Early history of Ambocoeliinae (Brachiopoda) in the Silurian (Ludfordian) of the Prague Basin, Czech Republic

The species Metaplasia hemicona Havlíček in Havlíček and Štorch, 1990 is herein reassigned to Kosoplasia gen. nov. This taxon is considered to be the earliest representative of the subfamily Ambocoeliinae in the Prague Basin in Bohemia. It is assigned to the family Ambocoeliidae based on a ventribiconvex shell bearing a weak fold and sulcus, with weak plication on the flanks. Vestigial crural plates in this species are typical of the Ambocoeliinae but the presence of short dental plates and a distinct ventral myophragm are unusual in this subfamily. Kosoplasia hemicona (Havlíček, 1990) was the characteristic element of several benthic communities in the Ludfordian of the Prague Basin. Its maximum occurrence immediately preceded the euxinic event associated with the initial phase of the Lau/Kozlowskii Events at the beginning of the Mid-Ludfordian Glaciation. Considering its small size and almost monospecific accumulations at this level, the species likely represents an R-strategist. It thrived in slightly hypoxic/euxinic waters which were not tolerated by other brachiopods. Although Kosoplasia hemicona vanished during the maximum of the euxinic event, it rapidly returned as the effects of this event diminished. In subsequent benthic associations Kosoplasia hemicona was abundant in association with dayiids, rhynchonellides, strophomenides and other brachiopods, but always was the subordinate element. The species finally disappeared with progressive environmental amelioration during the end phase of the Mid-Ludfordian Glaciation.