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| Gordodon, a new non-mammalian synapsid described this year |
Beginnings and Endings
In the modern day, it's pretty easy to tell mammals and reptiles apart. But, if we go far enough back in time, that eventually ceases to be so true. A common misunderstanding is that mammals evolved 'from' reptiles, but, in reality, mammals and reptiles are separate evolutionary lines that have lived alongside one another since long before there were dinosaurs. At least, that's true if we use the modern definition of 'reptile' since, of course, the animals that mammals really did evolve from would have looked an awful lot like reptiles if we'd been able to see them in the flesh.Where we draw the line between these early not-yet-mammals and the real deal is inevitably an arbitrary one, but the usual distinction used when we're looking at fossils is a change in how the jaw is attached to the skull. The standard theory is that this happened to increase bit force, with the muscles shifting to accommodate a new, more efficient, arrangement, and rendering the original jaw joint redundant - freeing it up to do something else entirely. A new study using digital reconstructions casts doubt on this idea, indicating that the muscles didn't re-arrange themselves into the more efficient form until after the bones had already shifted. The researchers suggest that, instead, changes in the size of the jaw made it possible for the other alterations to occur without compromising bite strength, thus freeing things up for the longer term shift.
Another difference between mammals and reptiles is the structure of the backbone. In mammals, this is clearly divided into five different regions: the neck, the chest (i.e. the bit with the ribs attached), the ribless lumbar region, the sacrum (anchoring the hips), and the tail. In reptiles, the changes are much simpler, with no lumbar region, and, even in those that aren't snakes, there are often ribs of some kind along most of the length of the vertebral column. A new report this year suggests that the more dramatic changes in mammals appeared very early on, and were likely driven by changes in the function of the fore-limbs, only later spreading to the bones further back.
At the other end of mammalian evolutionary history, humans have been responsible for the extinction of a large number of species. While that's not in doubt, there is a long-running debate as to how early it started, and how much humans contributed to various early extinctions after arriving in, say, the Americas at the end of the last Ice Age. Long before we were in America, or even Australia, we were, of course, in Africa. A new study traces our influence on large herbivores in that continent, and concludes that the main die-off of such species there started 4.6 million years ago, far too early for even our ape-like ancestors to be responsible for anything on such a large scale. (In fact, our diet was probably more like that of the mostly-herbivorous chimps at the time). Instead, climate change is a more likely culprit.
Large Herbivores
Speaking of large herbivores, they don't get any larger today than elephants. Elephants are unusual in a number of respects, but one of the many odd things about them is the way that they replace their teeth. Most mammals develop a set of milk teeth early on, which are later shed to be replaced by the permanent adult set. But elephants instead have a conveyor-belt system, with new molar teeth appearing one set at a time, with the oldest pair falling out to be replaced by a newer one. Combining this with extraordinarily long molars, so that they have just as much grinding surface as an animal with more teeth at a given time, means that their teeth don't all wear down at once, with a new one available when necessary. (Although there's a limited supply - eventually the last one does wear down, and they die of old age).While this system goes back millions of years, it's only in modern elephants that it reaches its full potential, with the animal dispensing with the unnecessary premolar teeth altogether. A new study shows that this happened independently in African and Asian elephants, with their earlier representatives still having premolars. This, combined with other details of the fossil record, implies that Asian elephants left their original homeland in Africa twice, once before, and once after, this change in their tooth structure.
When elephant-like animals still lived in North America during the Ice Ages, there were basically two kinds: mammoths and mastodons. The general picture has been that part of the reason that these were able to share the continent is that mammoths were mostly grazing animals, feeding on grass, while mastodons browsed on softer forest leaves. A new analysis calls this into question, arguing that, while they did eat different foods, there wasn't really much difference in how tough those foods were. Probably, they ate what they could get, as modern elephants tend to, and, in the Ice Ages, the available options may have been more limited than they would have liked.
In a similar vein, it seems that ancient bison (Bison antiquus), the immediate ancestor of the modern sort, were also pushed into eating food that was less than ideal in parts of their range, but were adaptable enough to do so without going extinct. Fossil bison from southern Mexico ate far more leaves than grass, despite being grazers further north, seemingly without doing them any harm.
