EPISKELETOBIONTS ON PSEUDOATRYPA (BRACHIOPODA) FROM THE LOWER GENSHAW FORMATION OF THE MIDDLE DEVONIAN TRAVERSE GROUP OF MICHIGAN, USA
Pseudoatrypa (Variatrypinae) was recovered for the first time from the early middle Givetian Lower Genshaw Formation of the Traverse Group of Michigan. Two different morphotypes of this genus were identified based on their phenotypic traits - Pseudoatrypa morphotype A and Pseudoatrypa morphotype B. Pseudoatrypa morphotype A differs from morphotype B in its fine- to medium-size ribbing and absence of broad keel-like hinge structure as opposed to the coarse ribbing and broad keel-like hinge of morphotype B. However, geometric morphometric analysis indicates that the two morphotypes do not differ significantly in overall shape.
Encrusting organisms equally encrusted both morphotypes. The brachial valve was more heavily encrusted in both morphotypes, likely because of the atrypides’ life orientation, in which the more convex brachial valve was exposed while the brachiopod rested on its flattened pedicle valve. Microconchus worm tubes were the most frequent encrusters on both morphotypes; bryozoan sheets and hederellids were also common, and auloporid corals, cornulites, ascodictyon, craniid brachiopods, tabulate corals, pelmetazoan holdfasts and fenestrate bryozoans were less abundant. The commissure- and hinge-proximal location and growth patterns of auloporid corals suggests epibiosis with live Pseudoatrypa. A few other encrusting organisms indicate postmortem encrustation of brachiopods, but for the majority of encrusters, the live or dead status of the host is unknown.
Although the two morphotypes of Pseudoatrya from the Genshaw Formation differ in ribbing coarseness and hingeline structure, morphometric analysis does not differentiate between the two morphotypes. Furthermore, encrusting organisms do not display a preference for one morphotype over the other, regardless of surface texture of the shell.