Experimental tumbling of echinoderms – taphonomic patterns and implications
1. Przemysław Gorzelak (a)
2, Mariusz A. Salamon (b)
a. Department of Biogeology, Institute of Paleobiology, Polish Academy of Sciences, Twarda Str. 51/55, 00–818 Warsaw, Poland
b. University of Silesia, Faculty of Earth Sciences, Department of Palaeontology and Biostratigraphy, Będzińska Str. 60, 41–200 Sosnowiec, Poland
Despite a wide array of published actualistic studies on echinoderm taphonomy the detailed pattern of decay and disarticulation of their skeletons is still not well understood. Here we provide results of tumbling experiments using a rotating barrel filled with artificial seawater and medium-sized quartz sand to mimic physical forces experienced by echinoderms during trasportation in high-energy conditions. In particular, we determined semi-quantitatively transportation-induced rates and patterns of damage and disintegration of freshly killed ophiuroid, asteroid and crinoid skeletons that were not allowed to decay initially. Our experiments showed that echinoderm specimens disintegrated in a characteristic sequence toward an increase of the degree of disarticulation, abrasion and roundness or thinness of echinoderm ossicles. The sequence of disintegration in crinoids began with the partial disintegration of distal arms after 2 hrs (a time equivalent to ~ 1 km of transport). The initial split of ophiuroid and asteroid arms and crinoid cirri occurred after 24 hrs (~ 12 km) and complete destruction of the asteroid mouth and ophiuroid disc area occurred after 72 hrs (~ 36 km). The duration of transport necessary to promote initial fragmentation in asteroid and ophiuroid arms and crinoid cirri into isolated ossicles was 120 hrs (~ 60 km). The complete disarticulation of crinoid, ophiuroid and asteroid arms and crinoid cirri occurred after 312 hrs (~ 156 km) and 408 hrs (~ 204 km), respectively. Although it has been argued that the quality of preservation can be a poor index of post-mortem transport, echinoderms allowed limited initial decay in the presence of rapid and relatively constant physical disturbance, an approximation of the distance of transport can be made.
Our data demonstrate that articulated ossicles can remain for several days, sufficient time for long (even a few hundred km) transporation. This finding illustrates that articulated echinoderm remains do not necessarily imply low energy and highlights the importance of a reliable discrimination of autochthonous and allochthonous components of fossil echinoderm assemblages. Application of isolated fossil echinoderm ossicles in e.g. paleoenvironmental and paleoecological reconstructions may lead to serious misinterpretations and should be supplemented by observations of abrasion traces.