Sunday, 13 December 2015

The Dog Family: Fossil Dogs

Hesperocyon, the earliest known "dog"
Despite their variety, all dog species alive today are placed in a single subfamily of the wider historical dog family. Of course, what we consider a "subfamily", rather than some other taxonomic ranking, is entirely arbitrary, but it does indicate that what we have now represents just one branch of a much older family tree.

This living subfamily are the Caninae, and thus, strictly speaking, the term "canine" can be applied to all living dogs, even the foxes,  which are "vulpine" in more common parlance. The canines have been remarkably successful. Thanks to the existence of the dingo, dogs are one of the few terrestrial mammal families with wild representatives on every continent save Antarctica. (Although not the only one, of course... mice, in particular, get everywhere). That is, at it happens, entirely due to the canines, since the other forms of dog never left their continent of origin.

It's not easy to say why the canines succeeded while the other forms of dog did not. Among their distinguishing features are some modifications to the bone structure of their ankles, and, in many cases, to the length of their legs, which likely adapted them to become more efficient runners, better able to chase down prey. But, while that surely must have helped, there may also have been an element of luck involved, with the movement of the continents, and changes to climate that favoured more open, grassy terrain (ideal if you happen to be a running animal) coming along at just the right time.

If so, the canines certainly had to bide their time waiting for things to get just right. They are a surprisingly ancient lineage, dating back as far as 34 million years - about half the way back to the time of the last dinosaurs. For the great majority of that time they were found only in North America, and for about two thirds of it they were represented by just one genus: Leptocyon.

There were, to be sure, many species of Leptocyon, and they wouldn't all have looked identical - any more than all jackal species or all fox species do today. In general, though, they were small, slender dogs with long snouts and low foreheads. Although there was some variation in their size, including one species that was even smaller than the living Fennec fox, they typically stood around 25 cm (10 inches) at the shoulder, which is about the size of a bichon frise or a small Jack Russell, albeit longer and more fox-like in shape.

That they survived for so long with only minimal changes to their form is a testament to their adaptability. The shape of their skull suggests that they hunted small, fast-moving prey, while microscopic analysis of their tooth enamel also suggests a diet similar to that of living foxes. Which is to say, they were likely omnivores with a preference for meat over fruit, and that much of the meat that they caught belonged to things like mice, along with the odd insect.

It's not until around 10 million years ago that we see any broader diversification among the canines. That's the approximate age of the oldest fossil foxes, still restricted to North America at that time. But the real break-out occured with the dawn of the Pliocene epoch around 5 million years ago, when foxes finally escaped from the continent, and reached Asia. (I should, however, note one oddity here: one fossil fox from northern Africa has been dated to as far back as 7 million years ago, which, if correct, suggests that something was going on rather earlier then we're otherwise aware of).

The living canines
(Click to enlarge)
Oddly enough, it seems that foxes then went into something of a decline back in North America, maybe even dying out altogether for a while. In Asia, however, they proved very successful, spreading out to both Europe and Africa in fairly short order, leading to the great many species we have in the Old World today. About a million years later, early wolf-like dogs also left North America for Asia, although they seem to have taken a little longer to get all the way to Africa than the foxes did.

While we're not entirely sure quite where raccoon dogs fit in the dog family tree, they first appear around 6 million years ago, by which time they were already in Asia. At one point, a close relative of the living species reached as far as Europe, although claims of an African species may be a case of mistaken identity.

By this time, the non-fox canines back in North America had been diversifying for some time. An early example is Eucyon, which was about the size of a modern jackal, or a large beagle, and had a distinctly wolf-like shape. Probably more carnivorous than its ancestors, and hunting larger prey, although not to the extent of the modern wolf, Eucyon is yet another example of a dog that left North America, in it's case as much as 6 million years ago. While many of the rather fragmentary remains assigned to the genus may well actually belong to other creatures entirely (and not even necessarily to dogs), there is no doubt that it was widespread in Eurasia, and probably one of, if not the, earliest dogs to reach Europe.

Eucyon, or something much like it, may well be the ancestor of both the modern wolf-like dogs, but also of many, or even all, of the South American species. Like the wolves and foxes, the latter first appeared in North America, but they made their break-out rather later, heading south around 3 million years ago when the continents first joined up. Finding no other dogs in the south, or anything much like them, they quickly diversified, taking over many similar niches. This, perhaps, is why the South American foxes look so similar to their kin elsewhere, despite being more closely related (possibly via Eucyon) to the wolves.

Speaking of which, there is one final great migration to mention in the history of the canine dogs. It's the most recent of all, occurring during the Ice Ages, less than 1 million years ago... and it's in the opposite direction to all the others. True wolves first appeared in Siberia, and their arrival in Europe a little over 2 million years ago in the so-called "Wolf Event" heralded a dramatic change in European wildlife as the Ice Ages dawned, also bringing with it the frozen steppe that proved so habitable for mammoths.

But the big migration was the return of both wolves and foxes to North America, heading back over the Bering land bridge millions of years after their ancestors had travelled the other way. The result is that most North American species today are actually descended from Asian species, despite the continent having been their original homeland. Coyotes are likely an exception, descending from the native species Canis lepophagus, which had remained behind during the interim. Grey foxes are another, belonging to a lineage that never seems to have left at all.

But what of the other subfamilies, those that didn't make it through to the present day? There are, in fact, two of them, the early hesperocyonines and the later. and more diverse, borophagines. Both were natives of North America, with the latter never leaving the continent at all.

