CARBS LEAD TO GREATER GIRTH: ATRYPIDE BRACHIOPOD SHAPE AND ITS RELATION TO GAMMA RAY LOG ENVIRONMENTAL SIGNATURES FROM THE DEVONIAN OF MICHIGAN AND ALBERTA

The use of indicator taxa to discern environmental conditions is an oft-used tool in paleoecology. However, as external shape, which is often related to environment, is not necessarily indicative of taxonomy, many taxa are not useful for paleoenvironmental reconstruction. This suggests using morphology as a more direct proxy for environment, particularly, as unlike taxa, shape is less constrained by time or region. We explored this possibility for dorsibiconvex brachiopods within the Order Atrypida; these brachiopods are the most abundant members of many Siluro-Devonian communities but are not always useful indicators.

Atrypides from eight units within the Traverse Group (Michigan, USA) and from six units within the Waterways Formation (Alberta, Canada) were analyzed in dorsal, anterior, and lateral orientations using perimeter based morphometrics. Generalized Procrustes and Relative Warp analyses quantified shape variation between stratigraphically distinct populations from each region. The loading scores of the 1^st and 2^nd relative warp axes (which account for > 85% of shape variation) indicate that the majority of variation is the result of expansion or reduction within the dorsal/ventral dimension.

To determine if the observed shape variation was related to environment, t-test, Hotelling’s t-test, and Discriminant Function analyses were performed comparing 1^st and 2^nd axis relative warp scores between specimens from shale and carbonate dominated stratigraphic units; sediment type was inferred from gamma ray log signatures. All statistical results corroborate that shale and carbonate dominant populations differ significantly; biconvex forms were more common within carbonate settings, while dorsibiconvex forms were prevalent in shale settings. Moreover, these results were derived independently for the two regions and hold across multiple taxa. Given previous biomechanical work which suggests both forms lived attached by a pedicle, the dorsibiconvex form would have sat with its commissure oriented more perpendicular to the substrate, enhancing separation of inhalant and exhalant currents, increasing ambient flow in quiet water settings associated with these shales. These findings indicate that convexity of atrypides may be a useful environmental indicator, regardless of place or taxa.