Central Coast Critters: Arctodus

Hey there every peoples!

The Pleistocene is one of my favorite time periods. Especially the late Pleistocene, the timeframe often referred to as the “ice age”.  It feels richly exotic and yet strangely familiar. It was a time when animals of today mingled with long dead beasts. No matter how many mammoth and bison and camel fossils I see, I never get tired of the ice age. And perhaps the most charismatic are the carnivores, namely the saber-toothed cats, dire wolf, and bigass lion (which may have been a giant jaguar). But one beast has been the subject of particular scrutiny and fanfare, a creature who has proved most enigmatic. This creature is Arctodus simus, the giant short-faced bear.

Now let’s get the obvious question out of the way: how do we know this animal lived on the Central Coast? I can’t say for certain, since my information on fossils from this region is very fragmented. However I can extrapolate from nearby fossil sites. Arctodus is known from the La Brea Tar Pits in Los Angeles but this is not the only tar pit locality in California. I have found a few mentions that there were tar pits in Carpentaria, just south of Santa Barbara, which yielded ice age fossils. I have been unable to find any detailed information so I can’t say with any certainty if the short-faced bear has been found there. However I do know that it is known from the McKittrick tar pits in western Kern County. Since Arctodus is known from localities around the Central Coast, it’s not unreasonable to think it lived here as well.

 

Jawbone of Arctodus simus from the McKittrick tar pits, displayed at the California Living Museum

Arctodus has been the subject of much debate in recent times. It has often been thought of as a giant, ravenous predator that terrorized the megafauna of ice age America. But this idea, it seems, has largely been rejected. It’s almost like a reversal of what happened with Tyrannosaurus rex. People didn’t take kindly to T. rex being thought of as a scavenger, so they set out to prove it was a hunter. Arctodus suffered the opposite: people apparently had issues with it being portrayed as a hunter and set out to “cork its fangs” (to use The Lord Geekington’s lingo). But just how did this come to pass? Let’s take a look.

When Arctodus was first discovered, it stood out from other bears. While scientists were able to learn that it is related to the modern spectacled bear, this animal was very different. It was huge, larger than even the polar bears found to be roaming the Arctic. It has a short muzzle and stout canines, and it walked about on long lanky limbs. It ranged from coast to coast and from Alaska to Mexico. It’s short face and long legs were seen as convergent with felids and so it was envisioned as a pursuit predator using speed combined with its great strength to tackle bison and other large prey. As you can imagine, such an image stuck in the popular imagination, as exemplified in Kirk Johnson’s “Cruising the Fossil Freeway”:

Standing nearly six feet at the shoulders when on all fours, this 1,400-pound bruin was a long-legged killing machine that would make a grizzly look like a sissy. Given the size of their limbs and the nature of the Late Pleistocene landscape, short-faced bears were probably open-ground predators that chased, caught, and ate bison and horses. A bear that can run down a horse on an open field is a scary concept.

 

Skeleton of Arctodus simus at the La Brea Tar Pits, California

But this image was not to last.  Scientists began to take a second look at the “predatory uber bear”. Chief among them was a scientist at the University of Alaska Fairbanks, Dr. Paul Matheus. He performed an isotope analysis on an ancient Alaskan Arctodus bone and found that its diet was almost exclusively carnivorous. But this didn’t prove it was a predator, only that it was a meat eater. Matheus looked at the morphology of the bear’s legs and found that while they were good at supporting its weight, it lacked the joint dynamics and structural integrity to maintain a chase. In shorter words, while capable of impressive bursts of speed, Arctodus’ legs could break if it tried to turn too sharply due to its immense weight. Since herbivores employ a zig-zag pattern to escape, being unable to make sharp turns is a big problem for a predator. Matheus concluded that the bear’s long lanky limbs instead allowed it to move in a pacing gate, which is a very efficient way to get around. Matheus began to imagine Arctodus instead as a superb scavenger. Using highly efficient movement and a strong sense of smell, Arctodus could roam the countryside in search of carrion. But could Arctodus have survived on dead animals alone, something unheard off outside of soaring birds? Well maybe it did. Modern grizzlies are known to pirate wolf kills. But there were more than just wolves in Pleistocene America: saber-toothed cats, scimitar-toothed cats, giant lions (or jaguars), dire wolves, jaguars, and cheetah-like cats. Arctodus, Matheus argued, could have used its size and strength to scare any of these animals off their kills. And what if there was little left at the ice age dinner table? Matheus looked to the bear’s short skull and flat teeth. Shortening the skull allows for a stronger bite while flat teeth are ideal for crushing. What had been used as evidence for predation (a shortened skull) was now envisioned as a trait that allowed Arctodus to break open bones to access the highly nutritious marrow inside. Arctodus, he argued, was not a super predator but a super scavenger. And to be honest, I was hooked.

