A TEST OF MORPHOLOGICAL RESPONSE TO DWARFING IN A MIDDLE DEVONIAN BRACHIOPOD FROM WESTERN NEW YORK

Diminutive faunas are typically attributed to one or a combination of the following factors: 1) dwarfing due to retardation of growth, 2) accumulation of immature specimens due to sorting by water, and 3) accumulation of immature specimens due to a high juvenile mortality rate. Such dwarfed faunas are a common occurrence in pyritic shales of the Middle Devonian Appalachian Basin of New York. The purpose of this study is to test whether dwarfing of organisms in pyritic shales represents a change in size, shape, or both.

Ambocoeliids can be commonly found in both pyritic and gray shales in the Hamilton Group (Middle Devonian, Givetian), of Western New York. The occurrence of these organisms in reducing (pyritic) environments makes them suitable for this study, as dwarfing is commonly associated with pyritic shales. Normal morphological forms can be found in gray, calcareous shales representative of normal marine conditions. Neither depositional environment shows any sign of hydrodynamic sorting of organisms. Geometric morphometrics is utilized to test for differences in shape between dwarfed and non-dwarfed forms, and allows for a comparison of diminutive forms to a normal ontogenetic sequence obtained from gray shale. This enables a test of different hypotheses for the observation of dwarfism in pyritic shales; in particular diminutive adult forms and high juvenile mortality rates.