Most fossil bison taxa are based on size and shape of horn cores. Horn cores vary more widely than, for instance, teeth; but teeth cannot be used in bison classification because they show no distinct phylogenetic pattern. Due to higher variation in horn cores, large population samples would be necessary for between-group comparisons. Horn cores are genetically highly plastic. As an example of the difficulty of using horn-core characteristics as specific criteria, an examination of tabulations of 26 different characters measured by Skinner and Kaisen of four species of Alaskan fossil bison reveals a spectrum of morphological variation which falls within the accepted magnitude of variational limits of one species (B. crassicornis).

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