Revenge of the Meat-Eating Megatherium

Unknown Island has to be one of the crummiest dinosaur movies ever made. Though it came out about 15 years after King Kong debuted, the plot is broadly the same and executed with less skill – young filmmaker hires a crew to go to an island swarming with prehistoric life; mayhem ensues. The acting’s bad, the effects are bad, and, well, just look at the movie’s rendition of Megatherium:

That’s right folks – it’s a giant, man-eating ground sloth come to snack on Unknown Island’s lecherous captain before going on to beat the stuffing out of a clunky Ceratosaurus. (I love the “Aw, for me? You shouldn’t have!” look the monster gets before descending upon the hapless sailor.) Nevermind that Megatherium was a ponderous, plant-chewing mammal. Why let facts get in the way of b-grade movie effects?

But maybe the idea of a meat-eating ground sloth isn’t that absurd. True, the Unknown Island version was a malformed brute made out of a co-opted gorilla suit, but ground sloths and carnivory have been brought together before. Thomas Jefferson, polymath and third president of the United States, even identified isolated parts of a ground sloth as bearing the hallmarks of a carnivore, but that was before he really knew what he was looking at.

On February 10, 1797, Thomas Jefferson sent “A memoir on the discovery of certain bones of a quadruped of the clawed kind in the western parts of Virginia” to his colleagues in the American Philosophical Society. The animal in question had clearly been a terrifying beast. In addition to part of a femur, two lower arm bones, and several foot bones, three large claws had been sent for Jefferson’s inspection, and these weapons indicated that the animal was a predator larger than a lion. “I will venture to refer to him by the name of the Great-Claw or Megalonyx,” Jefferson decided, “to which he seems sufficiently entitled by the distinguished size of that member.”

Jefferson took great pride in the fact that the animal had been found in his home state of Virginia. French naturalists – Georges-Louis Leclerc, Comte de Buffon foremost among them – believed that the Americas were impoverished lands where life degenerated rather than thrived. The proof was in the lack of large animals – the New World had nothing to match the elephants, giraffes, hippos, and other behemoths of the Old World. But the discovery of Megalonyx – as well as the elephantine American mastodon – showed that North American creatures were every bit as impressive as their Old World counterparts.

In the case of Megalonyx, specifically, Jefferson compared measurements of the fossils to the equivalent measurements of lion recorded by the French naturalist Louis-Jean-Marie Daubenton. The Megalonyx was clearly larger in every respect, Jefferson affirmed, and “he stood as preeminently at the head of the column of clawed animals as the mammoth stood at that of the elephant, rhinoceros, and hippopotamus: and that he may have been as formidable an antagonist to the mammoth as the lion to the elephant.” Perhaps, Jefferson speculated, such beasts were still roaming the uncharted American interior, and he used everything from Native American rock carvings to the tall-tales of hunters to back up this idea. “[T]he bones exist: therefore the animal has existed”, Jefferson said, and he concluded:

The movements of nature are in a never ending circle. The animal species which has once been put into a train of motion, is still probably moving in that train. For if one link in nature’s chain might be lost, another and another mist be lost, till this whole system of things should [vanish] by piece-meal … If this animal then has once existed, it is probably on this general view of the movements on nature that he still exists

But Jefferson had to almost immediately retract his analysis of what the Megalonyx was. Though rightly steadfast in his rejection of “American degeneracy” and wrongly stubborn in his belief about the stable nature of species through time, the exhibition of a giant sloth skeleton in Madrid, Spain’s natural history museum led Jefferson to realize that his “Great-Claw” was not a cat at all. This was the skeleton of Megatherium – one of the most imposing of all the ground sloths, and clearly a giant-size prehistoric relative of modern tree sloths. “According to analogy then,” Jefferson noted in a post-script, “it probably was not carnivorous, had not the phosphoric eye, nor leonine roar” he had attributed to the ground sloth from Virginia. Jefferson cautioned that his beast might turn out to be different – a tooth or piece of jaw would solve the mystery – but ultimately Megalonyx was confirmed as a close, northern cousin of the great Megatherium.

