On the measurement of size-independent morphological variability: An example using successive populations of a devonian spiriferid brachiopod

More than 500 undistorted, unfragmented pedicle valves of Ambocoelia umbonata (Conrad) (Spiriferida, Brachiopoda) were recovered from each of four levels within a Middle Devonian fossil cluster. The fossil cluster, an ellipsoidal shell accumulation measuring one meter in diameter and 2 cm in thickness, was exhumed from an exposure of the Ludlowville Shale (Hamilton Group) of western New York. Size frequency histograms indicate that the brachiopod experienced very high levels of juvenile mortality, due, probably, to the effects of high bottom turbidity. Sedimentological and paleontological evidence indicates that the cluster represents a sequence of in situ benthic associations. Size-independent variation was estimated by calculating the eigenvalue of the minor axis on a log-transformed plot of pedicle valve lengths and widths. The eigenvalue technique eliminates the effect of allometrically induced shape changes and is applicable to multicharacter analyses of morphological variability. Size-independent variability among the larger individuals of Ambocoelia decreases in successively younger cluster populations. The decrease is not correlated with any observed or inferred change in substratum, diversity, equitability or turbidity.