Sloth layer == Guano layer

>35,500 Guano layer


12,050 ± 400 Sloth layer

Wood rat layer

23,540 ± 460 (Wood rat) -32,560 ±730 (Sloth) 36,200 ± 6000 (Sloth)

Sloth layer


Figure 11. Floor plan of Rampart Cave, Arizona, and stratigraphy of sloth dung. Roughly 95 percent of an unexcavated deposit was destroyed by fire in 1976. Adapted from Long and Martin 1974. Used with permission from Science, © 1974 AAAS.

Plate 3. "Dance floor" of sloth dung in Rampart Cave, April 1971. In 1976, the deposit was destroyed by fire. Photo by Eugene Griffen.

not to be confused with Mark Harrington, is a Canadian paleontologist and mountain goat expert who joined one of our Rampart Cave trips.)

In 1942 Remington Kellogg and his crew screened a large volume of sloth dung and packrat middens at Rampart Cave. They found only a few more species to add to Evans's collection of fossil vertebrates. In addition, archaeologist Gordon C. Baldwin, detailed to look for any indications of early humans, wrote, "There was not a single fragment of evidence to indicate that man had ever occupied the cave, either contemporaneously with the ground sloth or later" (Baldwin 1946). The oldest artifact that we found on our trip in 1969 was a bit of newspaper from the 1930s. A small headline read, "Fascists Bomb Mallorca."

Our goal in 1969 was to clarify an ambiguity resulting from the pilot study that I had published with Dick Shutler and Bruno Sabels a decade earlier. In assessing the overkill theory (particularly any version of it that involved rapid extinctions), it was vital to ascertain as precisely as possible both when humans arrived on the continent and how long thereafter the large mammals survived. The available radiocarbon dates indicated that the Clovis people, the First Americans, had reached this part of the continent approximately 11,000 radiocarbon or 13,000 calendar years ago. The youngest of Shutler's dates, based on both plant residues

Plate 4. Shasta ground sloth adult cranium and infant mandible, from Rampart Cave, with penny for scale. Photo by author of specimens in the National Park Service Collection, Grand Canyon National Park.

and humic acids in the dung, indicated that sloths had lived at Rampart Cave until 10,000 radiocarbon years ago. Recent findings, however, had begun to cast doubt on this and other late dates, as well as on much younger dates on various extinct large animals generated in the first erratic years of radiocarbon dating. Libby, for example, had obtained an 8,500-year date (lab catalogue number C-222) on sloth dung from Gypsum Cave. None of our ground sloth samples from caves in Arizona, Nevada, New Mexico, and Texas were as young (see table 5).

Now armed with improved methods, investigators began to challenge all Holocene dates on extinct animals. In southern Arizona, Vance Haynes found that mammoths associated with human artifacts in a kill or processing site were all of Clovis age, about 13,000 calendar years old, and that mammoth extinction had occurred closer to 11,000 radiocarbon years ago than the 8,000 years or less that the first Arizona radiocarbon dates suggested. Therefore, if Shutler's 10,000-year date, L-473A, at Rampart Cave was valid, the Shasta ground sloths had disappeared a thousand years after the mammoths. This was a puzzling discrepancy, as the ground sloths should have been, if anything, more vulnerable to hunters. They were ambulatory pin cushions, helpless beasts unless a predator came close enough to be ripped with those long claws at the ends

Plate 5. Stratified sloth dung and packrat middens, Rampart Cave, ca. 1969 or 1970. Photo by the author.

Plate 5. Stratified sloth dung and packrat middens, Rampart Cave, ca. 1969 or 1970. Photo by the author.

of its long arms. If people and ground sloths had coexisted for a thousand years, my version of the overkill theory was in trouble.

We obtained radiocarbon dates on 14 samples, collected mainly on the surface of the dung deposit. As shown in table 5, all but one of our dates were hundreds of years older than Shutler's. Our youngest date, A-1067, was 10,780 ± 200 radiocarbon years. The discrepancy could have meant that the original measurement was in error, either because dating technology had not been sufficiently refined or because the sample was contaminated. It could also have meant that we simply had not dated enough young dung balls the second time around to reflect the true sample range.

Much depended on an accurate determination of when the last dung was deposited, so we pursued all of these possibilities. We hunted down other deposits of ground sloth dung for further tests. After three decades we made another test. Pete Van de Water located and cleaned a fragment

TABLE 5 Radiocarbon Samples from Ground Sloth Caves

(solid carbon dates excluded)

TABLE 5 Radiocarbon Samples from Ground Sloth Caves

(solid carbon dates excluded)

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