Accumulation of Bones

At least three vectors of bone accumulation seemed to have been active when the cave was open in the early and middle Pleistocene. The most important of these was probably the propensity of wood rats (Neotoma spp.) to collect random items to incorporate into their middens (Betancourt et al., 1990). Collected items include carnivoran fecal pellets and raptor regurgitation pellets, which are frequently laden with the bones of small vertebrates (especially mammals) that the

predators ate. Wood rats also dragged isolated bones of large mammals into their midden sites, including bones as large as deer humeri or jaws, horse teeth, and podial elements of horse and elk-sized perissodactyls and artiodactyls. During excavation of Porcupine Cave by CM crews in the 1985-89 field seasons, bushy-tailed wood rats (Neotoma cinerea) were observed as far back into the cave as the Pit, and fecal pellets, active nests and middens, and urine deposits (termed "amberat" by some authors) were observed in localities throughout the cave. Active or recently active middens within the cave contained diverse plant remains (e.g., sticks, twigs, seeds), teeth of cows, raptor pellets and carnivoran scat (probably coyote), and in one case a dead wood rat. In fact, all of these items were contained in a single nest approximately 15 m inside the cave.

Direct evidence that this activity went on for decades includes the recovery of a tobacco can with a note dated to 1939 from an active wood rat nest in the Velvet Room when the room was first opened to humans in 1986; the can had been dragged in by wood rats from the cave entrance (see chapter 4). A radiocarbon date of 2180 ± 80 yr BP obtained on a Neotoma midden (the Trailside Nest) about 100 m north of the mine adit (at the site known as Trailside Entrance) confirms that wood rats have actively collected in the area for at least millennia (Barnosky and Rasmussen, 1988).

Evidence that wood rat activities contributed to rich accumulations of bone throughout the cave during the early and middle Pleistocene includes the following:

1. Abundant fossils, purportedly of at least five different species of Neotoma, including N. cinerea, are present in most of the excavated deposits in the cave.

2. Within Velvet Room strata excavated by CM were fossilized middens, characterized by moderately indurated, tannish white layers that feature abundant casts of the shape and size of Neotoma fecal pellets (figure 2.8).

3. Many of the larger bones bear paired incisor gnaw marks of a size appropriate to Neotoma.

4. The vast majority of the fossil bones are of a size that is appropriate for wood rats to incorporate into their middens.

5. Most of the fossils are of small mammals, with overrepresentation of teeth, skulls with broken crania, mandibles, and other elements resistant to digestion.

These characteristics, coupled with etching by stomach acids in some cases, imply that some bones passed through the di gestive tracts of carnivorans and raptors and were contained in fecal or regurgitation pellets before being dragged into the cave. In situations in which rocky outcrops provide roosts for raptors and/or denning areas for mammalian carnivores within the foraging range of wood rats (as at Porcupine Cave), wood rat middens include many bone-laden pellets. Over time, much of the organic matter except bones decays, and the resulting deposits can be exceptionally rich in fossils (Hadly, 1999).

The second most important collection vector may have been the direct activity of mammalian carnivores either taking prey into the cave or dying there. This mode of collection applies especially to some of the few bones that are too large to have been dragged by wood rats. Fossils of bears, badgers and other mustelids, coyotes, and wolves have been found in Porcupine Cave. Pleistocene denning activity is suggested by the presence of dentitions of juvenile coyotes. Extant relatives of all these carnivores use caves as places to bring carcasses of small mammals or parts of large animals that they subsequently gnaw or eat. From 1985 to 1991 it was not uncommon to hear coyotes howling near the cave; signs of black bear activity (e.g., tracks, overturned rocks) were infrequently evident near the cave entrance; and 20 m inside the cave the nearly complete carcass of a recently killed and partially eaten rabbit was found in 1986. Thus extant mammalian carnivores clearly use the cave, and there is no reason to suspect that their extinct relatives did not also use it when adequate entrances were available.

Very rarely in Porcupine Cave are fossil animals much larger than rodents represented by bones of a substantial portion of the skeleton. An exception is a single cranium of the camel Camelops, which was recovered by DMNH crews from Tobacco Road (figure 2.3). Because of its size, the skull possibly represents an animal that either fell into the cave through an intermittently open sinkhole, wandered in and could not find its way out, or was dragged in as a partial carcass by a large carnivore such as a bear.

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