Uhen, M. D. and Pyenson, Nicholas D. 2007. "Diversity estimates, biases, and historiographic effects: Resolving cetacean diversity in the tertiary." Palaeontologia Electronica, 10, (2) 11A–11A.
The number of cetacean genera varied greatly over their evolutionary history. Cetaceans originated in the Eocene and then rapidly diversified, reaching around 30 genera during the Middle Eocene. The number of genera crashed in the late Eocene, with only eight described from the early Oligocene. This time also represents the putative extinction of archaeocetes and the origin of Neoceti, the clade including extant cetaceans. By the late Oligocene, the number of genera recovered and had begun an expansion that peaked in the middle Miocene, at over 75 genera. Since that time, the number of genera has decreased to around 40 genera extant today. Ideally, generic counts reflect the actual number of genera in the past (generic richness), but our understanding of past diversity is confounded by many factors. Generic counts are potentially too high because of the prevalence of genera based on non-diagnostic material and taxonomic over-splitting. Taxonomic counts can also appear to be too high if the counts are made in time intervals that are excessively long. Conversely, generic counts are potentially too low because of fossil non-preservation; non-deposition, destruction, or obscured fossil-bearing, continental shelf rocks; and lack of collecting or publication effort. We evaluate each of these factors as a cause of bias in cetacean diversity. Individual causes may have significant effects on both true generic richness and the amount of bias. These factors must be addressed in any evaluations of past diversity, especially for clades represented by an abundance of fossils.
