#eutheria

African bush elephant (Loxodonta africana)

Cimolestans were one of the major mammal lineages that survived through the K-Pg mass extinction 66 million years ago. Closely related to early placentals, they had a burst of diversification during the first half of the Cenozoic and rapidly evolved into a wide range of specialized forms – some uniquely weird, and others convergently resembling more familiar modern animals like squirrels, bears, ground sloths, and hippos.

And one group known as the pantolestids were incredibly otter-like.

(Because synapsids love them some lutrinization.)

Palaeosinopa didelphoides here lived during the mid-Eocene, about 52 million years ago, in what is now the Mountain West region of the USA. It was similar in size to a small otter, about 1m long (3'3"), and had a streamlined body with a well-muscled neck, short powerful forelimbs, slightly longer hindlimbs, and a very long tail.

Inhabiting a subtropical lake ecosystem, it probably swam using both hindlimb paddling and otter-like tail undulations. Its strong jaws and teeth suggest it was specialized for crunching hard shellfish prey, but so far preserved gut contents have only shown fish bones and scales. Fairly large claws indicate it was also able to dig out burrows similarly to modern otters and beavers.

Although pantolestids were never particularly common animals they were quite widespread, expanding their range from their evolutionary origins in North America across to Europe and eventually into Asia. A cooling and drying climate at the end of the Eocene seems to have driven most of the group into extinction alongside all their other cimolestan relatives – but a few of the Asian species clung on slightly longer as the very last of their kind, with the last known fossils dating to about 28 million years ago in the early Oligocene.

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Juramaia – Late Jurassic (160 Ma)

Just who is this beautiful boy? This chubby, shrew-looking fellow? He’s only a few inches long, and he has a sharp little nose. His name is Juramaia, and if you went back about 120 million generations, your ancestor would look something like him.

Juramaia was discovered in Liaoning, China, in 2011. Recent enough that if you’ve been following paleontology for a while, you may have heard of him. China has, since the beginning of this century, been a treasure trove of important fossils. We first found feathers on a Chinese dinosaur named Sinosauropteryx. We also found Juramaia, who is absurdly important to our understanding of mammal history.

Juramaia is a basal Eutherian. Eutherians are, simply put, living mammals who aren’t marsupials or monotremes. In fact, this is the oldest known Eutherian to date. We can’t say for sure that Juramaia is our direct ancestor. In fact, he’s probably not, statistically speaking. But he’s close enough to give us tons of insight into our origins.

When the first true mammals evolved depends on how you define a mammal. The late Paleozoic was ruled by a group of animals known as synapsids. These were our earliest ancestors and cousins, sometimes called proto-mammals. Originally considered a subclass of reptiles, we now know that they diverged from basal amniotes around the same time as reptiles, and have much more in common with mammals, anyway. They’re still popularly referred to as mammal-like reptiles, though that term is outdated. The terminology is a bit confusing, since synapsids also include true mammals, but I’ll use ‘synapsid’ here to mean the same thing as ‘proto-mammal.’

So, even when synapsids first appear in the fossil record in the late Caboniferous, they look distinctly mammalian on the inside. Their skulls have holes behind the eye socket, a trait shared by all their ancestors (fun fact, we lost this trait; our skull temples are the recently-closed secondary holes of the skull). They don’t have scales, either, so if you see a scaly Dimetrodon or Gorgonops, the portrayal is either outdated or incorrect. Synapsids also developed a semi-erect gait, something halfway between that of an alligator and a deer, for reference. Synapsids also immediately show the variety of teeth mammals eventually came to be most known for in paleontology. Towards the end of the Permian, they even develop bristles and fur.

