Prehistoric Mammal Discoveries of 2020

The largest known Oligocene whale was
moved into the new genus Ankylorhiza
this year

Well, that year was... different. Fortunately, it didn't stop me posting and, indeed, a cancelled summer break meant that you got one more post than anticipated you lucky people, you. But as 2020 finally passes into the history books where it can bloody well stay, it's time for the end-of-year roundup of paleontological discoveries that didn't make the regular blog. As always, no theme, just a random assortment of journal papers that may, for all I know, be thoroughly contradicted by this time next year. But this is a collection of things that some scientist, somewhere, thinks that they demonstrated in 2020:

Marsupials

Marsupials today are a reasonably diverse group of animals; there's little externally visible similarity between a kangaroo and a koala, for instance (beyond the females both having pouches). But, on the other hand, they clearly are not as diverse as the much larger group of placental mammals, which includes everything from bats to antelope. It's fairly easy to see why a marsupial dolphin is unlikely to work, but it's less immediately obvious why they couldn't evolve hooves or wings. The most common theory is the need for grasping forelimbs when they climb into the mother's pouch after being born limits how far they can alter these limbs (I've covered this before). A study published earlier this year took another look at the shape of marsupial forelimbs and concluded that they vary too much for this to be the case. If they're right, other reasons apply - perhaps it's hard to fly with a baby in your pouch, or bats flying to Australia from elsewhere had already taken the available niche.

One type of marsupial we don't have any more are giant carnivores, such as the "marsupial sabretooth" Thylacosmilus, with its enormous, Smilodon-like canine teeth. (Technically, it isn't a "marsupial", in that it isn't descended from the last common ancestor of any of the living sort, but it's close enough for these purposes). And, on the subject of overturning common wisdom, a new analysis of the skulls and teeth of these animals suggests that they would actually have been fairly rubbish for killing animals in the way that we think Smilodon did - with a powerful bite to the neck - because the other elements of the skull aren't there to resist the resulting force. But, if they weren't like sabretooth cats, they weren't like anything around today, either. Possibly, the authors argue, they used the long teeth to disembowel a freshly killed carcass and feed on the soft internal organs, but, if so, how they killed it in the first place (they don't have adaptations we'd expect of carrion-feeders) remains a mystery.

Ungulates

Perhaps the most defining feature of deer is that they possess antlers and that antlers, unlike the true horns of antelopes, cows and all the rest, are shed and replaced once every year. There has been some debate as to just how and when this strange feature first evolved. This year, researchers used micro-CT scanning to examine the microscopic structure of fossil antlers from species going all the way back to the Middle Miocene 18 million years ago, when deer were just getting started. This unusually detailed and complete study shows that, even in the very earliest deer that we know of, the antlers showed signs of being rapidly regrown throughout life, presumably then, as now, on an annual basis. So it isn't something that gradually developed through the course of deer evolution, but something that's always been there. Which, of course, just pushes the ultimate question further back in time...

Among the largest of large herbivores today are the rhinoceroses. Today, they live only in the tropics, and are dwindling in numbers, but they once ventured into much colder climates. A study this year brought together a number of lines of evidence to examine the probable diets of the woolly rhinoceros (Coelodonta) and the forest rhino (Stephanorhinus) both of which inhabited Eurasia during, and between the Ice Ages. They showed that the woolly rhino mainly ate grass, but that it would eat shrubs at certain times of the year, and some individuals did so more than others (probably depending on the vegetation where they lived). The forest rhino, however, had a much more mixed and flexible diet, often browsing on river banks.

The hornless rhino Plesiaceratherium lived much earlier than this, inhabiting China during the Early Miocene. This year, an analysis suggested that males had much larger front teeth than females. Comparison with living rhinos implies that the males therefore likely competed for females, with the fittest mating with several partners. The same study estimated the size of a fully-grown adult as around 1.2 tonnes, similar to a modern black rhino. This implies that, despite the large number of fossils found at a single site, they were unlikely to be herd animals and probably had the more solitary lifestyle we expect of modern rhinos.

