Intraspecific selective survival within variably uniplicate Late Devonian brachiopods

Abstract

The central fold of the Late Devonian brachiopods Cyrtospirifer monticola, Cleiothyridina devonica, Rugaltorostrum madisonense, Sinotectirostrum banffense and Trifiodorostellum dunbarense (Three Forks Formation, Central Idaho) varies intraspecifically from nearly rectimarginate or weakly developed to chevron-shaped. Frequency of central fold morphotypes through succesSjve size classes indicates selective survival in favor of variants with a strongly developed central fold, although morphotypes with the most extremely developed central fold never numerically dominate the larger size classes of any species. Differential mortality among the morphotypes can be attributed to: (1) less resistance to shell fracture by morphotypes with weakly developed uniplicae which were preferentially culled out of the population by durophagous predators, (2) less efficiency of weakly uniplicate morphotypes in the generation of nutrient-bearing eddies against the downcurrent inhalant margins. Failure of the morphotypes with the most strongly developed uniplicae to numerically dominate adult (largest) size classes is attributed to the hydrodynamic instability of such shell geometries on current-scoured substrates.