TAPHONOMY OF ISOCRINID STALKS - INFLUENCE OF DECAY AND AUTOTOMY

Stalks of isocrinid crinoids are differentiated into cirri-bearing col umnals (nodals) and columnals lacking cirri (internodals). This skelet al differentiation allowed us to test whether stalk fragmentation is r andom or whether it occurs preferentially at a specific articulation. Our analyses indicate that the patterns of fragmentation in multicolum nal segments of extant isocrinids collected by submersible, by dredgin g, and in sediment samples, as well as those found as fossils, are non random. The preferred plane of fragmentation corresponds to the synost osis, the articulation between a nodal and the internodal distal to it . In isocrinids this articulation has a characteristic morphology and is the site of autotomy. Although stalk shedding by autotomy may contr ibute to the observed patterns, decay experiments on isocrinid stalks, both in situ and in the lab, suggest that post-mortem disarticulation also results in nonrandom fragmentation. Thus both processes, autotom y and post-mortem decay, contribute to the observed pattern of fragmen tation. Underlying both processes is the organization of soft tissues at synostoses.