Prvni vyskyt kraniidnich ramenonozcu v siluru prazske panve (kopaninske souvrstvi, ludford, ludlow)

Craniid brachiopods are generally rare in Silurian and Devonian rocks. Correspondingly, only few taxonomic works focused on this group, with a rare exception of Holmer et al. (2018) review from the Silurian of Gotland. Recently, several shells of a so far unknown craniid brachiopod have been observed in the Požáry stratotype section, Czech Republic. The shells represent the first observed craniids of Silurian age in the Prague Basin.

All specimens come from the Požáry stratotype locality in Prague-Řeporyje (Kříž 1999). In the section above the tunnel portal, layers of weathered sparitic limestone (the so-called “white beds”) of the Kopanina Formation (Ludfordian) crop out near the base of the rock wall. This part of the exposure corresponds to the upper part of the Ananaspis fecunda horizon (Kříž 1999). In the lowest weathered layer about 20 cm thick (Fig. 1) a rich brachiopod and crinoid fauna was found. The fossil assemblage is dominated by small to medium-sized brachiopods Lissatrypa fumida, Gracianella plicumbra, Cyrtia postera, Jarovathyris canaliculata, Cadudium sphaerulites, Metaplasia sp., Dayia sp. and Salopina sp. In addition to brachiopods, trilobites Otarion diffractum, Ananaspis fecunda, calyces and pluricolumnals of microcrinoids, isolated plates of machaeridians, gastropod shells, and other small fossils are abundant. Craniid brachiopods are generally rare, representing less than 1% of the total brachiopod fauna.

The described specimens are characterized by a small dorsal valve, having a broadly trapezoid to pyriform outline, well-marked anterocentral apex, and prominent concentric growth bands. By contrast to muscle scars that are not preserved, the porosity of the shell wall is well marked. Although valves are rather poorly preserved, in their outline and the macroornament are similar to shells of Urbicrania limbata (nomen nudum). Havlíček and Vaněk (1998) figured this and other similar species from the Dvorce-Prokop and Řeporyje limestones (Lower Devoinian, Pragian) in the Prague Basin, but without the formal description.

References

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