TAPHONOMY OF ECHINODERM OSSICLES IN COAL SWAMPS

Echinoderm fossils rarely occur in coal, although crinoids and echinoids have been reported from Cretaceous amber. Echinoderms do occur in coal balls, which are carbonate concretions from Pennsylvanian and early Permian coal seams that preserve plant debris at the cellular level. These fossils consist of disarticulated columnals and columnal fragments that were probably transported into the swamp by storms. Their presence in coal balls supports coal-ball formation in brackish or marine swamps close to the coast, rather than in freshwater.

Coal balls, which can contain ephemeral plant organs such as pollen tubes, probably formed rapidly. Thus, these concretions may preserve aspects of echinoderm taphonomy seldom seen in Pennsylvanian nearshore environments. Likewise, the interstereom pores of coal-ball echinoderms provide clues to coal-ball formation. The interstereom of coal-ball echinoderms may be filled with low-Mg calcite (LMC), pyrite, or organic carbon. Ossicles with interstereom filled by organic carbon generally host thrombolytic LMC, assumed to be microbial. Although most LMC cement in coal balls is crystalline, occasionally pores at the edges of coal-ball ossicles are filled with a mix of organic carbon and thrombolitc LMC that grades to micrite and finally to crystalline LMC cement. Crystalline LMC cement in coal balls may have originated as microbial micrite. Pyritized ossicles and ossicles with interstereom pores filled with pyrite likely result from diagenetic alteration of echinoderms in the swamp. However, ossicles with crystalline LMC filling the interstereom pores may have been cemented in shallow marine environments before being transported into the swamp. Many coal-ball ossicles have microborings or dissolution cavities, which may contribute to the large number of fragmented ossicles. Dickson and Saller interpreted holes in crinoid columnals as dissolution features. Features that we interpret as microborings range from 80 – 170 μm in diameter and generally open onto the perimeter of the ossicle or fragment. Other cavities in coal-ball ossicles may result from dissolution.