STANDING DIVERSITY OF FOSSIL GROUPS IN SUCCESSIVE INTERVALS OF GEOLOGIC TIME: A NEW MEASURE

Abstract

The total number of taxa of given rank of a fossil group known from each of several successive intervals of geologic time is commonly used as a measure of the relative taxonomic diversity of the group during the respective time intervals; this procedure is not justified, particularly not as a measure of standing diversity. For the common case of a fossil group with relatively long-ranging taxa, an estimate of the taxonomic diversity of the group at the midpoint of a time interval is proposed as a measure of its standing diversity during the interval, or, more precisely, its standing diversity at one point in time during the interval. This estimate is calculated for 1) genera of suspension-feeding bivalve molluscs in the Ordovician to Jurassic, 2) families of all major groups of fossil animals in the Cambrian to Tertiary, and 3) skeletonized families of shelf benthos in the Cambrian to Tertiary. All three groups show a similar pattern: an increase in diversity in the Early Paleozoic, a diversity low in the Late Devonian-Early Mississippian, a more pronounced diversity low in the Early Triassic, and a systematic increase in diversity in successive time intervals after the Early Triassic.