The taphonomy of a carboniferous lagerstätte : the invertebrates of the Bear Gulch Limestone Member

摘要

The Bear Gulch Limestone Member is proposed herein as the new name for the Bear Gulch Beds of central Montana, USA. This member contains articulated and exceptionally well preserved fossils.;Taphonomic investigation, employing scanning electron microscopy, electron dispersive x-ray analysis, element mapping, electron microprobe analysis and Raman spectroscopy reveals that original carbonate biominerals underwent early dissolution, whereas original apatite was converted to carbonate fluorapatite. Non-mineralised tissues were rapidly replaced by apatite and more rarely calcite, or are evident as tissue imprints. In some cases fossil morphology is preserved in organic action. The sediment lacks both clay and iron and so previous models for carbon preservation cannot be invoked. Geochemical analysis shows that calcite and quartz dominate the sediment.;Bottom water conditions were periodically inimical to benthos. Reducing environments within the sediment and relatively rapid burial rates must have assisted in the preservation processes, including macroscavenger inhibition.;Coiled cephalopods are found encrusted with Sphenothallus, 'microconchids', bryozoans and orbiculoid brachiopods. Sphenothallus demonstrates holoperipheral cover and preferred growth orientation on two cephalopods. This suggests that sphenothallids colonised the cephalopods in vivo. Articulated polychaete jaw apparatuses, Brochosogenys reidiae and Symmetrioprion n. sp. are described. The latter is the first record of the genus from the Carboniferous and the first assemblage of Symmetrioprionidae found with the remains of its body.;Halicyne montanaensis n. sp. is recorded from the deposit. Several specimens with different styles of preservation have enabled its identification as a new species of cycloid.