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| Albertocetus meffordorum, the post-cranial anatomy of which was described for the first time this year. |
But this year, instead of discussing just how many new kinds of bat we discovered in the last twelve months, I'm going to note that my posts on fossil mammals tend to be more popular than those on the living sort, and take a look at a partial assortment of scientific papers published on this subject in the last year that, for various reasons, didn't end up in my regular blog posts. So here goes.
Beginnings
Mammals have been around for a very long time indeed. For much of this time, however, they were quite different, in terms of their relationships, from the sort we have today. Today, over 90% of mammal species are placentals (with most of the rest being marsupials), but we have contradictory information on just how old this particular group is. Studies based on molecular evidence use estimated mutation rates to determine how long ago the different sub-groups of placentals must have diverged from one another to be as varied as they are today. These show that the very first splits within the placental family tree - and thus, how far back its roots must be - occurred way back during the Cretaceous period, when dinosaurs still walked the Earth. The problem is, we don't have any fossils of such animals.Sure, we have fossils of animals that clearly belong to the placental lineage, going back to the even earlier Jurassic period. But nothing that belongs to any group of placentals that are alive today, which means that we don't even know whether these earlier fossil species were literally "placental". A couple of studies this year helped to shed light on this contradiction, both looking at what happened around the time of the K/Pg extinction event - the whopping great asteroid that (possibly with some help from other sources) finished off the dinosaurs. One, using molecular data, showed that there was no sudden burst in the appearance of new kinds of placental mammal after the event, and that the various groups must already have been diverging for some time before it happened, possibly driven by the diversification of flowering plants around the same time.
This was almost immediately questioned, on the grounds that the same study seemed to show that toothed whales (among others) aren't really an evolutionary group, which they quite clearly are. A few months later, a different study tried to work out just how complete the relevant fossil record is, using mathematical techniques and evaluation of known fossils to estimate how many unknown fossils ought to exist. This concluded that the reason we haven't found any fossils belonging to known placental groups from Cretaceous strata is that they didn't exist, and that mammals really did undergo a rapid diversification only after the dinosaurs got out of the way. The debate continues.
On the subject of early mammals, however, one of the major pieces of evolutionary news this year was an analysis that showed that whatever mammals did exist during the Cretaceous (and there were certainly many) they were likely all nocturnal. Daytime living only became common, apparently, after the extinction of the non-avian dinosaurs, and probably first appeared among monkeys.
Endings
At the opposite end of the scale, where did all those cool and large mammals of the Pleistocene go? Why don't we still have sabretooth cats and woolly mammoths? Quite how much this has to do with human hunting and general expansion, and how much to do with the changes in climate as the Ice Ages ended has been long debated, and, once again, nothing this year provides a slam-dunk answer to that question. In fact, a study on the relevant extinctions in North America only goes to show how complex the question is; the large mammals in Alaska died out before humans ever reached the continent, so we aren't to blame there, whereas around the Great Lakes, they lived alongside one another for thousands of years, which at least means that we must have taken our time. Other places will inevitably be in between, making it hard to disentangle cause and effect.A similar study from Australia, however, showed that a particular kind of giant wombat (it weighed about half a ton) has fossils from areas known to be inhabited by Aboriginals at least 17,000 years after they first arrived. Which, as a minimum, shows that it wasn't humans' first arrival on the continent that finished that particular animal off, no matter what may have happened later.
Small Mammals
Not all extinct mammals were huge, of course, and most were quite small. Among studies on these relatively innocuous animals, one scanned the interior of a fossil skull belonging to a squirrel that lived over 20 million years ago to assess the size and shape of its brain. This showed that the animal's brain was was similar in size to that of living ground squirrels, but also that it was expanded in areas associated with either vision or general agility - both of which would be important if it had started living up trees. It may, then, have been just on the cusp of heading upwards.Another small mammal, Necrolestes from Miocene South America, lies outside of any living group of mammals, and may be the last descendant of a separate branch that arose even before the ancestors of placentals and marsupials diverged. A new analysis of its skull shows it to have been a burrowing animal that used its snout to push soil out of the way, with internal bony buttresses unlike those of any living species giving it extra rigidity. It also had alterations to its ear regions that suggest it was particularly good at hearing the sorts of low-frequency sounds and vibrations that it would need to while living underground.
Large Herbivores
Woolly mammoths have an advantage over many other prehistoric species of mammal in that they not only lived relatively recently, as such things go, but in frozen environments that sometimes preserve enough of their tissues that we can actually do the same sort of genetic analyses on them that we use to determine relationships among living species. (Albeit, as one might expect, with quite a bit more difficulty and resulting uncertainty). One such study published this year looked at the mitochondrial DNA of woolly mammoths across Europe, Asia, and North America, and showed that they had three distinct populations that didn't mix much, perhaps because of inhospitable ice sheets in the way. In a similar vein, an examination of the DNA of one of the very last woolly mammoths, sheltering on an isolated island off the north coast of Siberia as recently as 2,300 BC (so, younger than the Great Pyramid of Giza, then) showed just how inbred that final population was, with a whole slew of genetic defects showing up.Perhaps even more significant, we now have some detailed genomic data on the straight-tusked elephant Palaeoloxodon antiquus, which lived in Europe a quarter-of-a-million years ago. Analysis showed that, contrary to previous expectations, this long-gone species was closely related to African, rather than Asian, elephants. Which is interesting, because it's the first evidence that "African" elephants ever lived anywhere other than, well... Africa.
