THE BROKEN-STICK MODEL AS A NULL HYPOTHESIS FOR CRINOID STALK TAPHONOMY AND AS A GUIDE TO THE DISTRIBUTION OF CONNECTIVE-TISSUE IN FOSSILS

Lengths of crinoid stalk segments consisting of multiple columnals (pluricolumnals) from the Fort Payne Formation of south-central Kentucky (Mississippian) were tested for uniformity using simulations. The results indicate that stalk segment lengths are too uniform to have been produced by a random process operating on homogeneous stalks. The most parsimonious explanation for the uniformity of pluricolumnal lengths is based on analogy with stalks of living isocrinids. The stalks of these crinoids are organized into multicolumnal segments of approximately uniform length: columnals within each segment are connected by "through-going" ligament and "intercolumnal" ligament. The articulation between segments, however, consists of only intercolumnal ligament. Preliminary results from experiments on Recent carcasses indicate that the latter is the primary plane of disarticulation following death. The preservational pattern of the Mississippian stalks suggests that their ligamentary organization was similar to that of the isocrinids. Since the plane of primary disarticulation in isocrinids corresponds to the autotomy plane, Mississippian crinoids also may have been capable of autotomizing their stalks.