Out of the Amazon: A History of the Opossums

Tate's woolly mouse-opossum,
a marmosin species

When most people think of marsupials, it's likely that Australia is the first place to spring to mind; the land of kangaroos, koalas, and wombats among many others. In fact, even the "Australian" marsupials aren't restricted to that country, since they are also found further north, notably in New Guinea - which, for example, has its own species of wallabies. But, more significantly, marsupials are also found outside of Australasia, in the Americas.

The best known of these American marsupials is surely the Virginia opossum (Didelphis virginiana). This lives across most of the US outside of the western mountains and interior deserts, and is also found just across the border in parts of southern Canada, through the whole of Mexico, and well into Central America. It's the only species of opossum found north of Mexico, but it's very far from being the only species of opossum anywhere.

Why Animals Have Whiskers

One of the key defining features of mammals is that they have hair, or are at least descended from other animals that once had hair. The primary purpose of hair is to keep the animal warm, something useful for any warm-blooded animal, especially if it's small (elephants and rhinos, for example, while they do have hair, aren't really what you'd call "furry"). But, over millions of years of evolution, hairs have also evolved to carry out other functions, such as the protective spines of hedgehogs and porcupines.

Another example of specialised hair is that of whiskers. Technically known as vibrissae, whiskers are remarkably common in mammals. When we think of whiskers, it's likely that most people's thoughts jump immediately to the long, mobile, whiskers of cats and mice. But whiskers can also be shorter and less mobile than this, as we see in such animals as horses. Indeed, even if we look at a cat, whiskers are not restricted to the long ones on the snout; they also have whiskers on the eyebrows, on the cheeks and chin, and on their forelegs just above the paws. 

The Social Lives of Giraffes

Many of the cloven-hoofed mammals are herd animals, living in large groups that typically have dominance hierarchies and other relatively complex internal structure. We see this in both of the main families of such animals: the deer and the "cattle family" (which includes goats, sheep, and antelopes besides the obvious). On the other hand, there are certainly plenty of exceptions. Smaller deer, such as muntjacs, for instance, tend to be relatively asocial, apparently gaining more benefit by hiding from predators than they would from living in large and watchful, but visibly obvious, herds.

But it isn't just the smaller cloven-hoofed mammals that can lack sophisticated social lives, because the same also seems to be true of the largest of all such animals: the giraffe (Giraffa camelopardalis). At which point, I should probably take a brief diversion to explain what a giraffe is, and where it fits in the larger mammalian family tree.

Prehistoric Mammal Discoveries 2021

Lesmesodon, a weasel-sized hyenadont.
A new species from Early Eocene France was
 described this year

And so another year approaches its conclusion and the pandemic seemingly isn't done with us yet. I had to self-isolate after a positive test myself for a couple of weeks (no symptoms, though) but if there's one thing that doesn't interrupt, it's blogging, so everything went smoothly here. But now it's time for what's becoming a traditional look at the paleontological discoveries of the past year that didn't get covered here but are worthy of at least a brief mention.

Large Herbivores

When we think of vertebrate fossils, the first thing that pops to mind is almost certainly going to be a skeleton of some kind, or perhaps just part of one. But there are also such things as ichnofossils - fossilised remains of how an animal affected its environment that no longer include any physical part of the animal itself. Perhaps the most obvious of these are fossil trackways - footprints of long-gone animals preserved in mud or other soft material that has since turned to stone. A study published this year examined the tracks left by two species of fossil horse. One of them, a one-toed close relative of the living species inhabiting southern Canada during the Ice Ages, turned out to have been galloping at around 34 kph (21 mph), which is quite fast for its small size (perhaps it was running from something). More significantly, however, the three-toed Miocene horse Scaphohippus was using a relatively unusual gait called the "rack" typically only seen in specialised domestic breeds today. 

Miocene (Pt 30): Horned Armadillos and Marsupial Dogs

Borhyaena

At the dawn of the Miocene, South America had been an island continent for many millions of years, allowing many strange forms of mammal to evolve that were not found elsewhere. Few of these survived the creation of the Central American land bridge, allowing more familiar creatures from the north to replace them. Some of those that did, such as the ground sloths, failed to make it through the Ice Ages, but there are exceptions still around today - descendants of ancient and peculiar animals that first originated in South America.

Among these are the armadillos, a group that was already well-established by the Early Miocene and with many living species, including one that has successfully colonised parts of North America. Miocene armadillos were perhaps more varied than those alive today, largely due to the existence of the glyptodonts. 

Mouse Lemurs in the Mangroves

Well over 6,000 species of mammal have currently been described, many of which have only been confirmed as distinct species in the last few decades. There is undeniably a trend in the 21st century towards "splitting" of species along comparatively fine dividing lines, but it's also the case that this has been supported by advances in genetics and the like that have made it easier to tell when two groups of animals really are different, and not interbreeding.

But the corollary to this is that many of these species look extremely similar. It's often the case that, once a new species has been described on the basis of genetics, we can point to physical differences between it and whichever species we split it off from. But these are necessarily minor differences, perhaps previously thought sufficient to justify a designation as a subspecies, but more likely not noticed at all. Given the physical similarities, we'd expect that their diet, behaviour, and so on would also be similar - they are, after all, very close relatives.

All the World's Deer: A History of the Deer Family

Megaloceros

Over the past year, I have described no less than 53 living species of deer, seven of which are currently considered endangered and one of which, bar a few park escapes, exists only in captivity. And that's excluding Schomburgk's deer which. as I noted, probably went extinct in 1938, and certainly didn't last long beyond that if it didn't. It's the only known species of deer to have gone extinct for centuries, although it might not be the last to do so. But there are, unsurprisingly, given their large size and distinctive features, a great number of extinct fossil species of deer that we know about.

Further along in this blog I have posted a summary of the family tree of the living species of deer. This is derived from molecular and genetic studies, but it turns out to map reasonably well to what we would have guessed purely from looking at the animals and their skeletons, which is good news if we want to try and place fossil deer into it. The first thing that's apparent from the tree is that, as predicted back in the 19th century, there is a deep split within it, representing the two subfamilies: the cervines (at the top in the diagram) and the capreolines.