It's not just elephants that have benefited from this sort of study. An analysis of degraded proteins from fossils of the Ice Age European rhinoceros Stephanorhinus showed that it was, most probably, a close relative of the all-but-extinct Sumatran rhino, as well as of the decidedly extinct woolly rhino, and was closer to the living Asian rhino species than to the black and white rhinos of Africa. Studies on the mitochondrial DNA of the weird Macrauchenia of South America also confirmed, as expected, that it belongs to no living group of mammals, but that it was closer to rhinos and horses than to, say, antelope or llamas.
And, before we leave the large herbivores, I should also add the announcement of the first evidence that any marsupial, anywhere, migrated long distances on an annual basis; specifically, that the giant wombat Diprotodon may have migrated 200 km (125 miles) each year between Australia and New Guinea, when such a thing was still possible.
Carnivores
It' not just herbivores that have been the subject of ancient DNA analyses this year. We also had a study comparing the mitochondrial DNA of four sabretooth cats, one of them a Smilodon, and the other three specimens of Homotherium from both Europe and North America. This confirmed the long-standing belief from skeletal examinations that sabretooth cats formed their own distinct lineage within the cat family, rather than being relatively unrelated animals that just happen to look similar. The study also showed that there was so little difference between the three Homotherium samples that the same species likely lived on each side of the Atlantic, rather than there being a subtle distinction, like that between modern Canadian and European lynxes.Speaking of Smilodon, a study on the vast number of fossils recovered from the La Brea tar pits showed that they had a harder time hunting than dire wolves did, being more likely to injure themselves while doing so. They were also more likely to injure themselves on the shoulders or the small of the back, while dire wolves injured themselves more on the legs and neck; this is likely related to the animals' differing hunting styles, with the cats being ambush predators and the wolves chasing their prey down.
Analyses of the bones of the late Miocene bear-dog Magericyon from Spain showed what appear to be anchoring points for unusually strong and flexible neck muscles. These might well have increased their killing efficiency, and, perhaps at least as importantly, how quickly they could disassemble and consume a carcass - a useful trait when a sabretooth might try and steal your meal, as lions and hyenas do to leopards today.
Cave bears lived in Europe much later than either bear-dogs or sabretooths did. It's been over a decade since DNA analysis on these large prehistoric animals showed that they were a distinct species closely related to living brown bears, and now this information has been put to use examining how their brain size must have changed over the course of the evolution. It turns out that their bodies grew faster than their brains did, making the latter unusually small for the animal's overall size. This may have been because bodily bulk was particularly important in an animal that needed to build up enough reserves to sleep through the Ice Age winters, and to be large enough to avoid other predators so that, being relatively vegetarian itself, it only needed to outwit plants.
Elsewhere, an analysis of the ears of the primitive carnivore Hyaenodon showed that it might have been able to climb trees, which would be a bit surprising if true, since, as its name implies, it was otherwise similar to a hyena. And, much less surprisingly, isotope studies on the diet of the giant ground sloth Megatherium showed that, contrary to some rather strange (but serious) suggestions, it wasn't secretly a carnivore at all, and ate only plants.
Cetaceans
The evolution of the gigantic, filter-feeding baleen whales (the group that includes the blue and right whales, among others) has long been something of a mystery. How did they go from being a fairly normal sort of mammal, with teeth, to having the large, complex, and decidedly non-toothy filter-feeding apparatus they have now? They have certainly had it for a long time, as shown by a description this year of the baleen plates of the Late Miocene whale Piscobalaena, something that, due to its lack of bone, is rarely preserved in the fossil record.But we do know of early members of the baleen whale lineage that did not yet have these plates, and still had sizeable teeth in their jaws. It had often been supposed that they may have used these teeth to filter feed (as crabeater seals do today), before evolving the plates, but a new review of the evidence suggests that the teeth were too sharp for this to have been a plausible function. Instead, these early whales may have fed more as typical seals do, sucking in prey and slicing it up. How they got from that to sucking up krill remains a mystery.
One thing that apparently didn't change, according to another study this year, was their hearing. Early toothed whales of the baleen lineage had ear-bones that suggest that, like the modern species, they were already adapted to listening for very low frequency sounds. This may imply that low-frequency hearing was originally the norm for whales, and that it is the high-frequency hearing of sonar-using dolphins and modern toothed whales that is the more recent adaptation - the opposite of what was previously thought. At the very least, it shows that the adaptations to hearing low frequency occurred before the whales became exceptionally large, rather than the two events occurring together, as one might expect.
Synapsida is taking a break for the holiday period, and will return on the 7th January.
[Illustration from Boessenecker, Ahmed, & Geisler, 2017. Available under the Creative Commons Attribution License.]
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