Bone Accumulation in Caves

Bones are deposited at sites such as Porcupine Cave by four major processes (Andrews, 1990):

1. Animals die in the cave, particularly during hibernation, denning, or both.

2. Animals fall into the cave through a sinkhole at the surface.

3. Animals are taken into the cave by predators.

4. Bones are transported into the cave by wood rats (Neotoma) after having passed through the digestive tracts of carnivores or raptors or having become disarticulated from carcasses decomposing outside the cave entrance.

The latter two taphonomic scenarios in particular seem to have been important at Porcupine Cave, although the possibility of some animals falling into the cave through intermittently open crevices cannot be discounted. Such sinkhole accumulations have been documented many times at other cave sites (Buckland, 1823; Brain, 1958; Morris, 1974), although as yet no characteristic sinkhole accumulations have been recognized at Porcupine Cave. For example, White et al. (1984) pointed out that natural trap sites have a high concentration of carnivore skeletal material. Morris (1974) noted that natural trap sites are characterized by relatively complete skeletons in caves, deposited largely intact, unless subsequent damage has occurred by trampling, gnawing, or weathering. Neither a particularly high concentration of carnivores nor complete skeletons are found in Porcupine Cave.

As with other cave sites, predation was a major factor in creating the skeletal deposits at Porcupine Cave. Owls and raptors may contribute large amounts of small mammal bones to cave deposits. The regurgitated pellets of predatory birds contain bones with a suite of distinctive physical characteristics that distinguish them from the bones found in carnivore scat. Because some of these birds (the Snowy Owl, Nyctea

Snowy Owl Skeleton
FIGURE 9.2 Dimensions of measurements for DMNH 30076, Canis latrans skull, discussed in the text and illustrated in figure 9.7. (Drawing by C. Suzane Ware.)

scandiaca; the Barn Owl, Tyto alba; and the Great Horned Owl, Bubo virginianus) swallow their prey whole and have a shorter digestive tract than mammalian carnivores, these characteristics include minimal corrosive damage to the bones; skulls often nipped off behind the orbits; minimal damage to maxilla and mandible, with teeth often intact and still in the alveoli; and minimally affected articulation surfaces of the long bones.

Bones that have passed through the digestive tracts of mammalian carnivores, such as the coyote, Canis latrans, or the red fox, Vulpes vulpes, tend to exhibit severe damage from initial dismemberment and chewing, as well as from highly corrosive stomach acids acting upon them after ingestion. The characteristics of bones from carnivore scat in comparison with those from raptor pellets include extreme damage to maxillae and mandibles, including loss or partial digestion of teeth. Teeth and bones in the carnivore digestive system are subject to the effects of passage through a long digestive tract, where they spend much more time exposed to highly corrosive stomach acids. Since carnivores are known to chew the ends of bones to extract the nutrients, the ends of long bones are usually absent. The ends of long bones may also be absent owing to their vulnerability to the digestive process: the bone diaphysis erodes and the bones become rounded in appearance (Andrews, 1990:figures 1, 3).

The specimens described in the remainder of this chapter and others discussed in chapter 22 document many of these characteristics of predation. In addition, wood rats (and potentially other rodents) are implicated through such features as the presence of gnaw marks on the bones, and by the sheer numbers of fossil specimens. Mead and Murray (1991:124) note that "middens containing large numbers of bones [as do those in Porcupine Cave] are indicative of raptor pellet introduction." Dial and Czaplewksi (1990) have observed ground squirrels (Spermophilus), white-footed mice (Peromyscus), lizards, and snakes using wood rat middens; owls regurgitating pellets over and near the middens; and porcupines (Erethizon) bringing bones into the area of wood rat dens. All of these taxa have been found as fossils in the deposits of Porcupine Cave.

Additional potential modifications of bones in fossil sites are those that occur after the bones have been deposited at the site (Andrews, 1990; Lyman, 1994). Although trampling is

FIGURE 9.3 Dimensions of measurements for DMNH 26646, Canis latrans femur, discussed in text and illustrated in figure 9.9. (Drawing by C. Suzane Ware.)

not evident for bones at Porcupine Cave, crushing and/or fracturing of bones by falling rocks and the weight of accumulating deposits, as well as by human foot traffic, was probably not uncommon.

Sometimes the damage seen on a bone is not the work of disease or trauma, or of postmortem processes related to predation or scavenging, but instead was sustained during the excavation or handling of the fossil. By the time a bone that has been collected in Porcupine Cave moves through the process of collection, screen washing, picking, accessioning, and cataloging in a museum collection, there have been many opportunities to damage it. Such excavation-induced damage of bone was not characteristic of the specimens discussed in this chapter.

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