Shasta Ground Sloth
The Story of Its Discovery at Aden Crater in New Mexico
by Jay Sharp
One day about 11,000 years ago, as the last Ice Age drew slowly to a close, an immature shasta ground sloth – a pale yellowish animal roughly the size of a modern black bear – found disaster in southwestern New Mexico. The lumbering creature, probably by misstep or from predatory pursuit, evidently stumbled and plunged into a fumarole, or gas vent, at the rim of Aden Crater—a low-lying volcanic lava cone located in the 500-square-mile Portrillo Basalt Field, southwest of Las Cruces.
Although, apparently, the sloth somehow survived the fall initially, it would find no escape. "A most appalling death trap," Professor Chester R. Longwell, Yale University, called the fumarole after exploring it in 1928. The sloth would perish, the remains of its last meal still in its paunch.
How was the shasta ground sloth's carcass discovered after its plummet into such a forbidding place so many millennia ago?
In late 1927 or early 1928 three young men – C. Ewing Waterhouse, Wilson Esterly and Carlos Rushing – came to the Aden Crater planning to explore the fumarole. They were either boy scouts, according to the internet site Desert Diary, or musicians, according to the Berkeley Daily Gazette, February 1, 1929. Perhaps they were both. In any event, they came prepared for adventure, but they could scarcely have anticipated what they were about to find as they lowered themselves by rope to the floor of the fumarole.
"The descent into the pit is difficult..." said Longwell, quoted in Richard Swann Lull's "A Remarkable Ground Sloth," a scholarly report on the animal. "...it is necessary to use a rope, taking advantage of occasional irregularities in the wall for foot rests. The descent is nearly vertical for the first forty feet. From the first landing the pipe continues down irregularly by a series of steep slopes, nearly horizontal stretches, and vertical drops, and the diameter varies greatly."
After a tortuous passage, Longwell said, he came to a "large room some fifteen feet in width by thirty in length." Like other rooms, he said, "This also contains a large quantity of bat guano." It was on the floor of this chamber that Waterhouse and his friends – to their surprise – had come upon the sloth skeleton "almost completely buried in the dry, loose guano, and at a distance of about one hundred feet vertically below the mouth of the pit." Along the way, they may have also seen the more recent remains of coyotes and a bobcat lying atop the guano.
Waterhouse, an obviously bright young man who had guided Longwell to Aden Crater and the fumarole, had earlier notified Yale University's Peabody Museum of the find by a letter dated February 25, 1928. He had enclosed photographs and drawings that alerted the museum to the fact that he and his two friends had come upon something extraordinary. "...it was at once evident," said Lull, "that a ground sloth had been discovered in a remarkable state of preservation..."
It would, in fact, draw national attention. In its January 7, 1929, edition, Time Magazine reported that after the sloth died, "...the indifferent bats dropped their guano on its dead body. Good for modern paleontology was their filthy covering. It preserved the sloth-bones, teeth, tendons, hide and even a food ball in its stomach." Time said that the sloth lived "1,000,000 years ago, certainly 500,000." The Berkeley Daily Gazette, by comparison, said that the sloth was "believed to have lived 50,000 years ago..." This was, of course, before the development of radiocarbon dating technology, which indicated an age in the range of 11,000 years.
A Window to the Past
The Peabody Museum – realizing that the partially mummified sloth offered an important window to the past – promptly made arrangements to acquire and study the remains. "The specimen was complete," said Lull, "the bones being held in articulation by their original ligaments and tendons. There are also present some of the periosteum [connective tissue covering the bones], patches of skin, and the mucous membrane lining the hard palate, as well as some muscle fibers." Even some of the animal's coarse yellowish-colored hair had been preserved. Astonishingly, the sloth had broken none of its bones in its fall into the fumarole. They remained largely preserved, suggesting that the animal had been able to search for an escape before it died.
A Striking Animal
Now, thanks especially to the find at Aden Crater, as Bjӧrn Kurtén and Elaine Anderson said in Pleistocene Animals of North America, "More is known about the external appearance of Northrotheriops shastensis than any other ground sloth..."
A mature adult shasta ground sloth – one of the smallest of the giant ground sloths, all now extinct – measured more than seven feet from the tip of its nose to the tip of its tail. It stood more than three feet high at the shoulder. It likely weighed around 400 pounds. (By comparison, a contemporary species of giant ground sloth – the Eremotherium – measured some 20 feet in length and weighed a ton.)
Like other members of its taxonomic family, the shasta ground sloth had a relatively small head with prehensile [grasping] lips and tongue, something like a modern giraffe. It had a long flexible neck. It had long and relatively slender forelimbs, muscular hindquarters and a muscular tail. It had, on its forefeet, long claws, which it may have used to dig food plants from the soil and to pluck edible fruit from thorny plants such as the prickly pear as well as to defend itself from predators such as the sabertooth cat or the dire wolf. Standing upright to take foliage from shrubs and trees, the animal likely used its back legs and tail much like a tripod, giving it stability and reach. Walking, it probably moved clumsily, with a waddle, on the knuckles of its front feet and the heel and outer edges of its hind feet.
A Very Different World
The young shasta ground sloth's territory in southwestern New Mexico featured – as it still does – low mountain ranges, scattered volcanoes, massive lava flows, rocky and sandy soils and playa lake beds, but measured by the climate, the plant and wildlife communities and the late Ice Age human community, its world looked very different from what we see today.
Reflecting the waning Ice Age, the sloth knew a climate that was much cooler, with mild winters and summers and moderate rainfall. The playa lakes held water through the year. The sloth foraged in a plant community that resembled an African savannah, or a grassy open woodland. The land bore a blanket of grasses such as bush muhly, alkali sacaton and several gramas with scattered stands of trees such as pinyon pine, juniper and liveoak interspersed with various species of arid-land vegetation such as prickly pear cacti, agave, yuccas and desert globemallow.
