POPULATION STRUCTURE AND ORIENTATION PATTERNS OF UINTACRINUS IN THE UPPER CRETACEOUS (SANTONIAN) OF THE WESTERN INTERIOR USA

The stemless crinoid Uintacrinus socialis Grinnell occurs in dense aggregations of many individuals preserved in thin, compacted beds. Taphonomic evidence indicates that these aggregations resulted from mass mortality followed by surficial disarticulation and microbial sealing, rather than by gradual accumulation. Therefore we treated aggregations as “snapshots” of Uintacrinus life assemblages from which we could derive information about population structure. We examined 25 Uintacrinus slabs in U.S. and European museums as well as newly collected material from the Niobrara Formation of Kansas and the Mancos Shale of western Colorado. Calyx diameters were measured for populations on 10 slabs having the best preservation. Size frequency diagrams revealed close similarity in mean diameter and size range for all populations except one from the Niobrara in which individual size is significantly smaller. All histograms are unimodal and bell-shaped. Cumulative frequency vs. size diagrams confirmed that distributions are normal and unclustered. These size distributions suggest that aggregations most likely represent single age-cohorts. Although specimens are preserved in a variety of orientations, most lie parallel to bedding. Orientation of horizontal specimens on 12 slabs was measured by dividing each aggregation into quadrants. Significant directional trends in one or more quadrants were found on 10 slabs, and on five slabs there was a tendency for a spokelike alignment of crowns with arms directed towards the center of the aggregation. Eddy currents may have produced these centripetal patterns; other slabs show alignments indicative of unidirectional flow. Bottom currents are consistent with passive filtration feeding by Uintacrinus, and may also have been related to processes of mass mortality that introduced lethal conditions.