Morphological Shape, Episkeletobiont Analysis, and Life Orientation Study in Pseudoatrypa cf. lineata (Brachiopoda) from the Lower Genshaw Formation of the Middle Devonian Traverse Group, Michigan: A Geometric Morphometric Approach

Abstract

Atrypids examined from the lower Genshaw Formation of the Middle Devonian (early middle Givetian) Traverse Group include a large assemblage of Pseudoatrypa bearing a rich fauna of episkeletobionts. We identified two species of Pseudoatrypa—Pseudoatrypa lineata and Pseudoatrypa sp. A based on ornamentation and shell shape. Qualitative examination suggested that the former had fine-medium size ribbing, narrow hinge line, widened anterior, gentle to steep mid-anterior fold, a more domal shaped dorsal valve, and an inflated ventral valve in contrast to the coarse ribbing, widened hinge line, narrow anterior, gentle mid-anterior fold, arched shape dorsal valve, and flat ventral valve of the latter. Geometric morphometric analysis supported two statistically different shapes (p < 0.01) for the two distinct species. This study further examined these atrypids to investigate the influence of morphology on episkeletobiont settlement on the two Pseudoatrypa species. Among the 343 atrypid hosts examined, nearly 50 % were encrusted by episkeletobionts. Common encrusters included microconchids, bryozoan sheets, and hederellids. Less common encrusters included auloporid corals, cornulitids, tabulate corals, Ascodictyon, craniid brachiopods, and fenestrate bryozoans. Hederellids, auloporid corals, cornulitids, and tabulate corals encrusted a few living Pseudoatrypa hosts, but determination of pre- or post-mortem encrustation by the majority of episkeletobionts is equivocal. In a very few cases, episkeletobionts crossed the commissure indicating the death of the host. Some episkeletobionts, microconchids and the sheet bryozoans, were more common on Pseudoatrypa lineata, which exhibited more dorsal–ventral convexity than Pseudoatrypa sp. A. Perhaps, P. lineata may have provided a larger surface area for episkeletobiont settlement relative to Pseudoatrypa sp. A. In both the host species, encrustation was heaviest on the convex dorsal valve. This suggests that most of the encrustation occurred in a reclining, dorsal-valve-up life orientation of both species, in which the convex dorsal valve was exposed in the water column and the ventral valve remained in contact with the substrate. However, life orientations of these atrypid species could not be confidently predicted simply from the location preferences of episkeletobionts alone, as the life orientation of the host would also have been a hydrodynamically stable orientation of the articulated shell after death. Most episkeletobionts encrusted the posterior region of both dorsal and ventral valves of the two species, which suggests that the inflated areas of these valves, when exposed, favored the settlement of most episkeletobiont larvae.