 |
| Zalamdalestes |
Large Herbivores
Woolly mammoths (Mammuthus primigenius) are among the best-known of all prehistorical mammals to the general public and, aided by the fact that some lived recently enough to be preserved in permafrost, they are also amongst the best-studied. But there is still more to learn about the details of their lives and habits. A study published this year looked at the detailed isotopic composition of a fossil mammoth that had died about 12,000 BC in Alaska, showing that she was female, and had migrated over 1000 km (600 miles) during her lifetime, having originally been born in the Yukon. The area in which she died was popular with mammoths, but also with humans - she may have died peacefully, but the presence of people where mammoths congregated may not be a coincidence.
Ice Age North America was also home to early species of bison, which were replaced by the modern sort only around 10,000 years ago when the climate warmed. Like the mammoths, these are often recent enough that we can do detailed isotopic analysis on their remains, revealing details of their diets in life. A study this year looked at ancient bison (Bison antiquus) and their own predecessors (Bison latifrons) from the lowlands of Florida to the mountains of Idaho, showing that they had a more flexible and less heavily grass-based diet than often assumed, with those in California and Colorado even switching to different foods with the seasons.
They weren't alone, since another study did the same for the steppe bison (Bison priscus), the Eurasian counterpart of the American species. Their diet, too, turned out to be less heavily grass-based than thought, allowing them to inhabit woodlands as well as pure steppe land. Nonetheless, it was less flexible than that of living European bison, which may explain why they died out when the climate warmed. Related to bison, a genetic study revealed some of the population history of aurochs (Bos primigenius) showing that North Asian and European populations merged during the last Ice Age, retreating to Spain and Portugal at its height before recolonising the continent as it thawed. Some bulls from that resulting population were later bred with domestic cattle descended from a separate Middle Eastern group of aurochs, eventually giving us the cows we have over most of the world today.
Not a fossil species, because they are still around, but another study traced the history of fallow deer (Dama dama) in the same manner. They also retreated to the Balkans and Turkey during the Ice Ages (as well they might, being forest animals) but, while they spread out across the Mediterranean early on, they didn't recolonise northern Europe of their own accord. Instead, the founding population in Britain was brought across by the Romans, and still shows genetic connections with Italian fallow deer today.
The success of fallow deer came at the expense of older species. Haploidoceros is known only from Iberia and southern France, but seems to have been successful there. A new study shows that early members of the species ate a significant amount of grass alongside their preferred diet of leaves and browse, allowing them to co-exist with other deer (mainly, but not exclusively, red deer) by eating different food in the same woodlands, but that they later became more restrictive, allowing fallow and roe deer to displace them.
Staying with Iberia, but moving back further in time, we have a study of that well-known Spanish animal, the hippo. Specifically, the ancient hippopotamus (Hippopotamus antiquus) which new research reveals to have arrived in what is now Spain around 1.7 million years ago, and to have survived there long enough to mingle with the modern species 450,000 years ago. That died out in the country around 100,000 years ago; two Spanish sites even show signs of human butchering of hippo carcasses, although it doesn't follow that they hunted them directly, rather than scavenging. The species reached Italy even earlier and a study published this year on remains from that country estimates that it would have weighed around 3 tons in life - twice the average weight of the modern species.
The woolly rhinoceros (Coelodonta antiquitatis) died out with the end of the Ice Ages, but there remains ongoing controversy as to how much that was due to climate change and how much to the obvious spear-wielding suspects. A new study of the population history of the species favours the former as the main culprit, but notes that low-level human hunting eventually trapped the rhinos in southern areas outside of their preferred habitat, hastening an extinction that might otherwise have taken longer. Before that happened, woolly rhinos lived happily in Siberia, and the recent discovery of a mummified animal in the permafrost there confirmed for the first time that the animal had a fatty hump on its neck and withers that must have allowed it to store nutrients during the long, harsh winters. Cave painting had previously suggested the existence of such a hump but we had never found remains good enough to prove it before.
Megadolodus was a 3-toed animal living in northern South America around 12 million years ago. A member of the litopterns, it would have been slightly smaller than a modern wild boar, but the shape of its teeth suggested that it likely had a similar diet to those animals. This year, a new analysis of a fossil from Peru suggested that might not be entirely accurate, with far less soft fruit in their diet than would be true of typical pigs. The authors suggested that babirusas might make for a more accurate comparison. On the other hand, the much earlier Protolitopterna, despite belonging to the same "herbivorous" group, seems to have been more of an omnivore, eating at least some animal matter.
Carnivores
While most isotopic studies are conducted on fossil bones, a new method for analysing teeth was used on fossil cave bears (Ursus spelaeus) for the first time this year. This was able to uncover details of its life history, showing that cave bear cubs began to take a small amount of solid food starting at 3 months, and were fully weaned by ten months, so that they must have stopped suckling by the time of their first hibernation.
The 11 million-year-old panda Kretzoiarctos was previously only known from Spain, but a new specimen was described from Germany this year. Examination of the wear patterns on its teeth suggest that it did not eat tough plant food like bamboo. Instead, their diet may have been similar to that of the living spectacled bear, which mainly eats flowers, bulbs, and fruit but will also eat meat when it gets the chance.