Fossil studies also show that an entirely different sort of large herbivore, kangaroos, greatly increased in diversity as grasslands spread across Australia in the Pliocene, much later than suggested by studies based on molecular evidence from modern species. The same study also suggested that short-faced kangaroos, which are now extinct, were actually doing quite well on the increasingly dry Ice Age continent... until humans turned up, which, if true, puts us rather more in the spotlight there than we might have been in Africa.
Small Mammals
Not every fossil animal was a deadly predator or a huge, lumbering, herbivore, and many fossil discoveries relate to creatures that we might often overlook in the grand scheme of things. Back in 2002, a 12 million-year-old fossil was uncovered near Barcelona that initially had palaeontologists excited, because they thought it was an ancient primate; they were rather disappointed when closer examination proved that it was merely a rodent.But eventually, once they had nothing better to do, they removed some more of it from the encasing rock, and revealed its wrists... which were a lot more interesting than they had expected. That's because the new fossil, Miopetaurista, turned out to be the oldest known fossil of a flying squirrel. Or, at least, the oldest known fossil that we definitively know could, in fact, glide, since we do have some teeth that we think probably belonged to something similar. Significantly, the wrists of this animal were almost identical to those of a group of living nocturnal giant flying squirrels found in Southeast Asia, suggesting that their gliding ability was fully developed very early on. This supports estimates from molecular studies that the first flying squirrels may have evolved over 26 million years ago, further back than we might otherwise have guessed.
With rather better flying abilities, a new species of fossil bat was described this year, hailing from New Zealand of all places, and hailing back around 20 million years - it's the first new genus of bat described on the islands since the mid-19th century. Named Vulcanops, it was somewhat larger than its closest living relative, a rather weird, mostly ground-dwelling (!) bat also native to New Zealand. Like that animal, this one appears to have been an omnivore, eating both fruit and insects, although the fine details of its diet were likely different.
Back in 1967, famous palaeontologist George Gaylord Simpson uncovered some fossil jawbones in Kenya that he named Propotto, because he thought that the few parts he had looked like the bones of a potto, a kind of lemur-like animal native to West Africa. Subsequent examination showed it to be a kind of bat... but a new study this year contradicts that, demonstrating that Simpson was right all along. Sort of. Because, if the study is right, this isn't a potto, but a relative of a different lemur-like animal, the deeply weird and slightly creepy-looking aye-aye. And that's significant, because aye-ayes only live on Madagascar, and were assumed to have evolved there... so being found in Kenya is a bit of a shock.
Strangeness in South America
Modern sloths are slow-moving tree-dwelling creatures, so that the idea that there were once giant ground sloths is an odd one. You might think that tree sloths must have a common ancestor, a kind of ground sloth that moved up into the trees, with some kinds subsequently losing more toes than others, but this has long been thought to be merely parallel evolution from two entirely separate kinds of early sloth - a fact further confirmed this year by DNA analysis of the extinct ground sloth, Mylodon.However, if the idea of a ground sloth seems odd to us today, the idea of a semi-aquatic sloth seems even stranger. Yet this is exactly what Thalassocnus, an animal that once lived along the west coast of South America was. Wading through shallow seas to graze on seaweed, this bizarre animal turns out to have undergone thickening of its skeleton, especially the bones inside the nose, as it became steadily more adapted to living in the water. This may have at least partly been to increase its density, making walking along the bottom easier, and similar adaptations are seen in manatees. This would not, however, explain why the bones in the nose are thicker, since they aren't large enough to make much difference in that respect; that parts remains a mystery.
Another mystery is what sort of animal dug large fossilised burrows found in central Argentina, and thought to date back over 5 million years. And, when I say large, I mean that some of them were 150 cm (5 feet) across, indicating that whatever lived in them was certainly larger than a typical rabbit. The few bones found inside them may have been washed in by floods after the burrows were abandoned, but suspects include giant armadillos and, yes, ground sloths.
