FURTHER STUDIES ON LATE EOCENE SHARK COPROLITES FROM THE YAZOO CLAY IN NORTH LOUISIANA

Coprolites, generally fossilized excreta of organisms, have long been found in Cenozoic sediments of the central Gulf Coast. Unfortunately, comprehensive studies of these coprolites, especially those of marine vertebrates, have been very limited. An initial study of shark coprolites from the upper Eocene Yazoo Clay in 2002 described 400 specimens. Continued, systematic collecting in the Yazoo Clay produced over 750 additional shark coprolites. The late Eocene shark coprolites (n = 1,164) are recognized primarily by their distinctive shapes and characteristic features. Shark coprolites were classified as spiral, scroll, or indeterminate based primarily on external morphological features. In spiral and scroll shark coprolites that are complete or reasonably complete, the length is much greater than the width. Prominent external characteristics were utilized to differentiate between spiral and scroll shark coprolites. Spiral coprolites tended to have moderately distinct to well-defined twisting patterns externally. Major external features of the scroll coprolites were the moderately incised to deeply incised, very distinctive folds and grooves. The key differences in the spiral and scroll coprolites are the twisting features versus fold and groove features and the orientation of these structures (width versus length). Specimens that did not have the features of spiral or scroll coprolites were identified as indeterminate. Approximately 98.5 percent of the coprolites were classified as either spiral (546) or scroll (600). The initially collected coprolites were described in detail based on length, width, weight, density, appearance, coloration, and morphology. Analysis indicated that the mean length, width, and weight of the spirals were significantly less than the mean length, width, and weight of the scrolls but with no significant density differences. Randomly selected spiral and scroll shark coprolites (n = 10) analyzed using x-ray diffraction indicated a composition of moderately crystalline fluorapatite [Ca₅(PO₄)₃F] and showed no compositional differences between the spiral and scroll types. Extensive shark tooth data from the site coupled with information on shark anatomy and excretory characteristics provided more specific identification of the coprolites as to possible producers.