Bivalve Species Duration, Areal Extent and Population Size in a Cretaceous Sea

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Abstract
Geographic distribution, species duration, and population size of 41 bivalve species which coexisted during the uppermost biozone (Sciponoceras gracile zone) of the Cenomanian were measured. Average species duration and correlation between geographic distribution, species duration, and population size were computed for five groups: total, infaunal, epifaunal, cemented epifaunal, and uncemented epifaunal bivalves. Average species duration for all bivalves is less than 2 Myr. Infaunal bivalves averaged slightly higher, epifaunal slightly lower. Positive correlation was measured between species duration and geographic distribution for all five groups and statistical significance of the relationships was demonstrated for all but infaunal forms. In general no correlation was found between species duration and population size, and between population size and geographic distribution with the following exceptions: species duration and population size were significantly correlated for infaunal bivalves and uncemented epifaunal bivalves, and population size and geographic distribution were correlated for uncemented bivalves. Because these 41 fossil species coexisted at the same time and were distributed in the same space, differences in the relationships of species duration, geographic distribution and population among the various life habits indicate differences in ecological strategy and the evolutionary consequences of that difference.