Prehistoric Mammal Discoveries 2021

Lesmesodon, a weasel-sized hyenadont.
A new species from Early Eocene France was
 described this year

And so another year approaches its conclusion and the pandemic seemingly isn't done with us yet. I had to self-isolate after a positive test myself for a couple of weeks (no symptoms, though) but if there's one thing that doesn't interrupt, it's blogging, so everything went smoothly here. But now it's time for what's becoming a traditional look at the paleontological discoveries of the past year that didn't get covered here but are worthy of at least a brief mention.

Large Herbivores

When we think of vertebrate fossils, the first thing that pops to mind is almost certainly going to be a skeleton of some kind, or perhaps just part of one. But there are also such things as ichnofossils - fossilised remains of how an animal affected its environment that no longer include any physical part of the animal itself. Perhaps the most obvious of these are fossil trackways - footprints of long-gone animals preserved in mud or other soft material that has since turned to stone. A study published this year examined the tracks left by two species of fossil horse. One of them, a one-toed close relative of the living species inhabiting southern Canada during the Ice Ages, turned out to have been galloping at around 34 kph (21 mph), which is quite fast for its small size (perhaps it was running from something). More significantly, however, the three-toed Miocene horse Scaphohippus was using a relatively unusual gait called the "rack" typically only seen in specialised domestic breeds today. 

On the subject of which, a study of the genetics of multiple horse remains from across Europe and Asia showed that the animal was first domesticated somewhere around the lower Volga and Don valleys in southern Russia. From around 2000 BC onwards, this original domestic form rapidly displaced the indigenous wild populations elsewhere. This fits, as one might expect, with archaeological evidence of the spread of chariot use.

An analysis of the genetics of living rhinoceroses and three fossil species - including the famous woolly rhino - showed that the African and Asian species first diverged around 16 million years ago, likely after the two continents had collided, rather than having existed side-by-side in Asia before this date. It also turns out that they never had much genetic diversity, which likely isn't helping them much now when, of course, it's much lower due to the great decline in their population sizes. One of the other fossil species in the analysis, the mighty Elasmotherium of Siberia is usually reconstructed as having a remarkably long and powerful pointed horn, but this wasn't made of bone and so isn't preserved in the fossils. A new analysis this year prefers instead the view that it was a domed nasal lump that may perhaps have housed a resonating chamber to modify the sounds of their calls.

Insular dwarfism is a well-known phenomenon in which large animals trapped on small islands, over the course of a sufficiently long period of time, decrease in size so as not to exhaust their food supply too quickly. Thus, it has always been odd that the Cretan deer (Candiacervus), although usually the small size we'd expect, down to 40cm (16 inches) in shoulder height, had one fossil species living alongside the smaller forms that was much closer in size to a modern red deer. A new analysis suggests that the fossil wasn't some unusually large species after all, but was instead an individual suffering from pituitary gigantism.

That was determined on the basis of the microscopic structure of its bones, but another study on insular dwarfism was able to use genetic evidence. Analysis of a dwarf elephant from Sicily showed that its ancestors had diverged from those of normal-sized elephants just 400,000 years ago, so that its ancestors must have shrunk by as much as 4cm (1½ inches) in height each generation - although is clearly an upper estimate, and it may have been much slower. Another ancient DNA study showed that the Columbian mammoth of America descended from a hybrid between woolly mammoths and a previously unknown lineage that must have crossed the Bering land bridge even earlier.

Carnivorans

Genetic studies on long-extinct species seem to have proliferated this year. One that's particularly significant from a paleontological perspective was conducted on the remains of dire wolves to try and clarify where they fit in the canine family tree. This showed that dire wolves last shared a common ancestor with living wolves 5.7 million years ago much earlier than previously thought. Despite the opportunity for doing so there's no evidence that they crossbred with grey wolves or coyotes (as those two species do with each other), probably because they couldn't. The upshot of this has been to move the dire wolf from its traditional place in the genus Canis, alongside regular wolves, and instead resurrect the much older name of Aenocyon. It now looks likely that dire wolves descended from some native American animal, while the ancestor of grey wolves, coyotes, and jackals appeared separately in Asia.

