We’ve been dancing all around the start of the mammals – mammal-like reptiles, synapsids, pelycosaurs, therapsids. All related, but, generally, not quite mammals. And depending somewhat on exactly how you define a mammal, and how you look at the fossils, we just might have had real mammals by late Triassic time – we talked about those critters on September 23, with a question mark because it’s not completely agreed that they really were mammals.
By the Jurassic, the arguments are over. The eutherians, a group whose name means “true beasts,” include all the modern placental mammals – which means all mammals except the marsupials and their close relatives. Today, about 90% of all mammals, including humans, are placentals, and the rest are marsupials like opossums and kangaroos. The oldest eutherian is named Juramaia sinensis, a name meaning “Jurassic mother from China.” It was described and named in 2011 and dated to 160 million years ago, about the boundary between the middle and late Jurassic epochs.
|Juramaia reconstruction drawing by Nobu Tamura (source)|
The discovery and date of Juramaia pushed the age of mammals back 35 million years and also indicates that the placental and marsupial lines of mammals diverged at least that long ago.
Like so many early mammals, Juramaia was probably an insectivore and the bones in its feet indicate that it was probably arboreal, climbing through trees in search of prey. It was only four or five centimeters long – a couple inches – and it was still very much shrew-like in its basic plan. It appears that mammals maintained a pretty low profile – a small size – for many millions of years, and presumably that was a survival advantage at a time when the world was dominated by larger animals including dinosaurs. The image of little mammals scurrying through the underbrush and up in the trees, avoiding predators, is a pretty good one.
Another Chinese fossil from the Jurassic is a little more debatable as an early mammal. Haramiyids were herbivores that may be ancestral to extinct multituberculates, egg-laying mammals that in turn might be related to modern platypuses. Haramiyids are known from the late Triassic, and if it can be agreed that they were true mammals, that obviously pushes the origin of mammals back that far – but that agreement isn’t there, yet. Jurassic haramiyids from China were squirrel-like creatures with long, possibly prehensile tails like monkeys. Like Juramaia, they lived in trees. So it would appear that placental mammals and non-placentals were occupying the same ecological niches, at least in some places. Haramiyids were extinct by late Jurassic time, so far as we know.
You can imagine that preservation is challenging for mammal fossils. Generally, when an animal died, it might have been eaten by its predators or scavenged by other organisms, and even more likely, would have rotted away on a forest floor. So unraveling the early history of the mammals, during Triassic and Jurassic time, has been challenging.
I have links below to a couple really good articles by Brian Switek about mammals, including one on when mammals got fur. Spoiler alert – they probably got fur before they diverged into the modern branches of placentals and marsupials.
So mammals, true mammals, are finally definitely on the scene. This might be a good time to remind you of the scale of our calendar – it’s not a proper scale, because I want to spend a month with each of the periods of the Paleozoic and Mesozoic eras. If it was at a true scale, this first Jurassic mammal, 160 million years old, wouldn’t appear until December 19. The Precambrian wouldn’t end until mid-November, and everything we’ve talked about since February first would be concentrated into the last 6 weeks of the calendar.
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John William Dawson was born at Pictou, Nova Scotia, October 13, 1820. He was a prominent Canadian geologist, often considered to be one of the fathers of paleobotany because of his studies of Paleozoic plants in Canada. He was a professor at McGill University and was first President of the Royal Society of Canada.
—Richard I. Gibson
When did mammals get furry?
BBC report on Juramaia
Juramaia reconstruction drawing by Nobu Tamura http://paleoexhibit.blogspot.com/ http://spinops.blogspot.com/ http://www.palaeocritti.com , used under creative commons license.