Paleoecology And Evolutionary Trends In Gypidulid Brachiopods

Abstract

The distribution of gypidulid brachiopods in Siluro­-Devonian carbonates of the Appalachian basin is strictly limited by gypidulid specializations for poorly sorted sand substrates, normal salinity and good circulation. Gypidulid occurrences are limited to open shelf near wave base sands which are closely associated with trans­gressive barrier systems. Regressive phases of carbonate deposition lack suitable substrates for gypidulid habita­tion. Progressive adaptive morphologic change in gypidulid spondylium/beak shape parallels progressive decrease in restriction of the gypidulid environment. Canonical analy­sis and comparisons of mean shape ratios, reduced major axes and allometric curves are used to statistically define variation in spondylium shape and size in three successive populations of gypidulius. Statistical tests indicate significant differences in spondylium/beak shape and size between species. Persistence of significant change in growth (allometric) curves through time indicates genetic change in successive populations.