Sunday 17 December 2023

Prehistoric Mammal Discoveries 2023

It's the last post of the year for 2023, and that means it's time once again to take a brief look at discoveries from the last year in the world of fossil mammals that didn't make it into this blog. Because, while dinosaurs are undoubtedly popular, the study of prehistoric mammals is also a major field, aided by that, being (mostly) relatively recent they tend to be more numerous and better preserved. Of course, everyone's heard of woolly mammoths and sabretooth cats but there's plenty more out there and, if I'm going to zip through them at speed today, I'm also going to try and cover as wide a range as possible. So, let's get going...

Large Herbivores

While there have been several new species of fossil mammal identified this year, sometimes we can learn new information by re-studying old fossils with new methods. One of the most frequent questions from lay people about any new fossil animal is "how big was it?" and this isn't always easy to determine when all you have is an incomplete skeleton. A study published this year developed a new method for estimating the weight of fossil ruminants from the shape and size of the leg bones that had to support them. Using over 100 living species to calibrate their results, they estimated that the late Ice Age bison Bison antiquus weighed 800-880 kg (just under a ton), not so different from the modern species.

Pollen analysis from the teeth of an Irish deer (Megalaceros giganteus) that lived in the Netherlands during a gap between the last two Ice Ages indicated that it had been eating a diet of hogweed, chervil, chamomile, groundsel, and comfrey along with grasses, suggesting cool open woodlands rich in herbs. Elsewhere, another Irish deer was discovered in 35,000-year-old deposits in Catalonia, Spain, much later than they had been thought to survive in the area - its recent ancestors had possibly taken a trip around one end of the Pyrenees when southern France became too cold to inhabit.

A newly discovered assemblage of fossils of Neotragoceros, the earliest known member of the cattle family in North America, showed that their closest living relatives were the nilgai "blue bulls" of India, living in Oregon 6 million years ago. Meanwhile, Procobus, a Late Miocene animal from northern Greece, was shown not be a relative of reedbuck antelopes, as previously thought, but more likely related to the common ancestor of sheep and goats.

A fossil horse (Equus sivalensis) discovered in northern India was dated to over 2.6 million years ago, during the Late Pliocene; if true, this suggests that modern-type horses entered Eurasia earlier than thought, prior to the Ice Ages. Emigrating in the opposite direction, and perhaps not much earlier, a newly discovered fossil tapir (Tapirus arvenensis) from northern Spain seems to be surprisingly closely related to the modern tapirs of Indonesia and Southeast Asia.

Among the really large herbivores, mammoths have the advantage of having died out recently enough that we can do research on their remains that wouldn't be possible on fossils that had entirely turned to rock. Analysis of the strontium isotopes in the teeth of a woolly mammoth (Mammuthus primigenius) showed that it had migrated from winter grazing grounds in southern Poland to cooler climes 250-400km (155-250 miles) further north each summer for at least 12 years. Genomic analysis of other individuals was able to identify genes responsible for hair growth, fat deposition, and immune function that distinguished them from modern, tropical, elephants. Furthermore, analysis of the tusks of a male woolly mammoth was able to identify the presence of testosterone, deposited in such a way as to suggest it experienced the same periodic bouts of sexually-charged "musth" as living elephants do.

It isn't just woolly mammoths, either. Genetic studies on woolly rhinos (Coelodonta antiquitatis) showed that the Siberian populations diverged shortly after the first emergence of the species in Europe, and that, in times when northern Asia was covered in vegetation-free ice sheets, they sought refuge and diversified in northern China before later returning. Among non-mammoth elephants, studies on American mastodons (Mammut spp.) added further weight to the theory that they evolved on the continent, rather than being the product of an emigration of apparently similar animals known to have lived in Asia. A study on the skulls of the Miocene shovel-tusker Platybeladon indicated that it may have been the first "elephant" to develop a prehensile trunk, five million years before the modern elephant family.


This year saw a new description of a fossil jawbone previously found in Alberta, Canada, confirming that it probably belonged to a dire wolf (Aenocyon dirus), not a modern grey wolf as had been suggested; this would make it the most northerly known example of the species, and the only one from Canada. Elsewhere, an analysis of the skull of the early dog Eucyon from Pliocene France indicated a jaw strength suited to biting smallish prey that fights back, implying a diet similar to some of the larger modern jackals, and perhaps related to the rapid evolution of rabbits and hares at the time. Focussing on an entirely different part of the skeletal anatomy, a study of the penis-bones of three species of borophagine including the giant bone-crushing dog Aelurodon, showed that they mated in the same way that modern dogs do but were, for entirely mysterious reasons, distinctly curved.

