24 Jul 2015

Longing for the Giants

By Claire Eamer

In the long and complex story of evolution, organisms emerge and then disappear, never to be seen again. I understand and accept that. Honest. But, fair or not, I regret the loss of some organisms more than others.

I admit it. I miss giant ground sloths.

Don’t get me wrong. I’m not old enough to have met a giant ground sloth face to face. None of us is. The last lingering remnants of a great bunch of animals perished about 5,000 years ago, probably on a small Caribbean island.

Fossil foot of a Jefferson's ground sloth beside my shoe. C. Eamer photo.

While they were here, though, they were amazing. And really, really big. Well, some of them were big. Some species were as small as house-cats, but the largest ground sloths – Megatherium and Eremotherium – were the size of elephants. Hairy elephants, with thick hides dotted with bony plates, claws the size of bananas, and a propensity to stand up on their hind legs and maul substantial trees.

If it’s any comfort, they were vegetarian. Mostly. Some scientists think that Megatherium, at least, might have used those giant claws to grab a bite of meat to go with the veggies, especially when climate change dried the local landscape and thinned out the forests it relied on.

Greg McDonald, senior curator of natural history for the US National Park Service, is another sloth enthusiast. He recently delivered a talk on ground sloths, via Skype, to an appreciative audience in Whitehorse, Yukon.

The Yukon is, surprisingly, ground sloth territory. It’s actually where I first met a ground sloth – if not face to face, then at least face to skull. It was a Jefferson ground sloth, a member of the genus Megalonyx, and its ox-sized skeleton looms over visitors to the Beringia Interpretive Centre in Whitehorse.

For hundreds of thousands of years, Megalonyx ranged from southern Mexico to as far as Alaska and the northern Yukon. Megalonyx was unique, McDonald said, in being a wholly North American sloth. Although its ancestors came from South America, Megalonyx itself evolved in North America and never spread south of Mexico.
A sloth skeleton towers over visitors to the Beringia Interpretive Centre. C. Eamer photo.

That made it a late-comer in the sloth world. Like many other strange and wonderful mammals, sloths evolved in South America, in splendid isolation.

“For most of the age of mammals,” said McDonald, “South America was an island continent, much as we think of Australia today.”

The giant Megatherium roamed the ancient forests of South America in those days. And it had a curious assortment of cousins: anteaters, giant armadillos, and a bizarre creature called a glyptodont that looked like an ambulatory bone igloo. They all belonged to a group called Xenarthrans, which evolved exclusively in South America.

Once thought to be an odd evolutionary offshoot, Xenarthrans are now recognized as one of the major subdivisions in mammalian evolution, McDonald said. They prospered for tens of millions of years in South America and moved on to North America when the Isthmus of Panama began to form and link the two continents, just a few million years ago. (Or maybe more than a few million years ago, according to some recent studies.)

That was a momentous time in evolution, said McDonald. It’s called GABI: the Great American Biotic Interchange. Released from isolation, the Xenarthrans drifted northward, along with other distinctly South American animals, such as giant flightless birds with the hooked beaks of meat-eaters and the ancestor of the humble porcupine. At the same time, animals that had evolved in North America moved southward, including hunting cats with dagger-like fangs and those members of the camel family that became South America’s llamas and alpacas.

Ground sloths spread and adapted and changed on both continents for several more millions of years. They even took to the sea. Marine deposits in southern Peru have turned up a wealth of ancient fossil whales, seals, and sea lions – and entire skeletons of giant sloths. The sloth skeletons show clear evidence of adaptation to an aquatic environment. McDonald said it appears that the aquatic sloths were evolving into animals that went into the sea to eat sea grasses. Like the whales, they were reversing the classic sea-to-land journey of most mammals.

But that’s as far as they got. The aquatic sloths died out about 4 million years ago, and most of the rest of the ground
sloths didn’t survive the great megafauna die-off at the end of the last glaciation, about 12,000 years ago. A few small ground sloths lingered on isolated Caribbean islands, where 5,000-year-old sloth bones have been found in caves.

Today, the much-diminished sloth tribe has been reduced to the endangered tree sloths – a mere six species in two genera – eking out an existence in the shrinking rainforests of South and Central America.

The giants are gone. And, even though I never shared the world with them, I miss them. In fact, they inspired me to write my first kids' science book, Super Crocs & Monster Wings: Modern Animals' Ancient Past.

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