In a bizarre twist of fate, I was more fascinated by the concept of this animal as a giant trash picker than as an insatiable terror. To my feeble mind Matheus’ arguments made sense: being unable to pursue prey, this bear was well equipped to search the land for carrion and there were plenty of predators to facilitate carcass theft. To me, this scavenging hypothesis made Arctodus unique not just among bears but among carnivores in general. No other large carnivores could be perceived as subsisting on dead meat alone. But again, things made sense to my naïve thinking: it couldn’t catch prey because it couldn’t turn well (speed may not be essential to predators, but maneuverability sure is). Because this animal had flat teeth ideal for crushing and grinding (instead of the pointed teeth of T. rex) and also because there were plenty of other animals to fill the predator role, Arctodus seemed like a shoe in for the role no one thought possible: that of a super scavenger. This hypothesis even affected (or infected, I’ll let you decide) my way of looking at things. A couple years ago Alton mentioned that a paper presented by Blaine Schubert at the Society for Vertebrate Paleontology meeting discussed a study of changes in tooth shape and body size changes in Arctodus over time. As time went on, Arctodus got bigger and the anterior grinding teeth got relatively larger, suggesting a change in diet. I had suggested (naively, it now appears) that this increase in tooth size may have been an adaptation for breaking open bones. And recently I thought about another way of looking at the scavenging bear idea: damage on herbivore bones.

To the extent of my knowledge there are at least four instances where Arctodus may have been dining on the remains of the dead: A southern mammoth skeleton from Anza Borrego in California, a Columbian mammoth skeleton from Huntington Reservoir in Utah, a mammoth heel bone from Saltville in Virginia, and a mastodon femur from Kentucky (there may be more, but this is all I have read about). This may not seem like much but I’m going to go out on a limb here and compare it with bone pathologies of another kind: battle damage in Native Americans. In the cultural anthropology class I took over a year ago, we were given an article titled “Prehistory of Warfare”, which talked about warfare in stone-age societies. We can often tell if a person died in war by tell tale marks on their skeleton: fractures and crushed bone indicate clubs; narrow grooves were most likely caused by axes; and scratches in odd places, as well as embedded points, demonstrate death by arrows. Now interestingly, the highest incidence of war related death as indicated by skeletal evidence comes from California. While California is often thought of as a hippie paradise, as much as 30% of male remains from the first centuries A.D. had wounds and death blows resulting from violent attacks. It is also noted that half that number of women from the same period show similar injuries. The article goes on to say that “When we remember that not all warfare deaths leave skeletal evidence, this is a staggering number”. My point? Museum displays and classroom lectures alike talk about how unlikely it is for a bone to become fossilized. I think what could be done is look at all bones thought to have carnivore trauma and try to determine who made what. A high amount of damage attributed to Arctodus could indicate a high amount of scavenging. And like the ancient wounds, not all scavenging would leave traces behind on the bones. And when you consider all the bones that wouldn’t have made it into the fossil record, this could indicate Arctodus was doing a buttload of dumpster-diving. What’s also interesting to note is all the instances of inferred scavenging are bones from proboscideans, the biggest animals on the landscape. Now I don’t know if scavenging evidence on the Huntington Reservoir mammoth comes from tooth marks or the fact that a couple Arctodus bones were found alongside it. Though it’s presence at the site of a mammoth carcass may be some indication: Arctodus fossils have also been found alongside mammoths at Hot Springs, South Dakota and at Saltville, Virginia. But seeing as how paleontology is reliant on hard evidence, this entire paragraph is just stupid meaningless speculation on my part.

