TAPHONOMY OF IRREGULAR ECHINODERMS: TUMBLING EXPERIMENTS WITH MELLITA AND THEIR IMPLICATIONS FOR ITS PRESERVATION

The irregular echinoid, Mellita has been found scattered throughout Florida's Plio-Pleistocene fossil record. The fossils are preserved under a variety of different conditions and were identified using spines, fossil molds, test fragments, and complete tests. To mimic the physical processes of taphonomy, extant members of the same species as the Plio-Pleistocene Mellita were used in tumbling experiments. Experiments were performed, using single tests, multiple tests, “live” tumbling, and “dead” tumbling of M. quinquiesperforata. A test found dead and test fragments found at North Beach, FL were used as a comparison for the types biostratinomic processes induced by the tumbling experiments. Typical specimens broke in a fairly distinctive sequence with initial expansion of the lunules, followed by the destruction of the Aristotle's lantern. The timing of this event had a substantial control on the timing of further test destruction initially into several large pieces and the continued breakage of those larger pieces. All experiments were performed with water only or with the addition of sediment, in this case gravel. The fragments found in North Beach and those produced by the tumbling experiments appeared to be comparable in the pattern of disarticulation. Therefore, the tumbling experiments appeared to produce similar physical forces and processes as experienced by echinoids in under natural conditions, validated by similarity of the two sets of fragments. The literature suggests that Mellita contained morphological features, which strengthened the test and resulted in its ability to withstand, based on empirical observations, 50 hours of tumbling. This study's results suggest that fossilization of Mellita was most favorable under rapid burial conditions, which would remove tests from the taphonomically most active zone.