POST-MORTEM TRANSPORT OF BLASTOID THECAE

Blastoids are relatively common components of many Paleozoic marine communities. Although the morphology of these organisms has been studied extensively, little is known about blastoid taphonomy. This study demonstrates the nature of post-mortem transport of the blastoid theca on fine grained substrates. Because arm and stem material is known to disarticulate very quickly after death in crinoids, and because these structures are rarely preserved intact in blastoids, only blastoid thecae were used. Several different species of blastoid, each represented by a range of sizes, were examined in order to address the effect of thecal size and shape on the relative timing and mode of transport. Epoxy casts of fossil specimens were used, so that the density of the models approximated the density of thecae composed of unaltered stereom. Transport of the models was studied under both unidirectional and oscillating flow conditions. The results of this study are remarkably uniform for all thecal shapes and sizes. In unidirectional flow conditions, the theca reorients with the oral-aboral axis perpendicular to the direction of the flow, regardless of the initial thecal orientation, and rolls downstream in this orientation at very low current velocities. In oscillating flow conditions, the theca reorients to the same position and rolls back and forth parallel to the flow. When ripples are present, the theca rolls back and forth in the ripple troughs. Models that are constrained not to roll reorient with the oral-aboral axis perpendicular to the direction of the flow, showing that reorientation in not a function of the mode of transport. Density is the only factor which affects the velocity at which thecae reorient and roll downstream, although this effect is relatively small. The size of the theca had no effect on the relative timing or mode of transport over the observed velocities, flow conditions, or substrate types. All models were entrained in the flow at a much lower velocities than velocities at which sediment transport occurred. The results of this study show that blastoids will rarely be preserved in situ, regardless of thecal size or shape.