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Keywords:

  • Functional morphology;
  • concavo-convex brachiopods;
  • Rafinesquina;
  • Ordovician;
  • Indiana

The concavo-convex shape of strophomenoid brachiopods has been inferred to be adaptive to a free-lying state. Such functional hypotheses should be constrained by identifying the factors controlling morphology. A typical strophomenoid, Rafinesquina alternata, is used here to study morphological influences. Specimens of R. alternata were collected from ten localities in Indiana, USA. Beds are Upper Ordovician (Richmondian) mudstones and limestones of the Dillsboro and Whitewater Formations. Length and hingeline width of the pedicle valve were measured, and elongation (length divided by width) calculated for each specimen. Specimens were qualitatively identified as geniculate or arcuate. Regression analyses using stratigraphy (time), mudstone percentage (substrate), grainstone percentage (disturbance), ratio of Strophomena planumbona to R. alternata (competition), and length (ontogeny) as independent variables, were performed to determine the factors influencing morphology. Elongation was most strongly influenced by grainstone percentage and the S. planumbona ratio. Populations of R. alternata from grainstonerich intervals are less elongate than other samples. The lateral margins of transverse R. alternata may have functioned as sediment traps during periods of high turbidity. Alternatively, variation in elongation may be a character displacement due to interspecific competition with the related S. planumbona, which is inferred to have had a similar life mode. R. alternata specimens found in beds dominated by S. planumbona are more elongate than R. alternata from beds in which S. planumbona is rare or absent. Geniculation was influenced by stratigraphic position, suggesting an evolutionary trend, and by limestone percentage. Geniculate individuals are most common in muddier intervals, supporting the hypothesis that geniculation enabled R. alternata to employ an ‘iceberg’ strategy, ‘floating’ convex down on the soft muds. The habitat distribution of R. alternata is inconsistent with the hypothesis that concavo-convex brachiopods lived convex-valve-up, as suggested by Lescinsky (1995). Both elongation and geniculation may be examples of phenotypic plasticity.