Meaningful analysis of phenotypic variation depends upon the distribution of individual ages in the samples studied. North American bison paleospecies have tended to be founded upon isolated specimens, whereas populations have been more difficult to assign, a paradox partly due to variation in age and sex distributions. Great Plains archaeological bison kills yield mortality profiles that, together with dental evidence for seasonal restriction, document culturally originated catastrophic kills. Other archaeological samples give evidence for selective attritional killing of individuals over a wide seasonal spread. The catastrophic samples include all age groups but juveniles are under-represented in comparison with idealized mortality models. Attritionally hunted samples may focus upon certain ages according to cultural priorities. In both cases there can also be selection as to sex, given that large herds are dominated by mature females with immatures of both sexes, and that hunters can also be selective in pursuing individual bison. Two non-cultural mortality models show that natural samples, whether catastrophic or attritional, are differentiable from culturally initiated kills. The Hitching Post Ranch, Alberta, sample (ca. 3600 yr BP) is a natural mass kill showing tight seasonality and a classic catastrophic population curve highest in juveniles and with all age groups represented. The La Brea, California, sample (Late Pleistocene) shows attritional mortality with high numbers of juveniles and a lesser peak of old individuals; yet dental attrition also indicates clustered seasonality. This suggests a migratory bison population seasonally present at the trap. Attritional samples, with a higher proportion of old versus mid-maturity animals, display more robust horn cores, a taxonomically significant character for past authors. Frontal breadth and horn core direction are also age-dependent. Long-horned fossil bison likely had a longer period of horn growth, making horn-core spread age-dependent even into upper maturity, whereas shorter-horned late Holocene bison reached full horn core length in two to three years. There is a strong possibility that some taxonomically important characters were environmentally influenced at a phenotypic level.