|Eomaia scansoria - no longer the oldest known eutherian|
Since 2002, the oldest known fossil of a possibly placental mammal was that of Eomaia. It is remarkably well preserved, even including impressions of the animal's fur, and dates from 125 million years ago. The oldest known marsupial fossil, Sinodelphys, dates from around the same time and place, and there had been reason to suppose that the two lines separated not long before that.
But just how long ago is that? I have recently been using charts to show the age of various fossils within the Age of Mammals. That entire era has lasted (so far) 65 million years, so to get back to Eomaia we need to head back almost as far again as the whole period covered by the chart. It was a time before such famous dinosaurs as Tyrannosaurus and Triceratops had evolved, and the best known animals of the time are perhaps Iguanodon, Kronosaurus, and Deinonychus (on which the 'raptors of the Jurassic Park films seem to have been based, although they decided to use the cooler sounding name of a rather smaller dinosaur instead).
Placental Marsupials Monotremes
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The diagram above shows only the living groups of amniote, and leaves out a range of early mammals whose ancestry diverged from their relatives before the great placental/marsupial split. The only living group of which that is true are the peculiar monotremes, including the platypus, whose own evolutionary history remains somewhat mysterious.
However, while much evidence pointed to the ancestors of the placentals and marsupials having diverged around 131 million years ago, there was always some uncertainty about the date, and some recent studies had suggested that it may have happened much earlier. This evidence comes from concept of a "molecular clock". The idea is that, over a sufficiently long period of time, DNA in a particular group of animals tends to accumulate mutations at a more or less constant rate. Therefore, by counting up the number of genetic differences between two groups of animals, we can come up with a figure for how long ago they separated.
There is a degree of uncertainty in this, especially when we look at really long periods of time, as is the case here. The mutation rate may not necessarily have been constant over all this time, and calibrating the clock (that is, figuring out what the rate is) isn't always easy. Now, those scientists who had calculated a date for the divergence of well before 131 million years ago have been vindicated, by the announcement of a new fossil that shatters the record set by Eomaia. In a paper in Nature, Zhe-Xi Luo of the Carnegie Museum and colleagues report the discovery of Juramaia sinensis (the name means, roughly, "Jurassic mother from China"), a 160 million year old fossil.
This, of course, pushes us back much further back in time. Most of the dinosaurs we know from this exact epoch are also from China, and not so well known to the general public in the west, but they do include close relatives of animals such as Diplodocus, Allosaurus, Stegosaurus, and for that matter, Archaeopteryx, all of which lived around 155 to 150 million years ago.
An obvious question is how on Earth we know that these fossils represent placental mammals? And we don't, really, since the details of the reproductive system that define the group today don't fossilise. When a fossil is obviously closely related to some group of placentals alive today, we can say with some confidence that it, too, was a placental, but with anything this far back that just isn't possible. However, there are a number of features, especially in the detailed shape of the teeth, that are different between placentals and marsupials, and that do preserve in fossils.
This particular fossil is relatively complete, including the whole of the head and a little over half of the body, including both front legs. There are enough features here to allow us to be fairly confident that the animal is, indeed, an early member of the placental lineage, not of that which led to marsupials. Although we have no clear evidence either way, I personally suspect that this animal would not have been a true placental as we know them today; most likely it gave birth to tiny, undeveloped young, as modern marsupials do, and the more complex system of placentals evolved later on.
Since we know nothing of the placenta of these animals, when we're talking about early fossils of the "placental" lineage, we instead call them "eutherians". This word literally means "true beasts", and distinguishes them from the marsupial line, or "metatherians". So what we really have here is the oldest known eutherian, not - necessarily - the oldest true placental.
What of Juramaia itself? The animal was about the size of a mouse, with a slender body, a long snout, and numerous sharp teeth. The teeth were somewhat similar to those of living shrews, and like them, it probably fed on insects and other small invertebrates. Just as interesting, though, is the shape of the front paws (the hind paws are unfortunately missing). These have long toes, the bones of which have what appear to be attachment points for large tendons. Again looking to modern mammals for our comparison, this structure suggests that the paws were good at grasping.
This sort of grip is common in climbing, tree-dwelling, animals, and the shape of the shoulder bones further supports that idea. Although they lived much later, both Eomaia and Sinodelphys also have paws that suggest they lived in trees, much as opossums, tree shrews, and the like do today. While the dinosaurs prowled the ground, these early mammals sheltered in the trees, and it may even be that their first movement into the safety of the branches was a key factor in their early success, and in the origin of the two most successful mammal groups of the present day.
[Picture from Wikimedia Commons]