Interactions between ichnofossils, body fossils and lithology studied by computer tomography (Šárka Formation, Ordovician of the Barrandian area, Czech Republic)

Fossil-rich nodules of the Šárka Formation, colloquially called "Šárka Balls" or "Rokycany Balls", often resulted from the ichnological inhomogeneity of the substrate, probably softground to soapground containing silica gels (and thereby prone to rapid lithification). As such, they represent a possibly rich field for the study of interaction of body-fossils and trace fossils, in other words, between shelly and non-shelly fauna. One of the studied specimens was considered to represent the evidence of scavenging on dead gastropods. The ichnogenus Gastrochaenolites Leymerie, 1842 belongs to trace fossils that relatively often provide a tracemaker preserved directly in its trace (cf. Mikuláš and žítt, 2006). The superficial conclusions made for the specimen No CW68a-b was the following: an in-faunal gastropod communicated (possibly as a specialized predator) with the space above the bottom by a narrow straight vertical shaft. However, any search in the literature did not bring facts supporting the indicated hypothesis. Opposed to the "infaunal gastropod hypothesis", the gastropod shell is somewhat broken and even not fully complete. Thereby, we preferred the hypothesis that portrayed the gastropod as a source of food from unknown, probably hardly preservable organisms. Here, two scenarios have been possible: 1, the dead gastropod sunk into muddy bottom, and its presence remained indicated by escaping gases. Thereby, it is discovered by a "worm" (cylindrical shape of the trace), who achieved the prey so precisely that the secondarily originated nodule (formed during years or dozens of years) is virtually geometrically precise, since the geochemical barrier induced the form of the chamber with dead gastropod (its body has been nearly disintegrated, but the shaft served as a breather pipe). 2, the second possibility differs from the previous one as it does not require the so precise "hit" to the place of a dead gastropod using chemotaxis, which can hardly be entirely exact under the bottom water currents and other heterogeneities. The possibility consists of the idea that the discovered gastropod carrion could serve to its explorer to placing eggs (similarly as parasites of solitary bees; Mikuláš and Genise 2003). The grown-up offspring could use gravitaxe to achieve the bottom floor. Notably, we have not taken into consideration the possibility that appeared, after the CT imaging, true. In reality, the shaft changed its direction closely to the gastropod remain to continue to the non-broken (and "non-polluted") part of the nodule. The nodules containing superficial well-pronounced trace fossils of Stelloglyphus show, from the methodologic point of view, the very opposite situation. Provided we have only the data from the CT, we have had to consider the structure on the nodule surface inorganic, because it is not visible any indication of bioturbation in the centre of the presumed biogenic structure. In conclusion, when the specimen CW68a-b was studied by computer tomography, it was shown that the tracemaker intended not to gain, but to avoid the gastropod carcass. The method brought a simple but virtually unfeasible solution of the configuration of the sample. In another case (nodules with Stelloglyphus), the tomography gave much less information than a mere eye-observation. Both the methods are to be combined to bring reliable data.