Late Pleistocene Vertebrate Paleocommunities of the Lower Grand Canyon: Rampart Cave [Les paléocommunautés de vertébrés du Pléistocène tardif du Grand Cayon inférieur : la grotte Rampart].

Résumé

Although a number of Grand Canyon (GRCA) caves have been examined for late Pleistocene (Ice Age) fossils, the western end of the Canyon has been relatively ignored and understudied. Pleistocene migration into GRCA from the west would likely have been via exposed spits of land up to, and just beyond, Rampart Cave. Desert community species reach their eastern limits in this vicinity, making the area a past and present repository of transitional plants and animals. Rancholabrean-age deposits discovered in Rampart Cave during 1936 Civilian Conservation Corps work consist primarily of dry Nothrotheriops shastensis (Shasta ground sloth) dung. Bones and soft tissue remnants have also been recovered. It seems the sloth used the shallow cave intermittently for at least 30,000 years as a birthing den and perhaps a shelter. Other taxa known from the cave include Oreamnos harringtoni (extinct mountain goat), Desmodus cf. stocki (extinct vampire bat), Marmota flaviventris (yellow-bellied marmot - does not occur in Arizona today), Sauromalus obesus (chuckwalla), Felis sp. (mountain lion?), and Gopherus agassizi (desert tortoise). The cave is a veritable Ice Age Noah’s Ark! Fossils were also recovered from a 1942 expedition by the Smithsonian Institution, but were left largely unidentified and never published. I am in the process of identifying the 1936 and 1942 specimens, which will be published as part of my thesis. Of special interest to me is a skull that appears to be Coragyps atratus (black vulture). This vulture does not live in northern Arizona today. Approximately 70% of the 450m2 dung bed was destroyed by a fire that burned from about July 1976 to March 1977. Until this fire, Rampart Cave was the repository of the thickest and least disturbed deposit of stratified ground sloth dung ever known. A trench dug in the dung bed in 1942 acted as a firebreak and saved part of the deposit. In 1999, my crew and I created two new profiles in the dung deposit. The Mead/Carpenter Profile mirrors a previously excavated profile used in paleobotanical and environmental reconstructions by Dr. Paul S. Martin (University of Arizona, Tucson) and others. The fire destroyed this profile, thus our new profile will allow future research of the stratigraphy and deposition in Rampart Cave. The Winfree Profile is in an area never before excavated. During work on this profile, we unexpectedly discovered bones in an adjacent area reduced to ash by the fire. Elements from Gymnogyps californianus (California condor), Aquila chrysaetos (golden eagle), O. harringtoni, and N. shastensis were recovered and will be included in my thesis along with any vertebrate remains I recover while processing dung and sediments from certain excavated layers. A hard, degraded, white basal layer that appears to be a carbonate will be chemically analyzed. This layer is interesting because it contains a large amount of hairs (yes, hairs). The hairs may have belonged to some of the first animals to enter the cave over 40,000 years ago. A hard, black, crystalline layer just above the white layer will be analyzed as well. Assumed to be ancient "bat guano" by past researchers, I hope to find out for sure what this substance is.