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

merous publications, but still many details are unclear or have not been considered. It is usually discussed in relation to individual fossil groups (e.g. bacteria, brachiopods, graptolites, plants, trilobites, vertebrates) or types of fossil lagerstätten (cf. Seilacher 1970). In the Hemichordata, the preservation of the fossils is especially important, as the involved materials differ considerably between the two groups, the Enteropneusta (Fig. 1A) and the Pterobranchia (Fig. 1B–D). The results found in the fossil preservation are often interpreted from the visual aspects of the fossils only, and without consideration of the composition and reaction of the various materials involved in the decay processes and the subsequent fossilization processes over millions of years. Also a differentiation of originally preserved organic material and replacement of this due to later processes is often not considered. Briggs (2003) suggested replication of the morphology by rapid in situ growth of minerals as essential to preserve labile tissues, which means the loss of the original organic material through geological processes and replacement by secondarily formed minerals. Decay­retardation during early biostratinomic proces­ ses may increase the potential for the preservation of fossils, not just in organically preserved material, but also in mineralic skeletons or shells. Important changes, however, modify the fossils considerably during later geological processes. To gain insight into the preservation of fossils it is necessary to consider the changes through these geological processes and to understand the various paths necessary to produce the final results, to see what happens to materials during numerous geological processes in Earth history. The fossilization of the Hemichordata may serve as a very important and useful example here.