Clingfilm preservation of spiraliform graptolites: evidence of organically sealed Silurian seafloors

The Early Silurian graptolites Spirograptus turriculatus and Sp. guerichi are typically preserved in condensed graptolite shale lithologies as entirely flattened outlines; all whorls are visible and little or no sediment infills the originally cone-shaped rhabdosomes, as though the graptolites had been sealed in clingfilm, or plastic wrap, prior to burial and compaction. By contrast, the rhabdosomes of graptolites transported in turbidity currents typically are filled with sediment. The most likely reason for the clingfilm mode of preservation is encasement or covering of the graptolite rhabdosomes by marine snow and/or microbial mats prior to burial by clastic sediment and compaction on an anoxic seafloor. Experimental evidence reported herein supports this suggestion. The organic material that mediated the preservation of such graptolites was likely akin to, but probably physically stronger than, the delicate benthic flocculation layer of the current Black Sea floor. Like the latter, it probably formed significant microtopography at and just below the sediment-water interface and mediated geologic processes such as early diagenesis in graptolite shale lithologies. Keywords: graptolites, black shales, marine snow, microbial mats.