Re-evaluation of the "law of constant extinction" for ruminants at different taxonomical scales.

The "law of constant extinction," proposed by Van Valen, states that long and short-lived taxa have equal chances of going extinct. This pattern of age-independent extinction was originally inferred using the fossil record of several different taxa and relied on survivorship curves built from the literal reading of the fossil record. Van Valen's seminal work was mostly done at higher taxonomic levels, hence its prevalence at the species level could not be directly inferred. The surprisingly few subsequent studies done at the species level have challenged the prevalence of age-independent extinction, but those have, for the most part, failed to explicitly incorporate inherent biases of the fossil record. Using a recent Bayesian framework that accounts for several of those biases, including the fact that very short-living lineages might never make to the record itself, we showed that Ruminantia species present age-dependent extinction, where extinction probability decreases with species age. An analysis at the genus level suggested age-independent extinction but further examination suggested that the pattern might be more complex than previously reported by Van Valen. Our results indicate that different taxonomic levels may present different extinction regimes, which could justify the development of new macroevolutionary theory and methods.