Discussion

Prior to the discovery of Porcupine Cave, little information existed about early and middle Pleistocene avifauna from the intermountain West. Rogers et al. (2000) provided useful information with the report of 10 taxa from SAM Cave: Tachy-baptus cf. T. dominicus (Least Grebe), Phasianus colchicus (Ring-Necked or Common Pheasant), Troglodytes cf. T. troglodytes (Winter Wren), Corvini, Ammodramus sp. (sparrow), Junco sp. (junco), Passerculus cf. P. sandwichensis (Savannah Sparrow), Parus sp. (tit), Vireo sp. (vireo), and Asio cf. A. flammeus (Short-eared Owl). The new records from Porcupine Cave add considerably to the diversity of birds known from the region and provide important information on the composition of avian communities in the Rocky Mountains of Colorado. The taxa identified include species reported as fossils for the first time in North America (e.g., the curlew Numenius madagascariensis or N. arquata), as well as many earlier fossil records than previously known (table 12.1).

The environment surrounding the cave today consists of a mountain park (Herring Park) with grasses, shrubs, and scattered open stands of ponderosa pine, Douglas-fir, and juniper. The lower valley is drained by Herring Creek, and although no riparian forests occur along this creek, there are some moderately large stands of aspen in the vicinity. Many of the avian taxa identified from the cave currently occur in the area either seasonally or year round and represent a diversity of habitats, including wetlands and ponds, alpine tundra, and subalpine forest. Although all these habitats currently occur within a 100-km radius of Porcupine Cave, the avifauna in general suggests that, during the time the fossils accumulated, the environment surrounding the cave was characterized by cooler temperatures and moister conditions than today. Probably within the vicinity of the cave there were more extensive wet meadows or marshes. Many of the avian species recovered from the deposits (e.g., shorebirds, ducks, rails) may have been brought to the cave by avian predators (e.g., eagles, hawks, falcons, owls) that have also been identified from this site. Although the sample size is small, the larger number of grouse bones from the deposits (table 12.1) is similar to findings by Steadman et al. (1994b) at Rattlesnake Cave, Idaho. Those authors considered roosting raptors as the primary agent for bone deposition in Rattlesnake Cave, where the non-passerine avifauna was dominated by Centrocercus urophasianus (Greater Sage-Grouse). A similar interpretation may be applied here.

The avifauna from the early Pleistocene deposits (e.g., Mark's Sink) in particular indicate that wetlands, ponds, marshes, or a combination thereof were located near the cave in association with dry grassland, sagebrush shrubland, and subalpine forest (table 12.1). The presence of sora, phalarope, and gull, taxa not found in the other dated levels, suggests that aquatic and wetland environments may have been more extensive during the early Pleistocene than now. In addition, the abundance of sage grouse bones in these deposits further indicates that large areas of sagebrush shrubland occurred near the cave. Sage grouse are restricted primarily to sagebrush habitats today (Johnsgard, 1973). Species indicative of dry grasslands and tundra include the curlew, longspurs, and rosy-finch, whereas the presence of spruce grouse, blue grouse, and woodpeckers implies that subalpine coniferous forest was located near the site. These interpretations are complicated by the possibility that raptors identified from the cave, including falcon (Falco sp.) and owls, may have brought prey items to the cave from a few kilometers away.

Fewer avian fossils are identified from the early middle (~1.0-0.78 Ma) and middle (~780 Ka and somewhat younger) Pleistocene deposits compared to the early Pleistocene ones (table 12.1). However, taxa from the middle Pleistocene are similar to those of the early Pleistocene except that they lack some of the aquatic and wetland species. Only one bone of an unidentifiable goose and three of ducks were recovered from these layers, although the Red-winged Blackbird from the same deposits may also be associated with ponds and wetland environments. Most of the other taxa recovered from the middle Pleistocene are indicative of subalpine forest and shrub-land. These differences may be due to excavation sample sizes and could change as new material is recovered from the cave and processed.

Additional fossils from middle to late middle Pleistocene deposits (table 12.1) represent environmental conditions similar to those of the middle Pleistocene with two exceptions. Pinyon Jay and Brewer's Sparrow are more restricted today to lower-elevation shrublands and pinyon-juniper forests, where they currently breed in Colorado. These two species suggest that climatic conditions were warmer than today during the deposition of sediments in the upper levels of the Velvet Room, though rosy-finch also occur in these levels. It is possible that these sediments accumulated during a relatively warm interglacial interval when more extensive sagebrush shrubland and pinyon-juniper forests may have existed near the cave. Supportive evidence for this interpretation comes from the mammalian record, which includes Lemmiscus cur-tatus (sagebrush vole), Cynomys sp. (prairie dog), and Mustela nigripes (black-footed ferret) from the upper levels of the cave deposits (Wood and Barnosky, 1994; Anderson, 1996). Anderson (1996:279), following Barnosky and Rasmussen (1988), suggested that sediments from one room in the cave, the Pit, show that "alternation of fine-grained, light-coloured loess in the upper layers with dark brown, organic-rich clay pellets in the lower layers indicated dry interglacial and wetter glacial intervals, respectively." Uppermost sediments from the Velvet Room have characteristics similar to those of the upper interglacial intervals of the Pit. Lower Velvet Room layers do not have the brown clay pellets, though they are different in character than the upper Velvet Room layers. It is quite probable that long-term climatic cycles were responsible for the diversity of species and environments represented by the fossil vertebrate fauna (Wood and Barnosky, 1994).

Several taxa from the cave represent range extensions from their modern distributions. The presence of a large curlew in the early Pleistocene of Colorado is notable; it is the only Eurasian species in the avifauna. The expansion of wetland habitats during glacial intervals probably facilitated the expansion of this and other aquatic and wetland species during the Pliocene and Pleistocene. Cooler conditions during glacial intervals may also have favored the expansion of alpine tundra and subalpine forests in this part of the Rocky Mountains, allowing for Snowy Owls and Spruce Grouse to reside in Colorado and Wyoming during the Pleistocene. Additional fossils recovered from the extensive deposits at Porcupine Cave will undoubtedly refine these interpretations and continue to add to our knowledge of early and middle Pleistocene avifaunas in Colorado.

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