Within-taxon morphological diversity in late-Quaternary Neotoma as a paleoenvironmental indicator, Bonneville Basin, Northwestern Utah, USA R. Lee Lyman T , Michael J. O’Brien Department of Anthropology, 107 Swallow Hall, University of Missouri-Columbia, Columbia, MO 65211, USA Received 18 November 2004 Abstract Ecological data indicate that as the amount of precipitation in an arid areas increases, so too does mammalian taxonomic richness. This correspondence has been found in two late-Quaternary mammalian faunas from Utah, one from Homestead Cave in the Bonneville Basin. We use the remains of two species of woodrat (Neotoma cinerea and Neotoma lepida ) from Homestead Cave to test the hypothesis that as the amount of precipitation in an arid area increases, so too does morphological diversity within individual mammalian taxa. Morphological diversity is measured as corrected coefficients of variation and as richness of size classes of mandibular alveolar lengths. Coefficients of variation for N. cinerea are few and coincide with moisture history if temporally successive small samples are lumped together. More abundant coefficients of variation for N. lepida coincide only loosely with moisture history, likely because such coefficients measure dispersion but not necessarily other aspects of variation. Richness of size classes of N. lepida is high during the early and late Holocene when moisture was high, and lowest during the middle Holocene when climate was most arid. D 2005 University of Washington. All rights reserved. Keywords: Bonneville Basin; Morphological diversity; Neotoma spp.; Percentage-stratigraphy graph; Woodrat Introduction It has been found that as the amount of precipitation in an arid area increases, primary productivity increases and so too does the number of mammalian species present (Brown, 1973, 1975; Brown and Gibson, 1993; Meserve and Glanz, 1978; see also Abramsky and Rosenzweig, 1984; Rosenzweig, 1992, 1995; Rosenzweig and Abram- sky, 1993). The typical explanation for correlated increases in all three variables is that relative to arid areas with low primary productivity, those with high productivity will support more mammalian taxa because the taxa can be more specialized and still maintain sufficiently large populations to avoid extinction (Brown and Lomolino, 1998, pp. 776–777 and references). The more food energy available, the more it can be divided among different consumers. It is also known that the amount of intrataxonomic morphological variation reflects the way that a population occupies its niche (Van Valen, 1965). All individuals of the species comprising the population may be generalists, in which case, each uses the entire niche, or each individual is a specialist and exploits only a small portion of the population’s entire niche. The former occurs when all individuals are morphologically similar; the latter occurs when individuals are morphologically unique in one or more characters (Roughgarden, 1972). As Roughgarden (1972, p. 684) noted, the bdistribution of variability in an ecologically functional trait is relevant to understanding niche width. If a population has much variability in the trait, then it has many kinds of individuals who are taking resources from many different places on the resource axis.Q The potential adaptive advantage 0033-5894/$ - see front matter D 2005 University of Washington. All rights reserved. doi:10.1016/j.yqres.2005.02.013 T Corresponding author. Fax: +1 573 884 5450. E-mail address: lymanr@missouri.edu (R.L. Lyman). Quaternary Research 63 (2005) 274 – 282 www.elsevier.com/locate/yqres