The sloth belonged to a rich community of "mega" mammals distributed across southwestern New Mexico, including, for just a few examples, mammoths, camels, three-toed horses, short-faced bears, sabertooth cats, dire wolves and tapirs—all now extinct for reasons that have puzzled paleontologists for years. As Arthur H. Harris, "Plio-Pleistocene Vertebrate Fossils of the El Paso Area," said, "a virtual Noah's ark of vertebrate diversity [now extinct]" lies embedded in the late Ice Age sediments within the region.
As the last Ice Age wound down, the shasta ground sloth, some archaeological evidence suggests, may have been hunted by the earliest Americans, the Paleo Indians, said Michael Cannon and David Meltzer, Quaternary Science Reviews 23 (2004). Nomadic big game hunters probably moving as compact family units, the Paleo Indians had arrived at an unknown time from undetermined origins by uncertain trails. Armed with spears tipped with exquisitely crafted stone points, they shadowed the big game of the time. Given the opportunity, they drove their spears into the great animals, killing them for food, hides and bones. They also scavenged the bodies of the big game that fell to injury or illness. Some archaeologists think that the Paleo Indians may have been such a powerful force that they contributed substantially to the extinction of the Pleistocene big game, the "megafauna."
For sake of comparison, today, the winters in the region – now part of the northern Chihuahuan Desert – remain relatively mild, but the temperatures of summer range 15 to 20 degrees Fahrenheit higher. The rainfall averages only 8 or 9 inches per year, with much of it falling in violent, localized, late summer monsoonal cloudbursts. The playa lakes are often dry. The plant community, with the grass component heavily depleted by overgrazing and fire suppression, ranks as a desert scrubland. The pinyon pine, juniper and liveoak have largely disappeared. Creosote, honey mesquite, yuccas, various cacti, fourwing saltbush and a few other arid-land plants have assumed dominant roles. The wildlife community comprises much smaller mammals than those of the sloth's time. They now include, for example, mule deer, bobcats, coyotes, black-tailed jackrabbits, desert cottontails and numerous rodent species. Modern hunters come for sport, not for survival.
A More Detailed Picture
Over time, with the study and analysis of the Aden Crater skeletal remains as well as other discoveries – for instance, shasta ground sloth fossils and dung in various caves across the Southwest – a more detailed picture of the animal's history and life in a late Ice Age world has materialized.
The ground sloth evolved in South America about 30 million years ago. It migrated into North America about three million years ago, when the Panamanian land bridge emerged to serve as a corridor between the continents.
In the American Southwest, the shasta ground sloth – its appearance and morphology best revealed by the individual animal that tumbled into the Aden Crater fumarole – ranged across the savannah-like country, canyonlands and lower mountain flanks. Except during times of mating, the sloth may have lived a solitary life, typically emerging at night to forage. It apparently took refuge in caves and sequestered rock shelters, where it found protection from winter cold, summer heat and predation. There, too, the female may have found a birthing place and established a nursery for her young. She probably produced a single offspring from a pregnancy. Like the modern tree sloths of South America, she may have carried her young sloth clinging to her back for some months, until it grew large enough and experienced enough to fend for itself.
From studies of the food ball in the stomach of the Aden Crater shasta ground sloth and of the plant remains in the dung of other shasta ground sloths (for instance, at Rampart Cave in Arizona), naturalists determined that the animal fed on plants such as prickly pear cacti fruit, agave, yuccas, desert globemallow, Mormon tea, saltbushes and catclaw acacia, according to Harris. These also offer glimpses into the climate and plant community at the end of the Ice Age.
Road to Extinction
As the last Ice Age wound down, the shasta ground sloth joined other megafauna on the road to extinction, a phenomenon poorly understood by scientists. Some suggest that the animals may have died as climate change altered the species and abundance of the plant community, making it impossible for the large herbivores and, hence, the carnivores to sustain themselves. Others think the large animals became extinct largely due to predation by the Paleo Indians. Others have proposed that the animals died because of a virulent and widespread disease that infected various species. Still others think the animals died in a perfect storm of environmental change, human predation and disease. The cause of the mass extinction remains a mystery.
Whatever happened, during the few thousand years that led to the end of the last Ice Age, "33 genera disappeared from North America," said Harris. By contrast, during the preceding three million years, only "20 genera of large mammals are known to have become extinct."
Aden Crater's Shasta Ground Sloth Today
After study, Yale's Peabody Museum placed the Aden Crater's shasta ground sloth on display in one of its exhibit halls. University of Texas at El Paso's Centennial Museum has placed other animals and materials recovered from the fumarole in its archives. The Peabody and Centennial collections may eventually yield still more information about the Chihuahuan Desert lands of southwestern New Mexico in the future.
Not only did Waterhouse contribute significantly to the study of late Ice Age animals and plants in northern Chihuahuan Desert, he became a cultural force in the Southwest and Mexico. As Laura Hollingsed, Special Collections, University of Texas at El Paso Library, said in a brief biography, Charles Ewing "Bill" Waterhouse, born in 1905, became an accomplished architect, artist, photographer and jazz musician. He left his enduring creative fingerprints across the region as well as in Mexico and the Panama Canal Zone. He counted movie actor John Wayne and artist Tom Lea among his friends. He invested his legacy in his family. His son Russell R. Waterhouse became a well-known artist in Lincoln, New Mexico. His great granddaughter, Cara, married Greg Luffey, who taught music at the University of Texas at El Paso and played the bassoon in the U. S. Air Force Band.
My thanks to Greg Luffey, Laura Hollingsed and Yvette Delgado, all of UTEP, for providing information and photos for this article.
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