The borophagines appeared at almost exactly the same time as the canines did, and appear to be a sister lineage, descended from the same hesperocyonine stock. When taken as a whole, the differences between borophagines and canines are slight, being based mainly around the exact shape of the teeth, although they did retain small dewclaws on their hind feet. Individual species, however, were often more distinctive.

The very first borophagines, such as Archaeocyon, were, like the first canines, somewhat fox-like. They were about the same size as Leptocyon, too, although with longer, more slender bodies, and likely had a similarly omnivorous diet. Even this early, however, there was some variation in the group. The best example is Otarocyon, a very small dog (about chihuahua-sized) with a pointed snout and greatly enlarged ear chambers in the skull. If, as seems entirely probable, the animal's external ears were as large as the skeletal parts would suggest, then it must have looked remarkably like a living fennec fox, and it presumably had a similar lifestyle.

Not long after this, however, we see a change in the direction of borophagine evolution, with the animals apparently becoming less carnivorous, perhaps to avoid competition with the hesperocyonines, which were still going strong at the time. A typical example is Phlaocyon, a small fox-sized animal that has been found from Florida to Oregon which first appeared around 30 million years ago, and kept going until about 16. When it was first discovered, its teeth were so well-adapted to a mixed omnivorous diet that it was thought to be a kind of raccoon.

But if Phlaocyon ate a lot of plant matter along with its meat, its contemporary Cynarctoides was even more extreme. This had strange crescent-shaped crests on its molar teeth, unique among dogs, but surprisingly reminiscent of the sort of thing we'd expect to find on cows. Were it not for the fact that was, after all, a dog, we might even think that it was a herbivore. But then, if there's a bear that eats bamboo, is that really impossible?

Starting around 20 million years ago, the primitive hesperocyonines began to die out. This left room in America for a much more predatory dog, and while the early canine Leptocyon continued to bide its time, the borophagines rapidly moved in to fill the gap. From this point onward, new species start to get progressively larger and more carnivorous. By the time of Aelurodon, which lived in the central and western US about 16 million years ago, they were nearly wolf-sized, ate little if any vegetable matter, and had teeth strong enough to crack bone.

And they hadn't finished.

The apogee of big scary dogs was surely the borophagine Epicyon, which first appeared around 12 million years ago. There at least two species, but the largest, E. haydeni, stood about 90 cm (3 feet) tall at the shoulder, making it more the size of a bear than a dog. The largest dog that ever lived, it seems to have been a pure carnivore, and one of the top predators of its day.

Unfortunately for the borophagines, it seems to have been this specialisation that eventually ended their reign. As the climate changed around them as the Pliocene approached, they found it harder to adapt to a changing fauna and prey base, and the canines finally began to supplant them. The last of the borophagines was Borophagus itself, for which the group is named. At about 60 cm (2 feet) in shoulder height, it was far less dramatic than Epicyon (its likely ancestor), but its jaws were even stronger, equipped with heavy, modified teeth that could pulverise bone like a modern hyena. As such, it would have completely destroyed the carcass of whatever it hunted, gaining the absolute maximum of nutrition from the corpse. But the world changed, and it died out with the dawn of the Ice Ages, about 2 million years ago.

We know rather less about the hesperocyonines, the ancestral subfamily of primitive dogs from which the other two seem to have evolved. The earliest dog for which we have good evidence is Hesperocyon itself, which lived in the west of North America from about 40 million years ago, and which might be the common ancestor of all later dogs. It was the size of a small fox, with a slender body, a long snout and relatively short fore-limbs.

Hesperocyon had a number of primitive features, including the presence of five functional toes on each foot, rather than the four found in members of the other two families. Judging from the shape of its ankles, it did not walk fully on its toes, and spent at least some of the time walking on the soles of its feet. Taken together with the shape of its claws, this may suggest that it climbed trees, something for which modern dogs are not typically known, but that would fit with the lifestyles of the even earlier carnivores from which it had presumably evolved. It was clearly a successful animal for the time, surviving for over 10 million years, and leaving fossils in such numbers that it's just about possible it lived in packs of some kind. We even have fossils of its dung, which tell us that it ate rodents and rabbits, albeit presumably along with plant matter.

Later hesperocyonines became larger and more carnivorous, including the first dog to approach the size of a wolf, Enhydrocyon. Like the later borophagines, this had teeth strong enough to crack bone, and was therefore presumably something of a pure carnivore. It died out around 21 million years ago, leaving the way open for the borophagines to take over its role as a predator. It was followed by the last of the hesperocyonines, Osbornodon, which was even closer to a wolf in size, albeit with noticeably shorter legs, and presumably much less running ability. Another pure carnivore, it died out around 15 million years ago, surviving alongside the other two subfamilies for a surprisingly long time.

Dogs represent what we believe to be a particularly early branch in the history of the carnivorans. As such, there is no one group of living animals that are their "closest living relatives". They are equally related to, among others, bears, weasels, and even seals, although more distantly so to the cats and their kin. In the 40 million years since they first came down from the trees (which does, indeed, seem to be the habitat of their ancestors) they have given rise to over 150 identified species, across every habitable continent and living from the Arctic to the desert, and from the tropical rainforest to the urban jungle.

[Photo by Claire H., from Wikimedia Commons. Cladograms adapted from Lindblad-Toh et al, 2005 and Bardeleben et al, 2005.]

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