Hemichordata (Enteropneusta & Pterobranchia, incl. Graptolithina): A review of their fossil preservation as organic material

Abstract

Hemichordata and especially the Pterobranchia (Cephalodiscida and Graptolithina) can be regarded as useful examples to understand the fossil preservation of organically preservable (non-mineralized) fossils through geological history due to the commonality of their fossils in Palaeozoic sediments. The organisms consist of organic materials of three different types, the zooids (cellular tissues), the stolon system (recalcitrant matter) and the tubaria (recalcitrant matter). These materials differ considerably in their taphonomical changes and final preservational potential. While the softbodied zooidal or cellular tissues, similar to the bodies of the phylogenetically related enteropneust worms, are nearly impossible to preserve, the recalcitrant material of pterobranch tubaria is extremely common in the early Palaeozoic fossil record. The durable stolon systems of benthic graptolites are largely found in residues from the dissolution of carbonate rocks. Diagenesis, metamorphism and tectonics modify pterobranch tubaria within the sediment. Tectonic distortion and subsequent weathering may have the largest influence on the destruction or preservation of these fossils. Graptolite reflectance is used commonly in Palaeozoic sediments to determine the thermal maturity of the encasing sediments. These maturity investigations and comparisons show that organic materials of animals and plants behave similarly when subjected to geological modifications and, thus, can used to interpret taphonomical changes of fossil preservation in a consistent way. The review includes also an overview on fossil lagerstätten with important graptolite components in their faunal associations and the preservational interpretations derived from these. In this respect, it is important to determine preservational aspects carefully and to use precise terminology to describe the fossil material, to recognize fossil modifications and replacements, both in originally organically preserved fossils and in mineralic remains like shells or skeletons.