Understanding biases that affect how species are partitioned into higher taxa is critical for much of palaeobiology, as higher taxa are commonly used to estimate species diversity through time. Using the deep-sea record of coccolithophorid microfossils over the last 150 million years (myr), we demonstrate that sampling and taxonomic effort are important drivers of the species/genus ratio. An unexpected two-stepped change in the ratio of species to genera over the last 150 myr correlates strongly with changes in both the number of deep-sea sites yielding coccolithophorids that have been studied and with the number of taxonomists who have published on those sections. While some limited biological signature from major extinction events can be recognized from changes in the species/genus ratio, the numbers of sites and the numbers of taxonomists combined explain some 82% of the observed variation over long periods of geological time. Such a strong correlation argues against using raw species/genus ratios to infer biological processes without taking sampling into account, and suggests that higher taxa cannot be taken as unbiased proxies for species diversity.