Over in the world of anatomical remains, a pair of fossil pandas dating back 100,000 years for the first time showed that pandas of this age already had a 'false thumb' essentially identical to those of animals alive today. Living animals use these to hold onto bamboo, so, since that was already well-developed at that time, the herbivorous habits of this unusual carnivoran likely go back quite a bit further. Pandas are, of course, bears, a group that goes back a very long time indeed. This year, an analysis of the skeleton of the very early bear relative Amphicynodon, which was about the size of a domestic cat, showed that it was well-adapted for climbing, and likely spent most of its life in the trees. 

Among the cats, fossil leopards were discovered for the first time in Taiwan; they are found further east than this today, being known from Korea, but had not been known to have reached the island. Much earlier, and a continent away, leopards were thought to be responsible for the collections of hominid bones found in caves in South Africa. A new study from Sterkfontein shows that brown hyenas had gnawed on at least some of these bones and may have dragged them to their dens after scavenging on dead Australopithecines.

Analysis of the teeth of the scimitar-toothed cat Homotherium showed that, at least in Texas, it had a distinctly different diet from its close relative, Smilodon. Whereas the more famous sabretooth hunted in forests and gnawed on bones, Homotherium preferred open habitat, grazing, animals and ate only the fleshy parts, much as cheetahs do today. In fact, its favoured prey seems to have been young mammoths.

Speaking of Smilodon, three fossils of the animal were discovered together in Ecuador. A new analysis shows that two of them were much younger than the third, and show dental features that suggest they may have been siblings. If that's right, and the third, older, individual was actually their mother, Smilodon must have looked after their cubs for longer than most big cat species do today, which would support claims from some quarters that they hunted in prides like lions. Although they weren't siblings, since they died thousands of years apart, a pair of mummified cave lion cubs were recovered from a cave in Siberia. Their fur seems to have been paler than we would expect for lions of the living species, and to have become more so with age; this may have been useful in the snowy Siberian landscape.

Primates

An analysis of the teeth of Mesopithecus, a genus Late Miocene monkeys from Bulgaria suggests that some species ate leaves while others preferred harder seeds. Since their closest living relatives today are all leaf-eaters (which is unusual for primates), this implies that either the habit of leaf-eating arose twice among these monkeys or that it started before they arrived in Europe and one group reverted to a more typical diet afterwards. What most primates prefer to eat is fruit, but this can carry its own problems, as the discovery of the oldest known case of dental caries (tooth decay/cavities) reveals. This was in 54-million-year-old fossils of the very early primate Microsyops and was so common among the samples examined - 7% of the total - that it's likely that, in evolutionary terms, it had only recently switched to eating sugar-rich fruit.

Monkeys are not the only primates to eat leaves, however. The giant lemur Megaladapis from Madagascar did the same until it died out, perhaps as recently as the 15th century. New analyses show that they were even more adapted to leaf-eating than had been supposed, as well as placing them within the lemur family proper, despite some previous suggestions that they might have been closer to the sportive lemurs.

Other Land Mammals

Looking at the much smaller fossil mammals that often get overlooked in popular books, a survey of the last 50 million years of squirrel evolution showed that those species who adapted to life in the trees gradually increased their relative brain size over time. Today's red and grey squirrels top the charts in overall brain size along with the oriental giant squirrels, but all tree-climbing squirrels show a large cerebral cortex, doubtless related to the complexity of navigating the complex environment of high tree branches. In contrast, the closely related "mountain beavers" haven't increased their brain size at all during that time, and marmots may even have reduced theirs slightly in the time since they gave up their ancestors' arboreal lifestyle.

Analysis of the limb bones and muscle attachments of another rodent, the 80 kg (180 lb) South American Neoepiblema suggested that, while it mostly walked along the ground, it would have been unusually good at swimming if it really had to. Most fossils have been found near ancient lakes and rivers, so it probably got plenty of opportunity.