Jefferson’s carnivorous Megalonyx was the creation of misattribution. As soon as he realized that the bones probably came from a giant sloth, he backed off his initial hypothesis. Nearly 200 years after Jefferson’s description, though, Richard Farina and Ernesto Blanco proposed that some giant sloths may have eaten meat, after all. They called their paper “Megatherium, the stabber.”

Farina and Blanco’s paper focused on how Megatherium might have been using its arms. Classic reconstructions showed the sloth pulling down tree branches to reach juicy leaves, but an alternative view had the beast jabbing at other sloths in territorial fights, flipping over armored glyptodonts to get past their tough outer armor, and caching decomposing carcasses to feed on later. Based upon their reconstruction of Megatherium arm anatomy, the paleontologists proposed that the arms of the sloth were suited for quick, slashing or stabbing movements rather than the strength needed to bend or break tree branches, but this presented a puzzle. “The conclusion that Megatherium americanum must have been an efficient stabber seems clear,” they wrote, “but identifying the stabbed is less easy.”

Maybe, Farina and Blanco speculated, Megatherium was less of a formidable predator and more of an opportunistic scavenger. Their ability to deliver quick, claw-tipped jabs would have aided them in driving predators off of carcasses in environments where plant foods were scarce: “Even the very large, highly specialized carnivore [such as the sabercat] Smilodon might have given way if Megatherium americanum decided to take its prey.”

What an extinct animal is capable of and what they actually did are two different things, though, and there was no direct evidence of giant sloths stealing sabercat kills or tossing glyptodonts around.  Such behavior wasn’t impossible for giant sloths – herbivores such as cows, deer, and hippos occasionally scavenge meat or prey upon smaller animals – but fossilized signs of such interactions like Megatherium bite-marks on a carcass, claw scratches on a glyptodont shell, or a wad of fossil sloth shit with bone fragments inside would be needed to support the idea of meat-eating ground sloths shuffling around the landscape.

In fact, evidence from the bones of some North American giant sloth specimens grates against Farina and Blanco’s hypothesis. In 2007 paleontologists Christine France, Paula Zelanko, Alan Kaufman, and Thomas Holtz published a study on the carbon and nitrogen isotope signatures preserved inside the bones of extinct mammals which roamed western Virginia around 10,000 years ago. Simply put, the food these animals ate during their lives left tell-tail chemical signatures inside their bones, and the investigation of these traces have allowed paleontologists to partially reconstruct the diets and environments of long-lost animals. A femur representing Jefferson’s Megalonyx made up the giant sloth contingent of the study.

Though based on only one specimen, the chemical isotope values of the Megalonyx in the study showed that the sloth was feeding on the same food sources as the other herbivores. There was no indication that the sloth was regularly eating meat – a finding consistent with a similar, earlier study of a Megalonyx from Alberta, Canada and an isotopic investigation of the giant sloth Paramylodon harlani. There has been no definite sign that any giant sloth was eating meat as a regular part of its diet.

If conclusive evidence of meat-eating megatheria is found, though, I have to wonder if it might give these creatures a publicity boost. Megatherium and other large mammals were the stars of paleontology until the dinosaurs like Megalosaurus and Iguanodon showed them up in the 1840’s. Perhaps, should paleontologists find a giant sloth coprolite flecked with glyptodont shell or Megatherium scavenging damage on bones, some of the giant sloths might be transformed from slow, dim-witted browsers to imposing scavengers so threatening that even sabercats didn’t dare compete with them. At present, such evidence does not exist and it may never be found, but it is compelling to imagine a Megatherium crouched over a glyptodont carcass, treating the overturned mammal as one giant soup bowl.

References:

Farina, R., and Blanco, R. (1996). Megatherium, the Stabber Proceedings: Biological Sciences, 263 (1377), 1725-1729 DOI: 10.1098/rspb.1996.0252

France, C.; , Zelanko, P.; Kaufman, A.; Holtz, T. (2007). Carbon and nitrogen isotopic analysis of Pleistocene mammals from the Saltville Quarry (Virginia, USA): Implications for trophic relationships Palaeogeography, Palaeoclimatology, Palaeoecology, 249, 271-282 DOI: 10.1016/j.palaeo.2007.02.002

Jefferson, T. 1799. A Memoir of the Discovery of certain Bone of a Quadruped of the Clawed Kind in the Western Parts of Virginia. Transactions of the American Philosophical Society, 4: 246-260