An analysis of three species of carnivorous panda placed in a new genus, Huracan, with fossils found in Spain, China, and North America showed that they were suited to open habitats, being able to chase down their prey. While there is no doubt that pandas are a kind of bear, this is less clear for some other fossils first described this year. Lonchocyon, dating back to the late Eocene over 33 million years ago, was about the size of a large dog and likely killed its prey with a powerful bite... but it has features shared with both early bears and the related, but extinct bear-dogs, so it's unclear which, if either, it was more closely related to. Eoarctos is almost as old and, if not quite a bear, may be closer to them than to anything else alive today. A description of a newly discovered skeleton, shows that it had the unusual combination of limbs suited for climbing trees and teeth suited to eating shellfish; perhaps it just used the trees to hide from larger predators.

Among the cats, a new analysis of the elbows of the "American cheetah" (Miracinonyx trumani) reveals it to be as much like that of its actual relative, the puma/cougar/mountain lion as it is like its Old World namesake suggesting that, while it was undeniably fast-running, it may have been less like a true cheetah in its lifestyle than previously thought. On the other hand, a study on the jaws of cave lions (Panthera spelaea) confirmed that the males were, indeed, larger than the lionesses, much like their modern relative. 

A four-million-year-old fossil cat from the Tibetan Plateau has been proposed as an early relative of the modern clouded leopard and given a new genus, Palaeopanthera; it had previously been identified as a snow leopard. Further south, in Thailand, the newly described big cat Pachypanthera is not thought to be closely related to any living species, being older than any of the known "roaring cats" at up to 9 million years old, and having exceptionally powerful jaws and teeth. It was about the size of a modern lioness and the jaws are so strong compared with living species that it may have had some ability to crack bone like a hyena.

As for sabretooths, a new analysis of the hyoid bones of Smilodon suggests that, contrary to many depictions in fiction, it probably couldn't roar, instead making sounds similar to those of a cougar, but deeper in tone. On the other hand, the fossil paw of another sabretooth, Amphimachairodus, showed that it had been so badly injured as to be useless. This ought to have resulted in the animal starving to death shortly afterwards... yet the paw had partially healed. This implies that it must have hunted alongside others of its kind, which helped to feed it, and supports other lines of evidence that this sabretooth cat at least, was a pack hunter, not solitary as most big cats (other than lions) are today.

Other Land-dwelling Placentals

In recent years, a number of fossil species belonging to the subfamily of "African apes" (as opposed to orangutans and gibbons) have been discovered outside that continent, from the eastern Mediterranean. This has cast doubt on the long-held assumption that African apes originated in... well, Africa. A newly described fossil species, Anadoluvius, from Turkey, adds to the debate by being one of the oldest known members of the subfamily at 8.7 million years old, suggesting that, while humans clearly originated in Africa, their own ancestors may well have come from further north. 

Sahelanthropus is a known species of ape living somewhat later, around 7 million years ago. A new analysis of its arm bones suggests that it knuckle-walked like a chimp or gorilla, adding to earlier evidence that it was not bipedal, as had once been thought. It remains unclear whether it was related to the common ancestor of chimps or humans, or was actually an early gorilla. Gigantopithecus, an ape which lived in China during the Ice Ages, had amongst the strongest bites, and the thickest teeth, of any known ape species (exceeded only by "nutcracker man" Paranthropus) suggesting that whatever sort of vegetation it ate was particularly difficult to chew, possibly including tough roots that would inevitably have been mixed in with grit.

This year also saw the description of two new fossil squirrels (Junggarisciurus and Eopetes) from Xinjiang in China. They are the oldest known squirrels from Asia, dating back around 35 million years, and thus equal in age to the previous oldest squirrel fossils, which were found in North America. Those were ground squirrels, but these, although large at up to 2.7 kg (9 lbs), more closely resemble tree-living species, implying that, since their North American counterparts were not, the split between tree and ground squirrels could date back almost to the origins of the family. Elsewhere, studies of skeletons of the giant Menorcan rabbit Nuralagus rex - it weighed at least 8 kg (18 lbs) - showed that, as expected, it grew very slowly throughout its life, probably being able to afford to do so since it lived on only on an isolated island devoid of predators.

Two analyses, one on the limb bones and one on the teeth and jaws of various different kinds of ground sloth, came to similar conclusions. The researchers (and it's mostly the same ones) interpreted their results to mean that the nothrothere and megalonychid ground sloths, which are related to the modern three-toes species, would have been skilled at climbing, while the mylodontid sloths, related to the modern two-toed species, were specialised for digging and had an unusually broad diet. The giant megatheres, which are also related to the three-toed sloths were, perhaps unsurprisingly, purely ground dwellers. The discovery of the fossil of a Nothrotherium ground sloth that had died whilst in a late stage of pregnancy showed features on the foetus that imply adaptations to relatively rapid weaning and limbs that could easily have gripped onto its mother's fur to be carried about, as well as demonstrating that, like modern sloths, these particular ground sloths gave birth to a single young at a time.

