Weekly Spotlight: Agriotherium

17 04 2010

Good tidings and well-wishes!

Bears are actually very weird creatures. They’ve adopted a plantigrade stance, they’re heavily omnivorous carnivorans, and they’re easily the largest modern amniotes to engage in hibernation (though, it should be noted that many comparative physiologists maintain that the ursid variety of this behavior can be better described as “winter lethargy”). Even the way popular culture depicts them is eccentric: in many regions of North America, indigenous people live in fear of grizzlies (Ursus arctos horribilis) and/or black bears (Ursus americanus) while simultaneously cuddling and adoring their stuffed, manufactured counterparts.

So it should come as so surprise that the prehistoric relatives of these massive beasts were largely an odd lot, too. Of course, the most famous examples of bizarre ursid kinsmen are the famed ‘Bear Dogs’ (‘Amphicyonids‘) and the over-hyped giant short-faced bear (Arctodus simus), the latter of which will be discussed in greater detail later on. Another perfect, but little-known, participant in this trend is Agriotherium sp., a genus ranging from the Miocene to the Pleistocene in North America, Europe, Africa, and Asia.

Agriotherium africanus skeletal reconstruction.

Agriotherium has been historically considered a member of the Ursavini tribe: which includes and is named after Ursavus sp., the earliest known New World constituent of the Ursinae subfamily. The tribe is most easily recognized by their shared possession of small, simple anterior molars (which, in some species, are even further reduced), the relatively large fourth frontal premolars which were well-designed for shearing, and plantigrade stance (which distinguishes them from several earlier and contemporary groups of ursids such as the aforementioned ‘bear-dogs’). However, several recent authors have claimed that Agriotherium should instead be placed within the Ursinae itself.

Agriotherium schneideri mandible.

As for the genus itself, in volume one of “The Evolution Of Tertiary Mammals In North America”, Robert Hunt writes that, among other features, “Agriotherium‘s outstanding traits are its anteriorly shortened lower jaw… [and] rudimentary [second metacarpal] talon.”

In “Ardipthecus Kadabba“, the authors explain “the possible presence of three phases of agriotheriine radiation during the Miocene. In the first phase, Indarctos arctoides was the only known species in Europe throughout the Vallesian… until it was replaced, in the second phase, by Indarctos atticus, a species that possibly arose from its predecessor. The third phase took place in the early Pliocene with the contemporaneous appearance of Agriotherium in Africa, Asia, and North America… [The] extinction of Indarctos atticus coincided with the proliferation of Agriotherium, and this may indicate a replacement of the former by the latter. However, it does not necessarily indicate an ancestor-descendant relationship, since Agriotherium was already highly diversified across continents towards the end of the Miocene…[It’s been] argued that an ancestor-descendant relationship between Indarctos and Agriotherium is merely based on stratigraphic occurence and not founded on synapomorphies. Based on [the] study of Agriotherium intermedium from China,… [some] have concluded that Agriotherium may have descended from a Hemicyon group. As a result, the origin and affinity of Agriotherium remains uncertain.”

Upon beholding the comparatively-lengthy limbs and strong dentition of the Agriotherium species, many authors have attempted to reconstruct this beast as a “hypercarnivore” which would have theoretically chased down ungulates and other land mammals to feed its disproportionately-high metabolic rate, as seen in the following illustration.

Arctodus has undergone similar treatment from the paleontological and artistic communities, having easily acquired more ‘super predator’ hype than any other fossilized ursid genus. However, Cameron McCormick of “Lord Geekington” has written an excellent article which explains why this “Godzillafication” of the short-faced bear is almost certainly a gross exaggeration, citing such reasons as its short canines, somewhat laterally-directed orbits, and a host of cranial features which closely resemble those of the largely herbivorous Tremarctos. Agriotherium has been similarly assumed to have been an insatiable, agressive, and powerful killer, yet it shares essentially all of the features which have forced the scientific community to disregard Arctodus as a creature befitting of this description. While both bears, in all likelihood, did consume meat in addition to foliage, it’s probable that this protein-rich fodder was generally acquired by way of scavenging rather than active hunting, but I have no intention of breaking open that particular can of worms by discussing this further.

May the fossil record continue to enchant us all!

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7 responses

18 04 2010
Doug

For the record, that illustration came from the Smithsonian, so take it up with them.

Nat Geo ran a great program a year or two back about prehistoric predators and one of them was about Arctodus. They talked about how stable isotope analysis showed it was almost exclusively carnivorous. Then they make what sounds like a pretty good case that it got that meat through scavenging. They cite that while the long legs would have made it slightly faster than a grizzly bear, they were relatively lanky. During a chase, the legs would not be able to support the weight of the animal in motion. It could run straight, but if it tried to turn with the prey, it risked breaking it’s leg. They described how the long legs could instead be used in a striding gait, like a camel, to cover huge distances in search of food. They said that it could have been a cleptoparasite like Yellowstone’s grizzlies. It’s pretty interesting, but more biomechanics and biochemistry studies need to be done (something I’m considering making a research project someday) to figure out what that bear was doing. So far all we can say for certain is that is was bloody big!

19 04 2010
tanystropheus

Geez! I didn’t even realize that you’d taken the photo! (Rest assured, however, that I never entertained the idea of blaming you for its inaccuracies).

As for the isotopes, lord Geekington wrote the following diagnosis:

“carbon and nitrogen isotope evidence [indicates] that A. simus was highly carnivorous, but functioned primarily as a scavenger adapted for long distance walking/predator intimidation. The only known vertebrate obligate scavengers are large soaring fliers and while a large terrestrial scavenger is not energetically infeasible per se, the hypothetical niche could only have evolved in an environment without vultures (Ruxton and Houston 2004). Furthermore, the isotope range overlaps with Pleistocene brown bears (Ursus arctos) and it does not appear that a hypercarnivore and omnivore with a diet including terrestrial mammals can be distinguished from isotopes”

19 04 2010
Doug

no worries. not the first time i’ve found one of my pics on this blog.

meh, since when do i get anything right…

19 04 2010
tanystropheus

You’re one of the only people I know who actually photographs the mammalian displays at paleo museums, so you’re definitely in the right in that regard!!

19 04 2010
Doug

well the “meh” was in regard to the arctodus affair. as for the picture, that habit of mine has gotten more than it’s share of praise. My picture of the skimmer porpoise skull was actually featured on Tetrapod Zoology.

20 04 2010
Zach Miller

I think the comparisons to Arctodus are warranted here. And I really doubt that either animal was a hypercarnivore. I do so love bears, though, and it’s great to see an article about one of ’em.

I’d like some amphicyonid love next, sir!

20 04 2010
tanystropheus

I shall have to get right on that, my good fellow! 🙂

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