Geometric morphometric analysis of patterns of shape change in the Ordovician brachiopod Sowerbyella

The brachiopod genus Sowerbyella is an abundant component of many Ordovicia n paleocommunities of the eastern United States. Five successive population s from a limited geographic region in central Kentucky were tested for the response of brachial valve size and shape to alterations in paleoenvironmen tal conditions. The populations span the interval from Chatfieldian to Mays villian (M5-C3) including the majority of an Ecological-Evolutionary (EE) s ubunit previously described for brachiopods in this area. A marked diminuti on in size occurs subsequent to maximum deepening in the M5 sequence, and i s persistent for the remainder of the interval sampled. Shape follows a dif ferent pattern, with similar changes occurring each time deep-water facies occur in the area. Discriminant analysis shows a clear separation between p opulations sampled from nearshore and offshore facies along discriminant fu nction 1. End member populations in both nearshore and offshore populations diverge significantly on discriminant function 2 over periods of 6.5 and 3 .5 my, respectively, the longer period spanning the majority of the EE subu nit. This contrasts with stasis in valve shape in Devonian. brachiopods ove r an EE subunit of similar duration, but is in accordance with previous fin dings of less community-level stability in Ordovician EE subunits when comp ared with their Devonian counterparts.