Onshore-Offshore patterns of variability in geniculation in the Late Ordovician (Cincinnatian) brachiopod Rafinesquina alternata

Alexander, R.R. and Daley, G.M., 1994
GSA Abstracts with Program 26(3):1

Geniculation in the concavo-convex brachiopod Rafinesquina alternata may have: 1) reduced fouling the commissural gape, 2) generated food-bearing eddies against incurrent regions of the commissure, 3) hydrodynamically stabilized shells subsiding into the sediment, 4) reduced energy needed to right overturned shells, 5) anteriorly localized shell fractures, and 6) deterred a posterior purchase on the shell by predators. This species is pervasive throughout five successive transgressive-regressive cycles (C1-C5) delimited within the Cincinnatian Series and abundant in offshore, transition zone, shoreface, and intertidal facies. Frequency of geniculation, mean pre-geniculation length, and post-geniculation height of shells fluctuates through the cycles. Frequency of geniculation is lowest in offshore facies of the McMicken Member (C1: 0%), and the Ft. Ancient Member (C4: 9%), as well as the transitional zone facies of the Miamitown Shale (C1: 3%). Frequencies of geniculation in transitional zone facies of the Corryville Member (C2: 33%), Sunset Member (C3: 26%), Clarksville (C4: 41%), and Blanchester Member(C4: 35%) are intermediate between values for offshore and shoreface facies. Samples from shoreface facies of the Bellevue Member (C1) and Mt. Auburn Member (C2) have the highest frequency of geniculation (87% and 73%, respectively). Samples from the shoreface facies of the Oregonia Member (C3) have atypical intermediate values of geniculation (32%). Mean pre-geniculation length ranges from 29 mm to 34 mm and shows neither a directional trend stratigraphically nor a correlation with the onshore-offshore position of the population. Mean post-geniculation height varies from seven to ten mm. Samples from offshore facies have the shortest values. Zigzagging trends in mean pre-geniculation length and post-geniculation height cannot be explained easily by heterochrony due to predational selection. Geniculation was probably ecophenotypically stimulated by the current-sedimentation regime. Storm-prone populations in shoreface facies have the highest frequency of geniculation and tallest mean post-geniculation height.