retained in this particular primitive one.
As I said above, so far as we can tell, all eutriconodonts ate other animals. But this could be anything from insects, to shrimp, to much larger prey. Most notable in this respect is Repenomamus, the largest known mammal to live before the K/Pg extinction. This was something like a metre (three feet) long, and is known to have eaten baby dinosaurs, and perhaps even the adults of some of the really small species.
Repenomamus was a contemporary of Yanoconodon, and lived in broadly the same area, but clearly the latter isn't large to feed on non-avian dinosaurs. So it was probably eating small invertebrates or the like, but which ones? A comparative study of a range of Mesozoic mammals concluded that it may, like otters, have been semi-aquatic, and now this has been followed up with a really detailed look at its skeleton.
The study concludes that the animals had unusually flexible shoulder, elbow, and ankle joints, as well as short, powerful, fore-limbs. They probably walked with a sprawling gait, with their legs partially out to the side, as they are in a lizard, but the strength of the fore-limbs might suggest that they were good at digging, and there some similarities with gophers in that respect. However, they don't appear to have had the necessary muscle attachments to be really good at digging - although the soft soil of a river bank might not have been much of a challenge. Their small wrists and large flat fore-feet, apparently lacking the tough claws of a digging animal, look, the authors suggest, as if they would have been good for paddling through the water. The hind limbs look to be suited to scrambling about over rough surfaces, and they may even have been able to climb.
In fact, the authors suggest mink as the closest modern analogy to Yanoconodon, although the latter were only about half the size of the former. So, not quite otters, perhaps, but certainly spending a lot of time in the water, and perhaps feeding on some aquatic invertebrates or particularly small fish. The structure of the pelvis is such that, as expected, they were unlikely to be able to give birth like a placental mammal, but whether they laid eggs or were more like marsupials remains a mystery.
All in all, this adds to the diversity of this very early mammal group, existing long before the forms we are familiar with began to appear. We can add "semi-aquatic" to the range of possible lifestyles for eutriconodonts, alongside gliding (we actually have the imprints of gliding membranes for one species), tunnel-digging, climbing trees, and just running about on the ground.
Not to mention eating dinosaurs.
[Painting by Nobu Tamura, from Wikimedia Commons.]