I have rambled on at length about how fascinating the idea of Arctodus was a scavenger is, but it is all for naught. The giant, short-faced bear that ran around on long legs trolling for flesh apparently never existed. Recent studies have shown that everything that ever made Arctodus unique have all been faulty observations. First off, it wasn’t a giant. The bear’s odd proportions have made it difficult to pin down its size but the consensus now appears to be that the figures of 1800 pounds or more are unfounded. “Fossil Treasures of the Anza-Borrego Desert” notes that studies by a fellow named Kurten used full body length to estimate that females weighed between 550 and 614 pounds while males weighed in around 770-825 pounds. His femoral cross-section analysis put females at around 594-638 pounds. Even though it cites a specimen from Utah weighing about 1364-1452 pounds, it is noted that this is a particularly large individual. It has been argued that because Arctodus was lanky it was not as massive as other bears; therefore the estimates of one ton are erroneous. Many think now that Arctodus rarely exceeded 1200 pounds in males, making it no larger than a brown bear. But the “fang corkers” were just getting warmed up.

 

A comparison between the "giant" short faced bear and a modern polar (image by and courtesy of Daniel Reed)

A paper released a year ago really went to town on the short-faced bear. They found that it didn’t have as long a set of legs as people originally thought. They argued that it wasn’t proportionally different than other bears and that the long legs are something of an optical illusion caused by its short back (Brian Switek notes that modern bear skeletons do look a big leggy themselves). Given this new information, the arguments that its long legs allowed it to run down prey or cover vast tracks of land were now irrelevant. The researchers also found that it wasn’t a short-faced bear either. Again, when compared to other bears and other carnivores, Arctodus’ skull was proportionally no different. This too may be an optical illusion caused by its deep skull. And diet? They suggest that Arctodus best be viewed as an omnivore who occasional ate meat like modern brown bears. But what about those isotope studies that showed it almost exclusively ate meat? The authors contend that the specimens used are all from Alaska and thus only demonstrate that that particular population ate meat and thus should not be used to generalize an entire species roaming a continent full of different environments (Alaska is often portrayed as a steppe environment. Perhaps the Arctodus up there had to rely more on meat because suitable plant food was not available). Arctodus, the once mighty king of the ice age, was reduced to a run of the mill ursid.

I know this sounds pessimistic (did I mention I suffer a mental illness ?), but what is so special about Arctodus now? Everything that ever made it unique: size, long legs, a short face, a carnivorous diet (whether through active predation or scavenging), has all been shown to be false. Pretty much it becomes like any other bear. I mean, I get excited over bison or horse fossils, which are no different from their modern relatives. But they are special at least because they are found in places they no longer live in. To have an animal usually associated with far away exotic locations found right here makes them unique. But when it comes to Arctodus… so what? We still have bears in North America. Hell we have three species of bears. And it’s possible existence on the Central Coast wouldn’t too much of a boost either, seeing as we had the equally big and ecologically similar grizzly bear roaming the place in historic times (it’s how the town of Los Osos got its name. Its full name, Canada de Los Osos, means “valley of the bears”). It seems Arctodus’ only claim to fame now is that it’s an extinct bear and we got its biology and ecology horribly wrong. Had its fangs corked? I’d say it’s effectively been castrated.

 

Arctodus has come full circle ( from the new paper)

They say that the bigger they are the harder they fall. And Arctodus sure has fallen hard. It went from uber bear to ultimate scavenger to just another bear. Whereas T. rex survived its fang corking, Arctodus was knocked off his pedestal for good. But this is the way science works. It’s not about what you want to believe… it’s about what the evidence says. And the evidence says that Arctodus wasn’t all that different from other bears. I know I will probably get flak for not being able to still appreciate it. I still do, but for reasons I don’t understand, Arctodus has lost most of its grandeur and wonder. I will still enjoy seeing its fossils in my museum hopping, but I can’t help but feel like the ice age has lost one of its most fascinating aspects.

Till next time!

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Addendum: i have written a follow up post