Monospecific mass associations of Anaconularia anomala (Cnidaria, Scyphozoa) from the Upper Ordovician of the Czech Republic: Sedimentological and palaeobiological significance
Abstract
Five quartzose sandstone slabs hosting small groups or mass associations of Anaconularia anomala are described from the Upper Ordovician Letna Formation in the Prague Basin. The slabs contain from 4 to 59, mutually adjacent or contiguous conulariids ranging from 5 to 89 mm in length and situated on a single bedding plane.
Associated with the conulariids are small mudstone intraclasts and other fossils, mostly disarticulated brachiopods and trilobites. All 101 studied conulariid specimens are oriented parallel to bedding and show strong preferential alignment, with the apical ends pointing in the same general direction.
Sixteen of the conulariids terminate adapically in a probabe schott and/or exhibit a possible internal schott, while five specimens preserve one or two apertural lappets. The investigated conulariids lived in clumps and were buried catastrophically following alignment by unidirectional currents or flows acting on bodies that may have been leaning in the down-current direction and/or which had their center of mass displaced toward their apical end.
Neither these nor 3000 additional specimens of A. anomala from the Sandbian Letna and Zahorany formations show any evidence of clonal budding; however, the hypothesis that clumping resulted from asexual proliferation cannot be ruled out. Finally, the periderm of A. anomala was compaction-resistant and smooth, lacking both corrugation (transverse ribs) and nodes, but whether the angular groove present at the facial midline of casts is a mould of a midline sulcus or of an internal carina remains unclear.