Bears of the Pleistocene

[brotherbear]

twilightbeasts.wordpress.com/2015/01/08/paddingtons-dangerous-cousin/

North and South America were the last continents to be conquered by humans. We have been in Africa since we first evolved, Europe and Asia for over a million years, in Australia for about 60,000 years, but in the Americas for only about 15,000. Considering that reaching Australia required a treacherous ocean voyage but you could walk to Alaska without getting your feet wet via the flat, treeless, mammoth steppe of Beringia (with plenty of game to hunt en-route), why did it take people so long to reach the promised land? Some researchers have suggested that perhaps people did reach Beringia much earlier, but what they met there prevented them from penetrating any further. Along with the mammoths, cave lions, bison, and horses, Beringia had something else. Something that would have been completely unfamiliar to the humans who encountered it. Something seemingly crafted from our deepest, darkest nightmares. Arctodus simus: the giant short-faced bear may have been the most terrifying land carnivore our species ever encountered.
Today, the polar bear (Ursus maritimus) is the largest land carnivore but 12,000 years ago that title went to a member of the subfamily of bears known as the Tremarctinae. We still have one member of this unique subfamily left; the spectacled bear (Tremarctos ornatus). The only bear found in South America (including deepest, darkest Peru), this predominantly vegetarian teddy is technically the largest carnivore left on that continent, despite being a relatively puny 100-200kg. A sister species, the Florida cave bear (Tremarctos floridanus), also roamed Pleistocene North America. However, their close relative, the giant short-faced bears (genera Arctodus and Arctotherium/ Pararctotherium) were absolutely colossal beasts. Arctotherium/Pararctotherium (opinion is divided on how valid these generic distinctions are) has been found all over South America, even down into southern Patagonia. Arctodus simus has been found at sites all over North America, from Alaska and the Yukon Territory down to Florida and Texas. Whilst we have a mitochondrial genome from Arctodus and know it diverged from the spectacled bear during the late Miocene/early Pliocene, it is not known exactly how closely related Arctodus and Arctotherium/Pararctotherium are to each other. Ancient DNA work is happening right now that should give a handle on the complicated phylogenetics of these bears.

Arctodus simus was an 800kg monster. To give some impression of its size: while on all fours this bear could gaze directly into my eyes (I’m 6’2”). Standing on its hind legs an average bear could reach 12-feet (in fact, the site of Riverbluff cave in Missouri has claw marks 15 feet up on some side walls that were probably produced by a large Arctodus). Another site with Arctodus is Big Bear cave in the Ozark mountains. The articulated skeleton found there is impressive for the amount of information it left about short-faced bear biology. The animal, like many other cave finds, was a small female, which cumulatively suggest that Arctodus females denned (perhaps surprisingly, only one fossil Arctodus baculum has ever been recovered). The Ozark skeleton is also unique in the preservation of fossilised hair at the site.
What natural forces could possibly have conspired to produce a carnivore of such enormous dimensions? Most researchers think that the specialisations present in Arctodus (i.e. long legs, large size, short jaws), are adaptations to a life of extreme hypercarnivory. Long legs allow for efficient movement over a wide home range, necessary to locate carcasses, large size can act as an effective deterrent to other carnivores to scare them off a kill site (kleptoparasitism), and short jaws give extra bone crushing power. It seems that this hypothesis is backed up by good data. Stable isotope analysis of Arctodus remains show elevated ∂15 Nitrogen values- this indicates that Arctodus was consuming a very meaty diet, perhaps with a large component of caribou (Rangifer tarandus). However, as with most things in science, different researchers think that the opposite is true and have concluded that the morphology of short-faced bears indicate a life of herbivory with some omnivory. My money is still on Arctodus simus being one of the biggest and baddest animals ever to have lived.  
 
 How badass was it? Well, a very interesting pattern cropped up when palaeontologists were looking at late Pleistocene radiocarbon dates from eastern Beringia. They found that during time periods when Arctodus simus was present (i.e. 20,000-45,000 14C years BP) there is a noticeable lack of other predators (lions, scimitar cats, brown bears). Either the environment was selectively excluding everything but short-faced bears during this time period, or perhaps more likely, Arctodus simus was such an efficient predator/scavenger that there was simply not enough prey biomass left for other carnivores to get a look in. If this was the case, it is perhaps no surprise that human presence in eastern Beringia is only known from after Arctodus’ extinction. Maybe one day we will find an Arctodus coprolite with some evidence of the pre-pre-Clovis pioneers!

Written by Ross Barnett (@deepfrieddna) Reply