THE PAUCITY OF “MASS EXTINCTION” THEORY: AN ANALYSIS OF DIVERSITY CRISES

The concept of “mass extinctions” is imprecisely defined and lacks consensus. Events identified as “mass extinctions,” large and small, have little comparability, no operational definitions, and inadequate underpinnings in testable theory. Unusual drops in taxonomic diversity have traditionally been assumed almost exclusively to comprise rapid increases in extinction rates, with only scarce consideration (if any) of origination rates.

The theory of “mass extinctions” (such as it is) needs complete overhaul for at least three reasons. First, there are no definitional limits on the application of the term “mass extinction” with respect to time (overnight or millions of years?), geography (global, regional, local?), ecology (all oceanic habitats or any one?), or taxa (Linnean nomenclatural categories, at whatever non-comparable level, or arbitrary “percentages” of lost taxa?). These parameters require not absolute numerical limits but situational ones.

Second, rates of origination and their dynamic interplay with extinction rates are largely ignored in assessments of these events.

Third, there is a nearly complete disjunction between marine and terrestrial crises, because (1) there are 25 times more marine than terrestrial fossil taxa, so the land data become “swamped” by marine data, and their particular biotic and abiotic influences tend to be ignored; (2) it is generally assumed that the timing of “events” (often poorly documented on the terrestrial side) are contemporaneous, without decisive geochronological information; and (3) it is usually assumed that the correlative “cause” (or “causes”) of marine extinctions must have similarly affected land organisms, without adequate testing.

There is no decisive evidence that any of the “Big Five” marine diversity crises had any comparable contemporaneous biotic crisis in the terrestrial realm, where sequences are shorter and more episodic, faunas less diverse, depositional environments erosional, and sample sizes smaller.

Analyses of hypothesized diversity crises should be operationally and situationally defined and statistically normalized through the histories of taxa and biotas, and should always explicitly include both origination and extinction rates. The term “mass extinctions” should be abandoned and replaced by “diversity crises.”