Sunday, 15 December 2019

Prehistoric Mammal Discoveries of 2019

Nehalaennia, an 8 million-year-old rorqual
from the Netherlands, first described this year
As the year - and decade - approach their inevitable conclusion, it's time again to look back at a few palaeontological findings of 2019 that didn't, for whatever reason, make it into the regular Synapsida posts. As always, there is no theme to this list, just a sample of what seemed interesting linked only by when it happened to be published.

Marsupials


The most diverse group of marsupials alive today are the macropodoids, including the kangaroo family as well as a couple of families of much smaller, but physically somewhat similar, animals. These animals are well-known for their hopping gait, but there's actually considerable variation in how this is employed by different species living in different environments across Australia. It's generally been difficult to determine how far back this goes, but an ancient extinct family known as the balbarids have often been cited as still having been quadrupedal. A new analysis of an unusually complete skeleton showed that this might be overly simplistic, with the animal at least able to hop, even if it probably preferred to walk on all fours in most circumstances. Some close relatives also showed adaptations to tree-climbing, showing that, even 25 million years ago, kangaroo-relatives were already surprisingly diverse.

Marsupials are not unique to Australasia, and, in fact, first reached the continent from South America, where a number of species, mostly opossums, survive today. Opossums clearly don't move like kangaroos, but a few million years ago, the small metatherian Argyrolagus apparently did. It wasn't technically a marsupial, since it didn't descend from the common ancestor of living members of that group, but it was certainly on that side of the mammalian family tree. While this hopping motion had long been suspected, the new study provides much stronger evidence for this, as well as showing that the animal's forelimbs would have been good at digging, making them similar to certain kinds of modern hopping rodent, as well as some of the smaller macropodoids of Australia.

There's only one way that marsupials can have reached Australia from South America, and that's by crossing through Antarctica. A surprising number of marsupial fossils are known from that continent, albeit all from one small island off the Antarctic Peninsula, and now we can add a new one to that list. A fairly close relative of Argyrolagus, and slightly smaller than a modern rat, the teeth of Pujatodon suggest that it ate something moderately hard, probably nuts and seeds, although insects are also possible. When it lived, 50 million years ago, Antarctica was a rather different place than today...

Large Herbivores


In addition to being particularly large, elephants are also amongst the most intelligent of mammals, with brains even larger than one would expect for their already impressive bodily size. While, for much of their evolutionary history, the size of their brains kept pace with the increasing size of the rest of their anatomy, there seem to have been two bursts of evolution where brain-size increased more rapidly than one would expect. Around 26 and 20 million years ago (the latter after the mastodons had split off, explaining why their brains are smaller than similarly sized elephants and mammoths) these may correspond with dramatic changes in the climate, forcing the animals to adapt to survive.

When some elephants became trapped on small islands at least four miles or so offshore, over the course of millions of years, their descendants shrank, making them less likely to starve in a location with a limited food supply. This led to entirely new species, such as Palaeoloxodon falconeri of Malta and Sicily, which stood around 1 metre (3 feet) high at the shoulder. A new study has shown that the brains of these dwarf elephants did not shrink as much as their bodies, leaving them proportionately far larger than in any living elephant. Something similar, if less dramatic, seems to have happened to dwarf hippos in the same situation.

Large brains did not help save the Columbian mammoth (Mammuthus columbi) from extinction, with new evidence published this year adding to the evidence that it was humans, rather than the climate, that ultimately led to their extinction. The situation with the "Irish" elk (Megaloceros giganteus) is more ambiguous. Detailed analysis of the dates of its extinction in various places across Europe shows that they match very closely with the climate changes brought about by the Younger Dryas around 10,000 BC, which would have affected its food supply. But, not coincidentally, the same climate changes also brought humans into some of these areas, making it hard to tell which (if either) had the larger effect.

On the plus side, the giant one-horned rhinoceros Elasmotherium seems to have gone extinct much later than we previously thought, surviving until just 35,000 years ago in parts of eastern Europe and central Asia. Its highly specialised diet probably didn't help it, although that is late enough that humans may again be a factor.

Among hoofed herbivores, a new study confirmed existing suspicions that the Hagerman horse (Equus simplicidens), which lived in Idaho 4 to 3 million years ago was, in fact, a zebra, meaning that those animals long predate their arrival in Africa. Other studies support the theory that goats first evolved somewhere around Tibet, that early musk deer ate far more fruit than the modern ones, and that Ice Age peccaries sheltered in caves through the Missouri winter.