This year also saw the description of the oldest known fossil bats so far, assigned to the genus Icaronycteris. 52 million years old, these were found in Wyoming, and do not belong to any of the modern families, nor do they seem particularly closely related to any of them. Taken together with the fact that the next oldest bats are from France, and not that much younger, this indicates that even at this early date, bats were already diverse and widespread, probably having gone a rapid burst of evolution not long after the extinction of the non-avian dinosaurs.

Aquatic Mammals

An analysis of the inside of the skull of Potamotherium, thought to be a semi-aquatic ancestor of seals, was able to get a good idea of the shape of its brain. This showed that the parts of the brain used in modern seals to interpret sensations from their whiskers when searching for food underwater were already well-developed in an animal that presumably lived more like an otter. A similar analysis of the inner ears of anthracotheres, semi-aquatic animals related to both hippos and the ancestors of whales, showed that their sense of hearing became increasingly adapted to underwater life as they evolved, implying that their last common ancestor with hippos still lived on dry land, and their aquatic habits evolved independently.

Among cetaceans, a few species with unusual feeding habits were reported this year. A newly described species of the early sperm whale Diaphorocetus had small teeth like modern species, but had a different shape to the head suitable for rapidly sweeping from side to side to snap at prey - quite unlike either modern sperm whales or the large-toothed "macroraptor" sperm whales of the past such as Livyatan. The teeth of a newly described dolphin-sized animal from New Zealand, Nihohae, had multiple tusks projecting out horizontally from the tips of a long and narrow snout, quite unlike any modern cetacean. Since the teeth were unworn, they can't have been used to poke about for prey in the mud and they would not have been much use for biting; the discoverers' best guess is that it whacked at fish with a sideways swipe, rather like a sawfish.

On the other hand, the skull of the oldest known member of the modern toothed whale group, Simocetus, which lived 32 million years ago in the North Pacific suggests that it had already adopted a modern style of suction-feeding even though it had yet to fully develop ultrasonic hearing. Finally, no discussion of prehistoric whale discoveries this year can fail to mention an analysis of the skeleton of Perucetus collosus. Gaining weight to serve as ballast when diving to shallow depths, the skeleton was so massive that the report suggests that this may have been the heaviest animal ever to have lived, at up to 340 tons... and, yes, that would be heavier than a blue whale, even if the bodily dimensions were not quite so vast.

Marsupials and More

Analysis of the small Ice Age marsupial Bohra this year elaborated on its adaptations as a probable ancestor of modern tree-kangaroos. While the hind feet initially had a similar skeletal structure to those of the sort of kangaroos we normally think of, later species within the genus developed more flexible ankles suitable for climbing as forests became more extensive in Australia in the early Pliocene, 5 million years ago. The researchers argue that Bohra probably had a lifestyle similar to some prehistoric lemurs. 

While we're up in the trees, another new fossil, Lumakoala, was identified as belonging to what's probably an early koala. At no more than 2.5 kg (5½ lbs), if it's what it appears to be, it must have been one of the smallest koalas ever, and its molar teeth were still primitive, not yet highly adapted to munching on tough leaves. Meanwhile, a new analysis of Nimbadon, a Miocene tree-climbing marsupial that looked like a koala, but instead belonged to a now-extinct group, suggests that it ate a large amount of fruit instead of leaves, thus behaving more like a typical primate and likely having no direct competitors at the time. Which probably explains what it was doing up a tree when it weighed about 70 kg (150 lbs).

The marsupial sabretooth Thylacosmilus is notable for the bony flanges on its jaws that protect the exceptionally large sabres. But this isn't the only thing odd about it; its eyes were on the sides of its head, the sort of thing that's entirely reasonable on a horse that wants to keep a wide-angle view of its surroundings but that seems rather odd on a predator that wants to pounce on prey. A new analysis, however, shows that the exact shape of the eye sockets means that the animal would have had better binocular vision than might be expected and that the eyes had to be pushed to the side to make space for the bases of its enormous teeth. 

This year also saw the description of the tooth of a monotreme, distantly related to modern platypuses, from southern Argentina. This is significant, because, at around 70 million years old, it's well before the only previously known monotreme fossil from outside of Australasia - the mere existence of which had been a bit of shock when it was described in 1992.

Last of all, I have to mention an extraordinary fossil of Repenomamus, a badger-sized carnivorous mammal not closely related to anything alive today. The fossil shows it locked in deadly mortal combat with Psittacosaurus, a small bipedal early relative of the more famous Triceratops. Its teeth are firmly clamped onto the dinosaur's front leg and its forepaw is holding down its opponent's jaw. Had the pair not been suddenly buried in hot ash from an erupting volcano it's very likely that this particular dinosaur would have lost the fight.

Score one for mammals.

Synapsida is taking a break for the holiday period and will return on the 7th January

[Picture by Gabriel Ugueto, available under the CC BY 4.0 license.]

1 comment:

  1. Hope you have a restful break :)

    I realize I've lost track completely what happens in palaeoanthropology in recent years ...