Going even further back, where did rhinos come from? One common theory has been that they originated from tapir-like animals in the Middle Eocene. A number of new fossils from northern China push this back, with clear evidence of rhino-like animals well into the Early Eocene - too early to be derived from tapirs. The new picture, therefore, is that tapirs and rhinos split from a common ancestor around the dawn of the Eocene 56 million years ago, if not earlier, and that only a few early and primitive fossils cannot be placed on one side or the other of their common family tree.

There was also some revision of the fossil history of camels this year, with a study suggesting that the giraffe-like "high llamas" (Aepycamelus) are an artificial assemblage of different genera mixed in with the much smaller Procamelus, and that both are, in fact, llamas, not camels. Meanwhile, a fossil water buffalo (Bubalus murrensis) was discovered near, of all places, Moscow, where it apparently lived during a warm gap between the Ice Ages - even so, this suggests that they were rather better at surviving through the cold times than we might have thought.

Carnivores

There was some coverage in the popular press about a study that traced the ancestry of domestic dogs by examining the genomes of ancient specimens, showing that there were already five distinct lineages of dog as early as 11,000 years ago - long before the existence of agriculture and the domestication of other animals. The development of these lineages mostly followed the movement of humans across the globe, with one exception being the complete replacement of the original European dog breeds by introduced ancestors of the modern sort way back in the Neolithic. A similar study showed that a 9,500-year-old dog from Siberia was related to modern sledge dogs and likely bred for the same purpose, way back in the Stone Age.

Also on the subject of canines, a piece of jawbone discovered from China was identified as belonging to a dire wolf (Canis dirus) - a significant finding, if true, since, so far as we previously knew, dire wolves never left North America. A study on the far more numerous dire wolves of the Rancho La Brea tar pits in America examined the small bones of their throats. The authors concluded that the size and shape of these bones would mean that the howls of dire wolves had a noticeably deeper pitch than those of modern wolves. The howls of Ice Age coyotes may have been deeper, too.

The extinct bear-dogs are a lost family of carnivorous mammals, and we know of several examples. This year, a relatively complete fossil of Megamphicyon from the Middle Miocene of Spain was described. Previously known almost entirely from its teeth, we can now say more about what the rest of the animal looked like. The authors estimate it as having weighed around 600 kg (1,300 lbs), about half again the weight of a male grizzly. Despite this huge size, the shape of the legs suggested that it could run faster than a modern bear and that it may have been a fairly active predator.

Ancient dogs aren't the only fossilised animals that were subject to genetic analysis this year. A study on cave hyenas showed that they split from the ancestors of today's spotted hyenas in Africa 2.5 million years ago, around the start of the Ice Ages. Which means that they entered Europe at around the same time as humans did, and probably by the same route. However, the picture was muddied by the European and African populations having mixed again since their initial divergence, likely suggesting multiple crossing attempts. A similar study on a sabretooth cat (Homotherium) showed, unsurprisingly, that its ancestors diverged from those of modern cats over 20 million years ago, but was also able to locate specific genes thought to be related to daytime vision (so, not a nocturnal hunter like modern cats, then) and the intelligence required for well-developed social behaviour.

Rodents

Beavers are amongst the largest rodents alive today, but they used to be even larger. The giant American beaver (Castoroides) was close to six feet in length and may have weighed as much as a small bear. It lived during the Ice Ages, but a DNA study this year showed that it last shared an ancestor with modern beavers around 20 million years ago. The reason that that may matter is that giant beavers were, like the living sort, semi-aquatic and, unless such a trait somehow evolved twice, this helps to confirm suspicions that beavers have lived in such a manner for at least that long.

Giant beavers did not, however, so far as we know, cut trees down and the same study, using DNA from living species, suggests that that habit may have started 7 million years ago, towards the end of the Miocene - much earlier than previously thought. The oldest clear evidence we have for beavers doing this dates from just 4 million years ago, and applies to the relatively small species Dipoides. A different study, based on isotopic evidence, showed that this beaver ate a significant amount of woody plants along with more aquatic vegetation. This makes it more likely that beavers initially evolved to feed on such plants before they began using them to construct dams.