By the Triassic period, we have the Cynodonts, a group so close to mammals that they’re easily mistaken for them. We aren’t quite at mammals yet, though. Cynodonts are pretty uncontroversially considered proto-mammals, but they’re definitely considered transitional. It’s when you move a few million years up that things get dicey. Animals like Megazostrodon and Morganucodon appear in the late Triassic, and they’re almost indistinguishable from a shrew or mouse. Plenty of people consider Megazostrodon and company to be the most basal mammals. This gets into what I talked about in Westlothiana’s writeup. These animals are so close together, and so close to the symbolic gulf between proto-mammal and true mammal that it’s almost impossible to come to agreement on which it is. In The Ancestor’s Tale, Richard Dawkins has a policy of not getting too hung-up on labels when dealing with these animals, and I think that’s a good rule of thumb.

Juramaia, though, is unambiguously a mammal. It has every trait we associate with the class today, and was a card-carrying member of the group that would eventually become the majority of mammals we know today. Most mammals looked a lot like Juramaia during the Mesozoic, with a few glorious exceptions. Most of our 100+ greats grandparents were relegated to exploiting the lower niches of nocturnal insectivores. Make no mistake, though, we flourished under the footfalls of dinosaurs, and spread all over the world. We made the most of that strategy for 100 million years, until there was room to diversify. If you can believe it, we—that is, mammals as a whole—retain a lot of holdovers from our stint as tiny nocturnals. Juramaia is an animal with just about all of those traits.

Think about some defining features of mammals. I don’t necessarily mean technical stuff, like bone structure and all that. That’s irrelevant to this particular point. You’d be surprised by how many of those traits are the result of 100 million years as pseudo-shrews:

Most mammals have dull coloration. Most of us are brown or tan or gray or black. Being the same color as the dirt or decaying leaves is very advantageous to a tiny animal that sleeps all day and spends all night worrying about being eaten by other animals. And remember that dinosaurs probably hunted primarily with eyesight, considering how good birds are at it. The color of our fur is a relic from when we needed camouflage to survive.

Your typical mammal has scent as their strongest sense, with hearing close behind. On top of that, they have terrible eyesight. It can be hard to remember that since we’re an exception, but it’s true for almost all non-primate mammals. Nocturnal animals tend to have poor color vision, trading off detail for the ability to see in as little light possible. Scent and hearing are really good traits for an animal that can’t and doesn’t need to see that well, too. It’s also worth mentioning that mammals on the whole have a well-developed sense of touch. It’s weird to think that other animals aren’t as good at feeling things as us, but it seems to be true.

Even fur and warm-bloodedness might be holdovers from Mammals: Nights. They might have served to keep us warm in dry places, where it would get much colder at night. Since we weren’t sleeping at night, it was important to have a mechanism to stay warm.

All of these points, are bundled together in a theory called the “Nocturnal Bottleneck.” There isn’t much evidence of nocturnality in other groups of animals back in the day, which suggests mammals pretty much had that niche on lockdown. Juramaia displays all of these traits, and it’s easy to see how an animal like this eventually diversified into the mammals we have today, who, despite having all kinds of shapes and roles, still retain some traits that betray their nocturnal heritage.

So... I’ve been very busy. I still am busy, and don’t really have time for drawing. But I thought I’ll let you guys know I didn’t leave this blog completely. So here’s the last completed artwork I did, back in October, when I thought about doing inktober. Well, that didn’t work out. This guy is Miracinonyx trumani, based on, IIRC, Mauricio Anton’s skeletal reconstruction of this guy, and inspired by a photo of a cheetah running at top speed.

And here’s something I started about two or three weeks ago. I’ve been working on it on and off when I have a bit of spare time, but now I really don’t have any time to finish it, so it’ll probably be quite some time before it’s done.

And that’s, of course, another Thylacoleo carnifex, because I’m just never happy with how I restore this guy. Plus, it’s one of my favourite extinct creatures of all time, so I keep coming back to it. All that said, I think this one is going to be my favourite reconstruction of it once I’m finished, unless I royally mess up in the process.

So, there, I’m alive, I just don’t have any spare time for art :(

Central European boar (Sus scrofa scrofa)

Silky anteater (Cyclopes sp.)