Anthony D Barnosky

University of California, Berkeley

The arvicolines are a taxonomically diverse assemblage of rodents that includes the voles, lemmings, and muskrats and their extinct kin. The importance accorded by paleontologists to arvicoline rodents stems in large part from the widespread recognition of their utility as biochronologic tools. The accelerated evolutionary rates within at least some lineages of arvicolines, their impressive capacity for rapid reproduction and dispersal, and their consequent abundance in the fossil record combine to provide temporal resolution on a finer scale than is available through the biostratigraphic study of other terrestrial faunal groups. It is for this reason that, throughout the history of excavation of the various deposits in Porcupine Cave, special attention was paid to the recovery and identification of arvicoline remains. The documentation of these remains is the primary focus of this chapter.

Here we present a summary of the fossil arvicoline rodent specimens recovered from each locality in the cave and explain the dental characteristics that were used to identify them. The Carnegie Museum of Natural History and University of California Museum of Paleontology excavations have been completed, but the Denver Museum of Nature and Science maintained an active excavation and research program in Porcupine Cave until recently; we include DMNH materials that were picked, sorted, and made available to us as of May 1998. For all collections, only those specimens identified to at least the generic level are included. The stratigraphic distribution of taxa in the localities for which stratigraphic control is available and a complete list of all specimens identified to at least the generic level are provided in appendix 19.1. A listing of which species occur in which sites, presented as minimum numbers of individuals (MNI) and number of identifiable specimens (NISP), appears in tables 10.1-10.13. Nondiagnostic teeth and postcranial elements are not included in this chapter, but they amount to thousands of additional specimens.

This chapter supersedes all previously published accounts of occurrence and taxonomic assemblages of arvicolines in Porcupine Cave (Barnosky and Rasmussen, 1988; Wood and Barnosky, 1994; Barnosky et al., 1996; Bell and Barnosky, 2000). The taxonomic identifications for individual specimens, which in some cases may differ from those listed in previous publications, are listed in appendix 19.1. The bio-chronologic implications of the arvicolines are discussed in chapter 7.

The wealth of material from Porcupine Cave prevents us from presenting detailed quantitative analyses for all arvico-line groups at this time. In some cases we note our observations and perceptions of patterns discernible in the Porcupine Cave record, but there is clearly much work yet to be done with these fossils. The abundant sample sizes of some taxa certainly offer opportunities for further research on morphological variation, morphological change through time, and patterns of variation in enamel microstructure. Similarly, future workers studying mammalian community structure and the impacts of climate change on the faunas preserved in Porcupine Cave will find the arvicolines a fertile area for continued investigation.

The significance of the Porcupine Cave material extends beyond the sheer numbers of specimens and their inherent value for this and other studies. The occurrence at 2900 m elevation of such diverse assemblages of taxa both constitutes a fascinating insight into the biogeographic history of these animals in Colorado and presents new challenges for the biochronologic interpretation of high-elevation faunas. Moreover, the Porcupine Cave faunas include taxonomic associations of many species not previously known to have been sympatric in distribution, and they provide new elevational records, geographic range extensions, or both for some species. At least three of the taxa reported here (Pliolemmus antiquus, Ophiomys parvus, and Mictomys vetus/M. landesi) are not known from cave-derived faunas elsewhere.

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