Panochthus is a genus of South American glyptodont, giant relatives of armadillos with dome-like shells. In addition to their shells they are known for possessing large clubs on the ends of their tails, the shape of which helps to distinguish between the eight known species. A new study looked at how these shapes changed over time, and showed that those in later species, said to resemble a Viking sword, would have been more deadly but required careful aiming, since they had to slash horizontally rather than being waved about indiscriminately as would be the case for the simpler clubs of earlier species.

It had traditionally been assumed that, like their surviving tree-dwelling relatives, ground sloths were exclusively herbivorous. A study this year, however, showed that their bones had an isotopic signature that can only be explained by eating at least a small amount of meat, perhaps scavenging on the odd carcass that they happened to come across, since it's unlikely that they would have been active hunters. Elsewhere in sloth-world, the three-ton North American ground sloth Lestodon was this year shown to have unusually large lower canine teeth in at least some specimens, which has been interpreted as a sign of possible sexual dimorphism. If that's what it is, it must have been used in fighting, suggesting that males competed for mates.

Kangaroos are among the most iconic of marsupials, and known for their hopping gaits. But not all kangaroos necessarily moved like this. The large "short-faced" kangaroo Protemnodon, which may have weighed as much as 170 kg (375 lbs) had forelimbs that suggest it habitually walked on all fours, using bipedal bounding much less than modern kangaroos do.  On the other hand, Wallabia kitcheneri, a close relative of the living swamp wallaby of eastern Australia seems to have been adapted for climbing through trees (although not necessarily very well). Which is odd, as while tree kangaroos are, in fact, a thing, it wasn't closely related to them.

Marine Mammals

Whales are, it's fair to say, big. Among the largest are the balaenids, a family which today contains just four living species, the three kinds of right whale and the bowhead whale of the Arctic. A study of living and fossil species shows that most balaenids actually shrank during the Pliocene, but that the two living groups developed their current enormous size separately. Right whales seem to have come first, perhaps due to changing nutrient levels in the oceans, while the ancestors of the living bowhead achieved their growth spurt after entering the Arctic 3 million years ago.

Balaenids feed on plankton (although not, in their case, much in the way of krill) but other cetaceans are more predatory. Typically, these are the dolphins, porpoises, and smaller whales such as orcas, but sperm whales are an exception in terms of size. Back in the Miocene, relatives of sperm whales were even more fearsome and the 7-metre (23-foot) Zygophyseter was no exception. An examination of the attachment points of its jaw muscles shows that it could exert a force of 10 kN (about one ton) at the back part of its mouth. This is easily enough to feed on, say, smaller whales.

There are a couple of major questions in cetacean evolution, and there was progress on both of them this year. One is how the toothless baleen whales evolved from ancestors that clearly had teeth. While baleen doesn't fossilise, some early members of the relevant evolutionary lineage have been speculated to have developed the filter-feeding plates before they fully lost their teeth. CT scans of the jaws of the 30-million-year-old Aetiocetus provided strong evidence this year that they fit this description, showing the channels through which nerves and blood vessels would have developed to supply both structures simultaneously.

The other question is how and when they first became fully aquatic. A study of the bones of an early whale (originally identified as Platyosphys, but the authors of the study are less convinced that's what it really is) showed that despite living 36 million years ago, it was already 10 metres (33 feet) in length and, more significantly, had a bone structure already part of the way along to the comparatively light form seen in modern whales... unusually far advanced for a whale of its time.

And, since not all marine mammals are cetaceans, it's also worth mentioning the rare discovery of a fossil manatee from South America. A member of the previously known genus Potamosiren, the Eary Miocene date of the fossil, and the environment in which it was found supports the theory that manatees were associated with freshwater environments from very early on, and only later returned to the shallow seas where the Florida manatee is found today.

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Synapsida is taking a break for the holiday period and will return on the 9th January

[Photo by Norbert Micklich, G. Gruber, from Wikimedia Commons.]