Sunday 19 June 2011

The Bone-Crushing Dogs of Oregon

Aelurodon, a borophagine dog
The mammals are an ancient group, far older than the birds, and, by certain definitions, as old as the reptiles. But the true rise of the group - the period often called the Age of Mammals - only begins with the extinction of the dinosaurs, 65 million years ago.

But 65 million years is an incredibly long time. In comparison, mastodons and sabre-tooth cats both died out round about 10,000 years ago, not even 0.1% of that great timespan. Over the course of those millions of years, many groups of mammals (and, of course, of other animals, too) came and went, and the faunas and landscapes of the world changed time and time again. If sabre-tooth cats prowled the countryside just a few thousand years ago, you can see that you don't have to go back very far to find a world that looks, in many respects, quite different to that of today, and the further back you go, the greater the differences become.

For example, compare Africa and North America today. Africa has lions, elephants, leopards, dozens of different kinds of antelope, rhinos, hyenas, giraffes, hippos... the list of large and cool-looking animals just goes on. North America, while not truly impoverished, has far less in comparison. There's pronghorns, a few types of deer, three bears, wolves and pumas and, in terms of big animals, not a lot else. But it used to be like Africa just a few tens of thousand of years ago, never mind millions. America had elephants - or at least mastodons - along with many more large and fierce cats, and plenty of odd-looking antelope like creatures, and more besides. If you could go on a safari trip to the North America of millions of years ago, there would certainly be plenty to see.

This 65 million year span is conventionally divided into seven epochs. The last of these, in which we currently live, only starts with the end of the last Ice Age around 10,000 BC, and is therefore so short that you can't even see it on the chart I've used here. Most of the others are so long that they need to be divided into much shorter chunks for them to be of any real use when we try to get a picture of what appeared when.

The Miocene is the fourth of these epochs, and its name means "less recent" because there were less familiar looking animals alive then than in the three epochs that follow it. It is, in a sense, the last epoch before the sorts of animals we see today began to really gain a foothold. One illustration of the way that this happened is the story of the dogs, and a recent review of local fossils by Eric Ekdale and Timoty Rowe of the University of Oregon provides a snapshot that feeds into the bigger picture.

Oregon today has coyotes, wolves, and two species of fox - the red and grey. The wolves have only recently returned to the state, and are still struggling to gain a foothold, but the other three species are reasonably secure. Like all living dogs, all four of these animals are true "canines", in the true technical meaning of the word (which includes foxes, which, in more common parlance, we would call "vulpines"). But these are not the only sort of dogs that have ever lived.

"True"     "True"
 Dogs      Foxes        Borophagines
  ^           ^              ^
  |           |              |
  |           |              |         Hesperocyonines
  -------------              |                ^
        |                    |                |
     Canines                 |                |
        |                    |                |          Bears,
        ----------------------                |           etc.
                  |                           |            ^
                  |                           |            |
                  -----------------------------            |
                                |                          |
                          (First dogs)                     |
                                |                          |
The group of "true" dogs (technically "canins" - note the missing 'e') includes the grey fox, among others that we traditionally think of as foxes. The red fox of Europe and North America really is a fox, though!

As the chart above shows, the canines - modern dogs and foxes - lived alongside the borophagines ever since both groups diverged from the more primitive hesperocyonines at around the end of the Eocene. During the Miocene, however, the borophagines were by far the dominant group, with there being (so far as we know) relatively few species of true dogs or foxes around at that time. The fossils in the review date from around 10 million years ago, just as the balance was starting to switch, and the borophagines began to go into decline. The hesperocyonines had already gone by this point, apparently out-competed by their relatives during the previous Oligocene epoch.

I've mentioned before that many fossils are very incomplete, and give scientists very little to go on. This was the case with many of the fossils in this review, which included five teeth, one foot bone, and one leg bone, all from different animals. The researchers were fairly confident that these all belonged to borophagines of some sort, but had no way of saying any more than that. For example, one tooth was a premolar. It was the wrong shape to belong to a cat, too well-developed to belong to the only other sort of large carnivore around at the time, and too large to belong to any sort of fox. That only leaves the borophagines, but beyond that, who can say?

Another pair of teeth, however, both seemed to come from the same animal, and had been described as belonging to a fox (Vulpes sp) in a previous study back in the 1960s. The authors of the review thought that that was a bit over-optimistic, not least because that sort of fox isn't thought to have appeared for another three million years. They do agree, however, that it isn't a borophagine, and therefore is a true "canine" dog. Although there's not really any way to know, it might have belonged to Leptocyon, which lived across much of North America at this time. Leptocyon was smaller than a red fox, and slightly larger than a kit fox, and probably looked quite like both of these animals.

Its significant that these remains were the only ones that belonged to something resembling a modern canine. With so many borophagine specimens, its may be that this animal - whatever it was - was rarer than they were, confirming the story that it was the borophagines that were the top dogs of their day.

The other three fossils all belonged to different species of borophagine. That means that, 10 million years ago, like today, Oregon had at least four species of dog, although there's every chance that there were more we don't know about. The most complete fossil belonged to a coyote-sized animal called Carpocyon, that had previously not been found this far north and west. Unfortunately, it was missing the skull. Palaeontologists like skulls; they're usually the most distinctive bit of an animal, and in this case, it meant that we can't tell exactly what sort of Carpocyon it was - there are at least two possibilities.

It's the other two animals that really show how the dogs of modern Oregon are no match for those of ten million years ago, and they're also the only ones we can identify the exact species of. One, Epicyon saevus, was perhaps about the size of a rottweiller, but the other, its close relative, Epicyon haydeni, was a really, really, big dog. It was larger than a black bear, and had powerful, bone-crushing jaws that could presumably have demolished the entire carcass of anything it ate, skeleton and all. Unlike Africa, North America never had hyenas, but it had the great pack-hunting Epicyon haydeni instead, which lived in the same way and was considerably larger.

It must have been one of the most fearsome predators of its day, and it's hard to imagine anything would have chased it away from a kill, except possibly a sabre-tooth. Apart from the large size, it probably looked more like a wolf than a bear or hyena, but was surely a muscular and impressive animal. These are the animals that once terrorised the wilds of Oregon.

[Picture from Wikimedia Commons. Cladogram adapted from Wesley-Hunt & Flynn, 2005]

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