The great majority of living mammals species are placental mammals, the marsupials representing what is, today, a comparatively small side-group. They are distinguished by the young gestating in the womb for a comparatively long period of time, taking in nutrients through a fully-formed placenta. There are other features that unite them, too, such as the basic number of teeth, although these are often obscured by the considerable evolution and change of form that has occurred in some placental groups to create animals as diverse as horses and dolphins.
The placentals have been the dominant group within the mammals for a very long time. When we look back at fossil mammals that lived since the extinction of the non-avian dinosaurs, we find that most can clearly be placed as placental. This obviously includes creatures such as sabretooth cats and mammoths, since we can place those in living families, but it also includes many extinct families, even where their other relationships may be obscure.
Often, when we push back through the fossil history of some well-known group, we run into the concept of "stem groups". These are animals that, while clearly related to the earliest ancestors of the modern group, don't seem to have left any surviving descendants; they are early twigs off the side of the relevant family tree that don't end up going anywhere. For example, take Stenoplesictis, an animal related to early cats that was not itself an actual cat and may, in fact, be equally related to at least two other mammal families (one extinct, and one living, but obscure).
The cat family is itself a branch of the order Carnivora, which also includes dogs, bears, and so on, along with all of their basal relatives. But if we look back far enough, we find stem groups to the order as a whole, too. Creatures such as Miacis were more closely related to carnivorans than anything else alive today, but they're not thought to be descended from the common ancestor of all living carnivorans, and don't count as examples of them. We can keep going.
It logically follows that the same thing must be true of the placental mammals as a whole; there must have been, at some point, animals that were not themselves placental, nor belonging to any other living group, that were, nonetheless, more closely related to placentals than they were to marsupials. Another way of looking at this is that there must have been, if we go back far enough, a "first placental" - and that animal must have had relatives, since it didn't come from nowhere. Perhaps some of those relatives died out shortly thereafter, but some may have left descendants of their own; the only requirement is that such descendants don't survive today.
And, we do, indeed, have a number of fossils that fit this description. We have to go a long way back to find them, but they do exist, and are collectively referred to as non-placental eutherians. Perhaps the best-known are the cimolestans and leptictidans, since both of these groups managed to survive at least halfway through the Age of Mammals, dying out during, or probably shortly after, the Grande Coupure. But there are others.
In 1975, Polish researcher Zofia Kielan-Jaworowska published a description of a fossil uncovered in the Gobi Desert of Mongolia, naming it Asioryctes. She originally considered it to be a type of cimolestan, but it is now considered to lie outside that group, perhaps representing an even earlier branch, and typically placed in its own order alongside its close relatives Kennalestes and Ukhaatherium.
Previously known only from a single rock formation in a desolate part of the Gobi, fossils belonging to the same species have recently been reported from another formation nearby, indicating a wider distribution in time than previously realised. Neither formation is well-dated, and it's not entirely clear which of the two is older, but it is thought that both date to between 84 and 72 million years ago. This is during the Late Cretaceous, and, for one point of reference, at least 4 million years before Tyrannosaurus rex first evolved. So, in mammalian terms, a long way back indeed.
The initial description was based on a skull about 25 mm (1 inch) long, but this was well preserved, making it possible to identify a row of sharp, pointed teeth. An interesting point here is that it had five incisors on each side of the upper jaw, and four on each side of the lower - no true placental has more than three on each side of either.
Unusually, considering the age we're talking about for an animal so small and delicate, a more complete skeleton was discovered a few years later - at the time, the oldest known postcranial skeleton of any placental-related mammal. This revealed that the feet do not seem well-adapted for climbing, lacking an opposable thumb for gripping. This doesn't entirely rule out the possibility that it ran along branches like a squirrel, but it weakens the case for tree-climbing, and, in any event, the formations in which the fossils were discovered look to have been treeless deserts at the time it lived. This contradicts our usual notion that the mammals living alongside the dinosaurs spent much of their time hiding up trees; even a particularly primitive example like this seems to have lived on the ground.
The deserts in question can't have been entirely barren, because several other fossil animals, including birds, lizards, non-placental mammals, and even dinosaurs such as ankylosaurs and oviraptors, are known from the same deposits, apparently living alongside these small creatures. Indeed, the sharp, cutting, teeth, suggest that Asioryctes was anything but herbivorous, even if, at the size of a small rat, it wasn't eating anything particularly large.
What we can't say, partly because we don't have a pelvis, is how it reproduced. It's not a "placental mammal" in the sense in which we define that term taxonomically, but that doesn't mean that it didn't have a placenta. It seems unlikely that the last common ancestor of all living placental mammals was also the first creature to develop that feature, so at least some of their close relatives must have had them, too. But, equally, the true placenta must have evolved since the split between the last common ancestor of placentrals and marsupials, so some of the stem eutherians must have lacked it.
On which side of that line Asioryctes lived is likely to remain an unanswered question.
[Photo by Robert Niedźwiedzki, from Wikimedia Commons.]
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