EXAMINING ENCRINITES AS FOSSIL CONCENTRATIONS: THE PALEOECOLOGICAL SIGNIFICANCE OF ENCRINITES WITHIN THE JOANA AND LODGEPOLE LIMESTONES, WESTERN UNITED STATES

Encrinites (accumulations of crinoid ossicles) are common components of Paleozoic and Mesozoic strata, and may be represented by thick, laterally continuous deposits termed regional encrinites by Ausich (1997). These echinoderm dominated deposits often are categorized as packstones or grainstones, but typically are not included in studies examining fossil concentrations since the ossicle grain size is commonly less than the traditional cutoff of 2mm, below which materials are considered to be matrix. Although crinoids inhabiting early Mississippian carbonate shelf and platform settings commonly reached sizes in which the stem ossicle diameter exceeded 5mm, because many of the accompanying grains (i.e. from the arms and pinnules, as well as grains that were broken and abraded) are less than 2mm, the encrinites formed during this time are still considered problematic from a fossil concentration viewpoint. However, considering grains less than 2mm in size only as matrix misses much of the paleoecologic information contained in these deposits, and does not adequately demonstrate the importance of the encrinites in an ecological context.

Due to the single crystal structure and characteristic general form of crinoids, ossicles and ossicle fragments less than 2mm in size can usually be identified (as crinoidal in origin) in the field with the aid of a hand lens, provided the grains are fairly unaltered or any replacement has preserved the fine scale physical details of the grains. This is the case for Early Mississippian encrinites of the Joana and Lodgepole limestones of the Great Basin and Rocky Mountain regions, which generally exhibit excellent preservation with little apparent alteration. Fine-scale examination of these units reveals the true nature of the encrinite deposits, which are composed almost entirely of crinoid material.

The utility and importance of analyzing Early Mississippian encrinites as fossil concentrations is clear when they are compared to Late Devonian fossil concentrations. This comparison shows a fundamental shift within the shelly/skeletal component of the level-bottom soft-substrate benthic realm from brachiopod dominated fossil concentrations in the Late Devonian, to crinoid dominated fossil concentrations in the Early Mississippian.