Perhaps the biggest item of mammalian paleontological news this year was the analysis of a 32,000-year-old frozen mummy of the cub of a large cat uncovered in Siberia in 2020. Astonishingly, this proved to belong, not to a cave lion or the like, but to a three-week-old Homotherium scimitar-toothed cat. This is the first time we have found more than the skeleton of a sabretooth, and it confirmed the shape and size of the neck and jaw muscles that we had already inferred from their attachment points on the bones of adults. Notably, the paws were wide, probably for walking on soft snow, and the ears smaller than in most cats, likely also as an adaptation to the cold weather.
Staying on the topic of young sabretooths, another study used fossil remains from the LaBrea tarpits to uncover the early life history of Smilodon. This showed that Smilodon cubs did not acquire the adult shape of the jaw until later than we would expect from looking at lions. This implies that were weaned later than lion cubs are, and that the development of the sabre fangs took long enough that mothers probably cared for their cubs for longer than lionesses do - perhaps a couple of years or so. A study on the sabretooth not-actually-a-cat-but-sort-of-related Barbourofelis indicated that that might not have hunted and killed in the same way that real sabretooth cats did, perhaps being less specialised; the authors point out that there may be more variation the habits of such animals than we often think.
Meanwhile, an analysis of the fossil dirk-toothed cat (a type of sabretooth) Meganteron from Spain shows that they survived in Western Europe for longer than we had thought, probably dying out with a climatic shift towards drier weather around 1 million years ago at the start of the Middle Pleistocene, although they survived for another 500,000 years in China.
An analysis of a 7-million-year-old Kenyan fossil of the running hyena Ictitherium suggested that its long, slender, legs resembled those of a maned wolf more than any other living species. This, the authors suggest, may indicate a similar lifestyle, allowing this early hyena to pounce on prey and to look over the tops of tall grass in open plains and savannah.
Another study this year looked at the relatively mysterious animal Triisodon, which lived in North America 60 million years ago - long before the last common ancestor of all modern carnivoran mammals. This concluded that it was one of the first large-bodied carnivorous placental mammals, weighing up to 44 kg (97 lbs) and with powerful limbs that would probably also have been useful for digging.
Other Land-dwelling Placentals
One of the key questions in primate palaeontology is how our ancestors evolved their bipedal locomotion. The traditional way of attempting to evaluate the locomotory abilities of animals is to examine the shape of their joints. For instance, a study this year on the partial skeleton of an unidentified species of ape from Kenya dating back 13 million years concluded that it still lived in the trees, but did not have the specialised leaping and upright swinging abilities seen in modern gibbons.
However, an alternative is to look at the shape of the inner ear, which contains the organs of balance that would be critical for both climbing and for walking upright. This requires well-preserved fossils, but research published this year on the 8-million-year-old Chinese ape Lufengpithecus used this technique to place its locomotor abilities almost exactly halfway between those of gibbons and chimpanzees, suggesting that it might represent an ancestral method of ape movement - a style from which both non-human great ape movements and human bipedalism could have evolved.
In addition to the inner ear, the inside of skulls can also provide another useful piece of information, in that they give clues as to the shape of the underlying brain. An examination of the skull of the 27-million-year-old Oligocene chinchilla Incamys from Peru showed that this early rodent's brain was larger in areas known to be associated with hearing and vocalisation. This indicates that the animal probably had acute hearing, and the researchers also suggest that the expanded vocal regions imply that it could make a wide range of sounds - something it would likely only need if it lived communally and regularly communicated with others of its kind.
In fact, we don't necessarily even need bones to infer that some animals lived in groups. Yaviichnus is a fossil, not of a specific animal, but of the burrows that it left behind. New research this year looked at the pattern of gnawing teeth marks on the walls of one such burrow in Mexico, concluding that they had been left by multiple members of the gopher genus Gregorymys around 30 million years ago. Not only were these gophers already living in colonies, but, judging from the size of the marks, two different species inhabited and enlarged the burrow, with one presumably having occupied the abandoned home of the other.
While sloths today live only in the trees, prehistoric examples lived on the ground and were typically much larger than their modern relatives. A study this year on the relatively small Ice Age ground sloth Nothrotherium confirmed earlier theories that it was at least semi-arboreal, good at climbing but spending most of its time out of the trees. At the time of its death, the individual in question also had arthritis.
Simomylodon, which lived about 5 million years ago in Bolivia and Argentina, was much larger, probably weighing over 230 kg (500 lbs) when fully grown. Examining the growth patterns in the skeleton of two individuals of different ages, researchers estimated that this particular ground sloth reached maturity at four years old, lived for around 14 years, and had a 9-month gestation period.
Both animals would have been dwarfed by Panochthus, a glyptodont armadillo-relative that probably weighed around 1.5 tons. Much of that would have been due to the heavy armour over its body, including a heavy bony tube around the end of the tail. Examination of an individual with a tail injury showed that this was consistent with it having used it to bash at enemies or rivals, perhaps in the same way that some dinosaurs used their thagomisers.
