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You think of fossil-hunting as something that takes place in faraway barren places: the Flaming Cliffs of the Gobi Desert, the windy wastes of Patagonia, the Dakota badlands. But not downtown Berkeley. That was the source of Specimen 78858 in the UC Museum of Palaeontology’s collection, though, a fossil I finally got to meet at last year’s Cal Day. It’s a massive thighbone, the femur of an extinct ground sloth that inhabited these parts in the Pleistocene Era, tens of thousands of years ago, and it turned up when the Berkeley BART station was being excavated. 

The species, depending on which sloth scholar you ask, is either Glossotherium harlani or Paramylodon harlani. Charles Darwin dug a Glossotherium, along with other former South Americans, out of the Bahia Blanca fossil beds in Argentina when the Beagle anchored there in 1832. The name, meaning “tongue-animal” (and I’ll get to that later), was coined by the anatomist Richard Owen, who later broke bitterly with Darwin after The Origin of Species was published. Harlani honors another nineteenth-century naturalist, Richard Harlan, who described the species from a jawbone found at Big Bone Lick, Kentucky. (Yes, a real place—a friend visited it during a business trip to Cincinnati. It’s a salt lick where sloths, mammoths, and other prehistoric megafauna left their skeletons.) Harlan’s more notorious younger brother Josiah, a rogue Quaker turned adventurer in feudal Afghanistan, appears to have been the real-life inspiration for Kipling’s “The Man Who Would be King.” 

But back to Harlan’s ground sloth: These ungainly creatures evolved in South America when it was an island, with a fauna of sabertoothed marsupials, giant flightless predatory birds, and odd hoofed mammals. Ground sloths show up in the fossil record long before the modern tree sloths, the only survivors of a diverse lineage: three-toed sloths like the one Dr. Maturin brought aboard HMS Surprise and Captain Aubrey won over with cake soaked in grog, and two-toed sloths. Some palaeontologists believe the two-toed and three-toed sloths evolved from distantly related ground-sloth ancestors, acquiring their tree-hanging lifestyles and specialized anatomies through convergence. Another South American sloth, the sea sloth Thalassocnus, became adapted to an aquatic life.  

When the Isthmus of Panama joined the Americas three million years ago, the sloths moved north as North American species—big cats, elephants, the ancestral llamas—headed south. Harlan’s ground sloth was one of several species inhabiting Pleistocene Southern California; after the extinct western horse and ancient bison, it’s the third most common plant-eating mammal in the La Brea Tar Pits. At 11 feet in length and 3,500 pounds, it was only a midsized sloth; its relative Eremotherium was elephantine. Apart from the Bay Area, Harlan’s sloth ranged at least as far north as Carson City, Nev., where there’s a fossil sloth trackway with 19-inch-long impressions in the yard of the state prison. 

Sloth tracks look oddly humanoid, but the creatures actually walked on the outsides of their hind feet; the foot was rotated so the sole faced inward. The gait of a sloth was at best a waddle. There were other anatomical pecularities. Ground sloths had a second set of ribs, linking the standard costal ribs to the breastbone. Some of their tail vertebrae were fused to the pelvis, forming a heavy-duty brace to support the beasts when they sat upright. And some species, including Harlan’s, had pebble-like nodules embedded in the skin of the back, armoring the sloth against sabertooths, dire wolves, and other predators. 

The sloths also had wicked-looking sickle-shaped foreclaws, used by G. harlani primarily for digging roots, by other species for snagging leafy branches. They would also have been formidable in defense. A flange on the cheekbone anchored massive jaw muscles for serious chewing power. Owen thought the sloth he christened used its tongue giraffe-fashion to gather food, hence Glossotherium. The flat grinding teeth of Harlan’s sloth suggest it was primarily a grazer.  

Those foreclaws misled Thomas Jefferson, our only palaeontologist president, into misclassifying an eastern species, Megalonyx jeffersoni, as some kind of giant feline. After the error had been corrected, Jefferson cherished the hope that some of the creatures might still survive in the unexplored West; he asked Lewis and Clark to keep an eye out for them. 

No such luck, though; the sloths were long gone. If you subscribe to Paul Martin’s controversial Pleistocene Overkill hypothesis, the big, slow, and probably none-too-bright beasts would have been easy targets for Palaeoindian hunters. Alternatively, they may have succumbed to changes in climate after the retreat of the glaciers. In any case, the youngest G. harlani remains from Rancho La Brea are 13,890 years old; some persisted for another four thousand years in Florida. 

For those who are inclined to believe such things, there are persistent rumors of something big and slothlike in the Amazon jungle. It’s reputed to leave oddly shaped tracks and a foul smell. So far, as with Bigfoot and other cryptofauna, tangible evidence is lacking.