The brachiopod genus Sowerbyella is an abundant component of many Ordovician paleocommunities of the eastern United States. Five successive populations from a limited geographic region in central Kentucky were tested for the response of brachial valve size and shape to alterations in paleoenvironmental conditions. The populations span the interval from Chatfieldian to Maysvillian (M5-C3), including the majority of an Ecological-Evolutionary (EE) subunit previously described for brachiopods in this area. A marked diminution in size occurs subsequent to maximum deepening in the M5 sequence, and is persistent for the remainder of the interval sampled. Shape follows a different pattern, with similar changes occurring each time deep-water facies occur in the area. Discriminant analysis shows a clear separation between populations sampled from nearshore and offshore facies along discriminant function 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 subunit. This contrasts with stasis in valve shape in Devonian brachiopods over an EE subunit of similar duration, but is in accordance with previous findings of less community-level stability in Ordovician EE subunits when compared with their Devonian counterparts.

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