Copemysand the Barstovian/Hemingfordian Boundary

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Abstract
Boundaries of land mammal ages have rarely been addressed. Now that North American land mammal ages are well ordered and reasonably well calibrated, it is fitting to seek more rigorous guidelines for the definition and recognition of their boundaries. Criteria for recognizing boundaries of land mammal ages are discussed briefly, with selected examples of problems in boundary recognition. The boundary of a land mammal age should be defined as (a) the appearance of a single taxon, preferably an immigrant, that is morphologically distinctive, widespread and well represented, taxonomically stable; and (b) strata where the boundary event is recorded should be well sampled, both above and below the level where the “boundary event” taxon appears. The Hemingfordian/Barstovian boundary is reviewed and the boundary redefined, based on the appearance of the cricetid rodent Copemys. This small mammal best meets the criteria for a “boundary event” taxon discussed in the text. The revised Hemingfordian/Barstovian boundary is recognized low in the Rak Division of the Barstow stratigraphic sequence in the Mud Hills of southern California. This is lower than previous designated boundaries and equates (more or less) to the lower part of magnetic chron C5CN, or approximately 16.8 Ma, about 0.8 Ma older than previous calibrations.