The giant ape Gigantopithecus, although living in Asia, was once thought to be related to living African apes, such as gorillas (and us). It's been suspected since at least 2013 that that probably isn't true, and a new analysis of proteins in its teeth published this year confirms that. Instead, they diverged from the ancestors of modern orangutans about 10 to 12 million years ago, long after those animals had left Africa.

Carnivores


The diet of cave bears (Ursus spelaeus) has been somewhat controversial, with disagreement as to just how much meat this larger relative of the modern brown bear really ate. Some previous studies have even implied that they were almost totally herbivorous, but a new one this year showed that both the wear patterns on their teeth and the mix of isotopes in their bones support the more popular theory that their diet was not that different from its closest modern relatives. This seems to have remained broadly the same even in the very last cave bears to die out around 24,000 years ago, some of the fossils of which show signs of human butchery.

Far too early for humans to be responsible, fossils of what may be the last panda to live in Europe (that's "panda" as in "it's closer to a giant panda than to anything else around today") have been dated to around 6.2 million years ago in Spain... not a place we would associate with pandas today.

Another common question in mammalian palaeontology is just why sabretooth cats evolved such large canines. One possibility is that, in addition to using them to kill, they might also have been used in fights with other sabretooths, most likely for access to mates. Two new specimens of Smilodon populator described this year had injuries to their skulls that certainly look to have been inflicted by other cats, although it isn't possible to know for certain. A study on the closely related Smilodon fatalis showed that, in that species at least, there's no reason to suppose that the teeth were signs of physical fitness that members of the opposite sex might find attractive - they were simply large in order to be deadly.

Small Mammals


One of the smallest mammals ever to have lived was Batanoides, a tiny shrew-like animal from Eocene California. The structure of its wrists suggest that it would have been good at climbing, perhaps even spending most of its life in trees, but it's otherwise fairly obscure. Indeed, partly because the bones of such a small animal were fragile and its fossils tend to be fragmented, we weren't even entirely clear what it was. The best guess was that it was more or less what it looked like; a shrew, or at least a close relative thereof.

A study this year, however, shows that it may not have been closely related to anything alive today, instead being a relative of animals called nyctitheres that died out over 30 million years ago. This would at least put it in the same corner of the mammalian family tree as shrews and hedgehogs, which is interesting as the nyctitheres have also been interpreted as flightless relatives of the first bats ("nyctithere" literally means "bat-beast").

When it comes to actual bats, the fossil record is largely rubbish, again partly because of their fragile skeletons. Nonetheless, a fossil of the 52 million-year-old fossil bat Onychonycteris was analysed this year. One of the oldest bat fossils known, the analysis showed that it was also the least aerodynamic bat we know of, suggesting that it had only recently (in evolutionary terms) got the hang of flying rather than gliding.

While today's rodents are also typically small animals, the giant beaver Castoroides, which lived in America until as recently as 9,000 BC, was around two metres (6 feet) in length. A new analysis has shown that, unlike its modern relative, it preferred to eat water-weeds to woody material, and probably never cut down trees to make dams with.

Cetaceans


At the opposite end of the size scale from shrews and bats we come to whales. Whales have increased in size over their evolutionary history, but it's generally been thought that the truly large ones first appeared around 8 million years ago, in the Late Miocene. However, a partial fossil from Antarctica first described this year suggests that at least one species measuring around 12 metres (40 feet) in length lived there as far back as 34 million years ago. While related to today's filter-feeding baleen whales it was primitive enough that it still had teeth, and probably hunted for relatively large, fleshy prey.

While they are not baleen whales, today's beaked whales have few, if any, functional teeth, and feed by sucking prey into their mouths and swallowing them whole. A new fossil from Denmark seems to have already fed in this manner, despite living alongside a previously known species that could still bite its prey with sharp teeth.

Unusual feeding strategies are far from the only unique adaptations of whales. Analysis of the ear bones of the extinct whale Olympicetus showed that it probably couldn't hear ultrasound, which, given its place on the toothed whale family tree, shows that that feature may well have evolved twice in these animals, despite its seeming oddity. Going back rather further, two new fossils of primitive whales from Africa catch part of the transition from a more seal-like animal to the streamlined form of a modern whale. One from Senegal had fore-flippers that could actively pull the animal along, rather than just be used for steering, while one from Egypt was slightly further along, generating motion by undulating its body, but still possessing hind-limbs and probably lacking a tail fluke.


Synapsida is taking a break for the holiday period and will return on the 5th January

[Illustration by Remie Bakker of Manimal Works, available under CC BY licence 4.0.]

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