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Rampart Cave in Grand Canyon National Park once held one of the most unusual fossil deposits in North America: a blanket of dung left by Shasta ground sloths. This National Park Service article, by a guest scientist with the Geologic Resources Division, describes the cave, what was found in it and how most of the deposit was lost.
Interior of Rampart Cave. NPS photo, September 1938.
The animal
Shasta ground sloths (Nothrotheriops shastensis) were one of several now-extinct ground sloth species that roamed much of North America. Small for ground sloths, these herbivores still reached about 9 feet in length and weighed over 500 pounds. Their remains and dung turn up in several caves, but not to the extent found at Rampart Cave.
Finding and losing the deposit
The National Park Service sent an archaeologist and a geologist to the cave in July 1936, after a junior foreman who had worked at another sloth cave near Las Vegas recognized its significance. The first excavations produced hundreds of fossils. The largest excavation took place in 1942, under the Smithsonian Institution’s National Museum of Natural History, though much of that material remains undescribed. The cave was vulnerable: despite a gate at the entrance, it was broken into in 1976 and a fire destroyed 70% of the dung deposits.
The sloth dung deposit in Rampart Cave. The apparent boulders are balls of sloth dung. NPS photo, September 1938.
The deposit
Rampart Cave is moderately sized, with a roughly H-shaped floor plan: two narrow passages, one about 140 feet long and the other, which opens outside, about 155 feet long, joined by a low broad chamber. Before the fire the dung averaged several feet thick, showing sloths used the cave for a long time.
| Layer | Contents |
|---|---|
| Lowest, oldest | Sloth dung from before 40,000 to about 24,000 years before present (“present” meaning 1950, the radiocarbon standard) |
| Middle | A “seam” of plant material about 13 inches thick left by packrats, matching the height of the most recent ice age, which suggests the sloths left the area then |
| Upper | Sloth dung from about 13,000 to 11,000 years before present |
Packrat middens, the remnants of packrat dwellings, and bat guano were found throughout. Plant fossils in the dung and middens record the ecosystem and climate over thousands of years and the switch from a building ice age to a warmer, drier interglacial period. The dung shows the sloths ate desert plants including desert globemallow, Nevada mormontea, saltbush, catclaw acacia, cacti, reeds and yucca. Some have suggested that, because such plants were thriving when the sloths vanished, extinction was due to overhunting rather than climate change. At least one piece of dung held roundworms and their eggs.
A Shasta ground sloth skull from Rampart Cave. These sloths had no teeth in the front of the skull, only in the cheeks. NPS photo, September 1936.
Other finds
Fossils include not just bones but hair, skin, sinew and other soft parts, mostly of sloths. Many bones of young sloths suggest the cave was a shelter or nursery. Other animals include tortoises, lizards, snakes, herons, vultures, condors, hawks, eagles, owls, falcons, rodents, rabbits, bats, bobcats, mountain lions, ringtails, weasels, skunks, horses, mountain goats and bighorn sheep. One unusual specimen is a goat forefoot with preserved muscle, cartilage and skin. Extinct animals besides the sloths include a vampire bat, North American horses and an extinct mountain goat. Unlike many other caves in the region, Rampart Cave shows no evidence of human use.
Bones from Rampart Cave, including horns and partial feet with preserved soft tissues. NPS photo, September 1936.
The park around it
Grand Canyon National Park, a World Heritage Site, covers 1,218,375 acres of the Colorado Plateau in northwestern Arizona, semi-arid land of raised plateaus and structural basins cut by steep-walled canyons, with forest at higher elevations and desert basins below. It is one of the most studied geologic landscapes in the world and holds a record of three of the four eras of geologic time. The canyon, cut by the Colorado River, averages 4,000 feet deep along its 277 miles, is 6,000 feet deep at the deepest and 18 miles wide at the widest.
Sources
- National Park Service, Geologic Resources Division, “Pleistocene Life and Landscapes—Grand Canyon”, article by Justin Tweet, guest scientist; the restoration drawing credited to a Wikimedia Commons contributor is left out; rewritten in hubnx's own words.
In these publicationsGrand Canyon National Park
Licence: CC0 1.0 (public domain) · Adapted from www.nps.gov
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