Water & Air
The great majority of living cetaceans inhabit the seas, and this is, as we would expect, thought to be where they first evolved. Some, however, live in freshwater, and these are thought to have evolved from marine ancestors, something that has evidently happened more than once. The fossil "dolphin" Parapontoporia lived off the coast of California between 6 and 2 million years ago, during the Pliocene, but it is significant because, rather than being a member of the true dolphin family, it is instead related to the recently extinct baiji, a freshwater species that inhabited the Yangtze River. Analysis of the structure of its inner ear this year revealed that it was likely already able to hear the distinctive narrowband high-frequency sounds that baiji used, despite living in an entirely different environment.
Besides the evolution of biosonar, another intriguing event in the evolution of cetaceans is the question of how some of them lost their teeth to become the giant filter feeders we have today. A study early this year looked at the early Japanese baleen whale Yamatocetus. Based on the shape of a bone in its throat, the researchers concluded that it fed in a manner similar to modern right whales, taking a range of small crustaceans, rather than the more specialised diets of modern blue and Bryde's whales, supporting the idea that this would have been typical of the first baleen whales more generally.
Coronodon, on the other hand, was a member of the baleen whale group that lived much earlier and, unlike its modern relatives, still had teeth. Living around 30 million years ago, during the Oligocene, a study this year of the wear patterns on its teeth suggest that it fed by simply snapping and biting at fish, rather than by either sucking them in or by filtering food through its teeth. This places it as rather more primitive than had previously been thought, although whether the path towards true filter feeding took longer than thought, or it was just a late-surviving member of an earlier group is another matter.
Going even further back in time, we come to the 40-million-year-old Perucetus, an early giant whale sometimes suggested to have been even larger than today's blue whales. A new analysis retains the blue whale's crown of "largest ever animal" by estimating that Perucetus likely didn't weigh much more than 70 tons, significantly less than the 100 tons of the modern animal.
Heading back another 10 million years, Onychonycteris was a much smaller animal known for adaptation to almost the opposite environment: it's the most primitive known fossil bat. A study this year showed that it would have been highly effective at gliding, even though it had already begun to develop the ability of powered flight. This supports the theory that bats originated in the trees, gliding from branch to branch before they learned to fly, rather than by running fast and jumping up from the ground, as is one of the more popular theories for birds.
Marsupials and More
Zalambdalestes lived during the Cretaceous period, when dinosaurs still ruled the Earth. Although it's more closely related to placental mammals than to marsupials, it's probably not derived from the last common ancestor of living placentals and so technically doesn't fall within their group. A new study on the bones in its neck showed that they were unlike those of any living mammal, giving it a remarkable ability to shake prey (presumably to stun it) or to suddenly yank worms out of tight crevices without hurting itself. The authors also suggest that the shape of the bones imply that they may have helped to anchor elongated spines on the animal's back and shoulders, perhaps giving it a hedgehog-like appearance.
Enlarged sabre-like teeth were not unique to placental mammals, but were also present in the South American marsupial Thylacosmilus and its relatives. A study this year suggests, however, that, based on other aspects of its anatomy, it cannot have killed in the way that sabretooth cats did. Instead, the authors think that it might have been a specialised scavenger, using the teeth to rip open carcasses before slurping up the soft innards with the help of a long tongue. If anything, they say that the "marsupial lion" may have more closely resembled the sabretooths, even if it didn't have the same dental adaptations.
Carolocoutoia would have been considerably less fearsome than either of these animals. A relative of opossums, it lived in Brazil 55 million years ago, and a new study showed that its unusual blunt teeth were adapted to a diet consisting of nothing but fruit, which presumably would have been common in the exceptionally dense jungles of the time.
The sparassodonts, on the other hand, were definitely carnivores. Sipalocyon was a member of this group of South American marsupials, living around 20 million years ago. A new study of the shape of its brain and of the bones around the ear show that it was not merely the size of a domestic cat but likely had similar visual and olfactory abilities, and had a hearing range resembling that of a Tasmanian devil; as a result, it probably hunted much as modern cats do.
Moving even further away on the mammalian family tree from ourselves, this year saw the description of a new fossil mammal, Patagomaia, that lived during the time of the dinosaurs 70 million years ago. We think of mammals of the time being mostly shrew-sized, but this animal would have weighed around 14 kg (30 lbs), making it more than twice the size of the largest known mammal of the day from the Northern Hemisphere. Quite what it was is debatable. The original authors decided that it was descended from the common ancestor of placentals and marsupials, without actually being either, but an alternate theory by other researchers placed it in a more remote branch (although still not as remote as monotremes). Given that we only have one hind leg and part of the hip, and not more diagnostic features such as the skull, it may be difficult to say which, if either, is correct.
Synapsida is taking a break for the holiday period and will return on the 4th January
[Picture from Arnold et al 2024, cropped from the original and available under CC-BY-4.0]
Merry mammalian Christmas!
ReplyDelete