EXPERIMENTAL TAPHONOMY OF CRINOIDS: A POSITIVE FEEDBACK FROM MODERN AND ANCIENT EXAMPLES

Experimental taphonomy is a strong tool to reconstruct early processes of fossilization of ancient organisms based on experiments on related modern forms. By identifying similar patterns of postures between fossils and modern analogues, we can learn how and why some characteristic patterns are made during burial and after death. If these expected patterns are not seen in the fossil forms, then we can infer that either the fossil forms did not possess similar characteristics to modern forms, or they behaved differently than modern forms. We are conducting experiments on a modern isocrinid crinoid Metacrinus rotundus to understand how fossil isocrinids were buried in ancient deposits. Living specimens of Metacrinus rotundus were captured from the depth of about 140 m in Suruga Bay, central Japan, and were transported to a 200 liter aquarium at Nagoya University. Two specimens were then buried alive in fine volcanic ash to simulate the pattern of fossilization, especially the postures and characteristic fragmentation of isocrinid crinoids. Examples of fossil Lagerstätten of isocrinids that we are observing are: 1) Isocrinus oregonensis occurring in tuffaceous siltstone beds of the upper Eocene - lower Oligocene Keasey Formation, northern Oregon, and 2) Isocrinus hanaii from the Early Cretaceous (Aptian) Hiraiga Formation of northern Japan. A Paleozoic crinoid Lagerstätte from the Mississippian Crowfordsville, Indiana is also compared. Specimens of Isocrinus oregonensis show a variety of stages of fragmentation, both while alive and after death, even from a single layer. So far we have the following results: 1) disarticulation at the cryptosyzygial articulation (ligamentary articulation) suggests autotomy during burial while alive, 2) arm fragments curled aborally is the normal posture after autotomy, with the contractile force of aboral ligaments stronger than that of the adoral muscles, and 3) the slow process of stalk ligament decay, requiring months, induces columnal disarticulation at the cryptosymplectial articulations below nodal. Different patterns of reaction to burial and fragmentation observed in Paleozoic crinoids suggest different organizations of arm and stalk ligaments and lack of muscles.