And, on the subject of giant armadillos, an analysis of the bony armour of some ancient species has revealed the first evidence of fleas preying upon these kinds of animals. Even the tank-like glyptodontids, it seems, were not immune from some of the smallest and most annoying of insects.
Carnivores
While Smilodon is surely the best known of the sabretooth cats, it was merely one of quite a large assemblage of the animals. Yet another genus was added to that roster this year, with Tchadailurus, a lynx-sized sabretooth from an ancient lake shore in Chad. Perhaps significantly, it is known to have lived alongside an early relative of humans, Sahelanthropus, sometimes claimed to be the first form to have walked upright.But not all fossil cats were sabretooths, and another analysis this year looked at the ear region of the fossil cheetah Acinonyx pardinensis, an animal with some skeletal similarties to leopards, despite not being that closely related to them. Modern cheetahs have significant modifications to their middle ears, thought to be related to their ability to maintain head balance and visual stability during a high-speed chase. The fossils, despite dating back barely more than 0.1 million years ago, did not have these modifications, implying that's they were a surprisingly late adaptation in cheetah evolutionary history.
Another group of fearsome carnivores were the great bone-crushing dogs of the Miocene and Pliocene, North American animals thought to have been the equivalent of hyenas in Africa and (formerly) Eurasia. At least, it's always been assumed that they crushed up the bone of their victims, or other carrion, because their teeth were perfectly adapted for doing just that. Now, however, we also have direct evidence, from actual 5 million-year-old fossilised dogshit uncovered in California. Not only did the dung reveal chunks of chewed up bone, the bones in question came from animals much larger than the dogs, suggesting that they must have hunted in packs. Their arrangement in what appear to be carefully placed piles also implies that the extinct Borophagus may have used them for scent-marking its territory.
Cetaceans
A number of discoveries this year shed light on how early whales and dolphins evolved from relatively normal looking ground-dwelling animals to the highy derived aquatic creatures they are today. Looking at the backbone once again, one new analysis this year looked at a whole range of animals along what is now quite a well-known continuum from walking to swimming. In particular, they looked at the flexibility of that ribless lumbar region, the existence of which in mammals has led to a whole range of adaptations not seen in lizards and their ilk (human bipedalism may well be another).It turns out that, after entering the water, this region of the spine became more flexible than it had been previously, allowing early whales to swim by undulating their bodies rather than paddling with their hind limbs. Later still, however, that part of the backbone became more rigid again, as whales switched from that undulating motion to just propelling themselves along by moving their tail flukes up and down, as they do today.
A bigger puzzle in cetacean evolution is presented by the existence of the giant filter-feeding baleen whales, of which right and blue whales are probably the most famous. One study this year showed that their ability to hear (and presumably emit) infrasound originated around the mid-Miocene, as evidenced by the shape of their inner ears, but the real question is how they evolved their odd feeding habit.
Around the middle of this year, a description the skull of Llanocetus, the second oldest fossil baleen whale known, was published. So early was it, in fact, that it didn't have baleen, and still made do with a set of teeth. That isn't unique, but what was interesting was the relatively large size of the animal - at 8 metres (25 feet) in length, it was about the size of a modern minke whale. That's actually kind of small for a modern baleen whale, but much larger than anything else that was around at the time. At least some whales, it seems, developed their great size before they developed their modern style of feeding.
And then, at the end of the year, came a discovery that was perhaps, even more significant. This was Maiabalaena, at 33 million years old, not much younger than Llanocetus but, crucially, entirely toothless. Yet, examination of its jaws showed no hint of the necessary adaptations for baleen either. Perhaps surprisingly, it seems that whales lost their teeth before they developed filter-feeding, and went through a stage when they had neither feature. Most likely, Maiabalaena was a suction-feeder, slurping up soft-bodied or small prey, just as many fish do today.
Only later did baleen whales develop baleen.
Synapsida is taking a break for the holiday period and will return on the 6th January
[Picture from Lucas, Rineheart & Celeskey, 2018. Available for non-commercial use under CC-BY-NC-SA license.]
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