Body mass distributions of mammalian species are a major focus of macroecological and macroevolutionary studies. However, these distributions may be obscured by taxonomic error, just like any other aspect of biodiversity. The key problem with taxonomy is that many currently used names are synonyms of each other or are biologically indeterminate. This article reassesses body mass patterns in the fossil record of North American mammals using the recently developed flux ratio method for estimating the underlying proportion of invalid names. Current name quality varies very strongly with body mass: small species names are highly unreliable, but names of large species have been evaluated thoroughly. The main reason is that there has been a dramatic fall through historical time in the average size of described species. Hence, there simply has not been enough time yet to reevaluate the names of most small species. This bias only accentuates the previously described bimodal diversity distribution for North American mammals, which suggests the existence of dual body mass optima—so not all evolutionary lineages converge on 100 g. The historical shift in the underlying quality and body mass of newly described species also differentially affects our picture of biodiversity in major taxonomic groups. On the one hand, ungulate and carnivoran names are much more likely to be invalid in the 1st place than are rodent and insectivoran names. On the other hand, most of the invalid names for large mammals already have been identified, but this is not true for the small-mammal groups. Therefore, the most fruitful strategy for future taxonomic research would be to focus on small- and medium-sized mammals.
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