Brian Switek Musings on Evolution, the Fossil Record, and the History of Science

Prehistoric “bear-dog” Amphicyon was built like a “bear-lion”

In surveying the diversity of living organisms, the 20th century evolutionary theorist Theodosius Dobzhansky did not see "a formless mass of randomly combining genes and traits." Instead he perceived pockets of discontinuity organized around available ecological niches - clusters of occupied "adaptive peaks" separated by rifts and valleys representing vacant spots in the natural world. Big cats, for example, formed their own mountain chain distinct from the cluster of dog species, and dog species were likewise separated from bears. By looking at the ecological and evolutionary topography of species, life's pattern would emerge.

In such a view, it might be supposed that a valley between two clusters of peaks might be an evolutionary "no man's land". There is no living creature intermediate between a dog and cat, for example, so it seemed apparent that there was no niche for such a creature among the Carnivora (thus barring its evolution). Yet, when we take the fossil record into account, the landscape changes - extant mountain ranges erode away and what are present-day valleys are thrust up into jagged peaks. The topography of the past does not neatly match that of the present - there is no set array of niches which bring about the same forms over and over again - and the extinct carnivore Amphicyon is an imposing embodiment of the disjunction between the past and the present.

Popularly called a "bear-dog", the several species of Amphicyon were large mammalian carnivores which ranged over much of the Northern Hemisphere (as well as points in southern Africa) between 20 and 1.8 million years ago. Their popular name comes from their anatomy and their place in the Carnivora family tree. They sat on the caniform ("dog-form") side of a great split among the Carnivora, and Amphicyon species had robust, bear-like features while also being well-suited to chasing after prey as a dog might. Interestingly, however, paleontologist Christine Argot has proposed that the popular name for Amphicyon might be a misnomer. It might be more accurately called a "bear-cat."

The basis for Argot's new study is a nearly-complete specimen of the species Amphicyon major recovered from the approximately 15-13 million year old rock of Gers, France. Complemented with a few parts from other Amphicyon individuals from the same deposit, this skeleton is perhaps the best available guide to the species, and Argot compared its bones to the corresponding anatomical puzzle pieces in the grizzly bear, lion, and wolf. By examining Amphicyon in relation to these three distinct kinds of living carnivore, Argot could set about determining what kind of hunter it might have been.

At the very least, Argot found that the "bear" part of the "bear-dog" moniker fits the prehistoric carnivore well. The shoulderblade of A. major, especially, was large and expanded much like a grizzly bear's, and even possessed an expansion of the bone called a postscapular fossa indicative of muscle attachments which are unique to bears among carnivorans. Its limbs were also very robust - much like a bear's - and the hands and feet of A. major were likewise suited to walking on the whole of these appendages rather than standing on tip-toe like cats and dogs.

The latter part of the "bear-dog" name did not hold up so well. In almost all the bone-for-bone comparisons, the wolf bones were far more slender than those of A. major. There is no doubt that Amphicyon and its close relatives shared a closer relationship with dogs than with cats, but the widest disparity between the species studied by Argot was between the extinct mammal and the living wolf. Clearly, A. major was a different kind of predator.

Interestingly, however, certain aspects of A. major anatomy resembled their counterparts in the lion, especially in the hips and spinal column. When looked at from above, for example, the hips of A. major don't have the same flaring expansions of bone seen in bears or dogs - instead, they have the more streamlined form of lion hips. Likewise, the first two vertebrae which articulate with the skull in A. major - the axis and atlas - most closely resemble the same bones in the neck of a lion. As Argot notes, this may mean that A. major was capable of moving its head through a similar range of motion as the big cat.

When looked at as a whole, it appears that the limbs of Amphicyon were very similar to those of a grizzly bear while parts of its spine and hips were lion-like (with its long tail being unlike that of any of the taxa studied). It appears to be a hunter well-suited to grappling with prey, and may have used its strong arms to wrestle its victims to the ground.

Despite the anatomical details of A. major elucidated by Argot, however, just what it hunted and how it did so is somewhat difficult to determine. The site from which the specimen Argot studied was found used to be a body of fresh water adjacent to a hot open-woodland environment where relatives of living mammals mingled with representatives of now-extinct groups. There were elephants, rhinos, saber-toothed cats, deer, chalicotheres, antelope, and other mammals, but Amphicyon major was the largest of the carnivores, being the size of a large leopard or a small female lion. Given what is known about the hunting habits of living carnivorous mammals, it is likely that it hunted prey as big as - if not bigger than - itself, although studies of the microscopic wear left on A. major teeth hint that it would not have been above chewing some plant food some of the time.

How it would have tackled its prey is presently a mystery. There were many carnivores present in the same habitat, and in places where this is the case today (such as the African savanna) predators are pushed to use different techniques and pursue different prey. Based upon limb proportions, it appears that A. major might have been an efficient runner which ran after its prey at relatively low speeds until it could catch up and grapple its victim to the ground. Then again, it has been suggested that predatory mammals during this time were primarily ambush predators which stalked their prey before leaping out of the bushes. Unfortunately, such hypotheses are difficult to test, but given the bear-like nature of its limbs I could easily envision A. major chasing after a prehistoric deer until it got close enough to bring it to the ground and deliver a crushing bite.

Had Amphicyon survived to the present day, it would occupy a strange place among Dobzhanky's ranges of adaptive peaks. While more closely related to dogs and bears than to cats, in form (and perhaps ecological role) its own evolutionary hill would have poked up between cats and bears. This is why the fossil record is essential to understanding evolution. Living species are all too easily shuffled into discrete components with considerable gaps between them. When the evidence of the fossil record is brought to bear, however, unique formations entirely unknown today spring up, sometimes serving as intermediate forms between modern discontinuities. It provides the context for the natural world which we find ourselves in.

Christine Argot (2010). Morphofunctional analysis of the postcranium of Amphicyon major (Mammalia, Carnivora, Amphicyonidae) from the Miocene of Sansan(Gers, France) compared to three extant
carnivores: Ursus arctos, Panthera leo, and Canis lupus Geodivertistas, 32 (1), 65-106