Measuring Late Quaternary Ursid Diminution in the Midwest

by Nagorsen et al. (1995), who found that the proposed Paleobiologists generally agree that within the past 10,000 yr relation appears to hold in that region. North American black bears (Ursus americanus) have decreased Along the same lines, Graham (1991, p. 237), reporting in body and tooth size. Some researchers infer that diminution work on late Quaternary black bears in the mid-continent, was gradual and continuous; thus, one might infer that a specimen states that ‘‘[l]ate Pleistocene black bears (Ursus ameriis old if it is larger than an average-size modern bear. Ursid recanus) are generally larger than modern black bears from the mains recovered in the 1950s from Lawson Cave, Missouri, that same geographic area.’’ Further, ‘‘[s]ize and morphological are larger than some modern bears have been reported to date to attributes have been used to define several fossil species and the late Pleistocene, but association with modern taxa, taphonomic subspecies of black bears’’ (Graham, 1991, p. 237). Graham considerations, and a radiocarbon date of 200 yr B.P. indicate that (1991, p. 244) uses a detailed morphometric analysis of the they are modern. Modern specimens from Lawson Cave and other upper second molar to classify a poorly provenienced blackparts of the American Midwest are relatively large compared to modern North American black bears from other areas, suggesting bear skull and mandibles from Bill Neff Cave, Virginia, as that many supposed late Pleistocene bears from the area might be a member of U. americanus, ‘‘[b]ecause of its cranial and modern also. q 1998 University of Washington. dental dimensions, which are larger than modern black bears from Virginia and most black bears form other geographic areas, it is quite possible that the Bill Neff skull is from INTRODUCTION either a latest Pleistocene or earliest Holocene population.’’ Graham’s (1991) detailed morphometric analysis and comMany paleobiologists believe that through the Holocene parison with modern specimens from the same region (Virvarious mammal species were represented by progressively ginia) is commendable, given the absence of stratigraphic smaller individuals. Kurtén and Anderson (1980, p. 184), and radiometric evidence of age. for example, state that ‘‘In the Holocene, there was a marked The general impression one derives from the literature is decrease in size in [black bears, Ursus americanus], a phethat size diminution of North American black bears was nomenon also seen in many other species of large mamgradual and continuous throughout the Holocene, and we mals,’’ and Nagorsen et al. (1995, p. 16) indicate that ‘‘the believe that a tendency exists to consider larger-than-average evolutionary trend in U. americanus has been a reduction black-bear-remains relatively old, across the continent (Gorin size, with Pleistocene Black Bears significantly larger than don, 1986, p. 398; Nagorsen et al., 1995, p. 16; Graham, extant forms.’’ This relation between size and age is often 1991, p. 237, 244). Clearly late Pleistocene black bears were assumed. Gordon (1986, p. 398), for example, states that large in certain regions; however, in many areas the fossil body size of western North American black bears continurecord for black bears is poor (including the Midwest and the ously and gradually decreased from 10,000 yr ago to the Pacific Northwest), particularly during the middle Holocene. present based on size differences observed in the faunal Did gradual, continuous diminution actually occur across record, despite the absence of evidence for mid-Holocene the continent? Or, has that time–space relation simply been bears. Of equal importance is that Gordon’s sample of late inferred? The ease with which gradual diminution is inferred Pleistocene bears comprises individuals from much of the in the Midwest results, in part, from the fact that few ursid American Midwest, Texas, and southern California, but none remains have been directly dated. Only two cases incorporatfrom the Pacific Northwest. On a continent-wide scale, then, ing directly dated specimens are known to us, and both are bears are assumed to decrease in size through the Holocene from the west coast of North America (Heaton et al., 1996; because ‘‘the data indicate that the late Pleistocene ancestor Nagorsen et al., 1995). was large . . . and that it has become smaller’’ (Gordon, Most North American black-bear remains assigned to the 1986, p. 398). Gordon’s (1986) assertion that late Pleistocene late Pleistocene have been indirectly dated on the basis of ursids were, on average, larger than modern ursids was retheir association with remains of mammalian taxa that became extinct at the end of the Pleistocene, but the strength cently tested with fossil evidence from the Pacific Northwest