The sewellel, or mountain beaver, is not at all related to true beavers, beyond also being a rodent. They are the last surviving member of an ancient family of rodents and dig burrows in remote and hilly terrain. An analysis this year of the ear canals of its extinct relative Mesogaulus showed that it was, as expected, also ground-dwelling and likely had a similar lifestyle. But the ear canals of the much older relative Prosciurus implied that it was probably as agile as a tree squirrel, supporting earlier suggestions that the group originally evolved to live up trees and only came down to the ground later.

Aquatic Mammals

A peculiar relative of modern river dolphins was described this year based on an Early Miocene specimen from Patagonia. Given the name Dolgopolis, it is unusual because it had no teeth, nor any sign that it could have grown any. The relatively short and wide snout, compared with living river dolphins, supports the suggestion that it must therefore have fed by simply sucking fish into its mouth and swallowing them whole. Other related dolphins from the time mostly fed in the usual dolphin-like manner, although there is some variation. It appears that this may have been a time of evolutionary innovation amongst the group, which has few living species today, surviving alongside each other because they fed on different prey.

Going further back, almost 50 million years ago to the Early Eocene, another study looked at the wrist bones of early cetaceans. They showed that the wrists of Ambulocetus, a dolphin-sized animal that still had hind limbs and toes on its forefeet, were already shifting towards the flattened, sideways-projecting form seen in their modern relatives. As this trend continued through the Late Eocene, such animals would have found themselves unable to move easily on land, effectively "locking in" their shift to a fully aquatic lifestyle.

An analysis of a number of new fossil walruses from Miocene California revealed that some of them belonged to a previously unknown genus and species, Osodobenus. It turned out not be closely related to other known walrus fossils yet, despite this, had the long tusks for which the modern animal is so well-known. Since many closer relatives of our current walrus had nothing of the sort, this must, it seems, be a case of parallel evolution - with this animal evolving the feature before the ancestors of the living one did.

And the rest...

Two lines of evidence published this year (by different researchers) tell us something interesting about the lives of ground sloths. A fossil site in Ecuador revealed a large number of specimens of Eremotherium that apparently died at around the same time, including both adults and older juveniles. The site had once been a shallow marsh or wallow, where the giant sloths may have been feeding before drought or some toxic event, perhaps from the accumulation of their own dung, killed them off. A similar mass collection was described from Argentina, belonging to the even larger sloth Lestodon, thought to have weighed over 3 tons when fully grown. They too, seem to have died in a sudden event, and included juveniles as well as adults. The findings suggest that both species must have lived in herds of some kind... something for which modern tree sloths are not known.

On the subject of ground sloths, yet another species, Nothriotherium, gained the dubious honour this year of becoming the first non-human Pleistocene mammal to be diagnosed with cancer, when it turned out to have an osteosarcoma in one of its legs. In more positive news, we have the discovery of new species of fairy armadillo, a group of rather odd animals with very little known fossil history. An early example of its kind, dating from the Late Miocene, it seems to have already been good at digging, but probably didn't spend most of its life underground as its living relatives do.

The main Caribbean islands have no living wild monkey species, but that wasn't always so, with some surviving until the arrival of European settlers. The oldest known is Paralouatta from Pleistocene Cuba, which seems to have been fairly odd for a New World monkey. New analysis of its bones this year seems to confirm its oddity, suggesting that it must have spent at least some of its time walking on the ground - every single American monkey species alive today dwells almost entirely in the trees.

Going really, really far back in mammalian history, we come to a new study on Morganucodon, which lived something like 200 million years ago (and I don't often get to write that many digits in an age in this blog). In fact, it's so far back that it wasn't really a mammal, just an early relative of the animals that later became mammals. The study looked at the microscopic structure of its bones, to see how blood may have flowed through them, used as a proxy for its metabolic rate and hence the key question: was it warm-blooded? It seems that mostly, it wasn't, although it could elevate its metabolism above what we'd expect for reptiles for at least short periods, an early step along the road to true endothermy.

And finally, a newly-named fossil from Venezuela that I'm going to mention purely because I don't get to say the words "giant vampire bat" often enough. What else would you want for Christmas?

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Synapsida is taking a break for the holiday period and will return on the 10th January

[Picture by "PaleoGeekSquared", from Wikimedia Commons.]