A new pustulose bivalve from the Late Ordovician of the Prague Basin (Czech Republic) and remarks on the diversification of pteriomorphids

Abstract

The new genus Alena Polechová, Zicha and Rak, with its type species Alena pustulosa nov. gen., nov. sp., is described from the Sandbian Letná Formation of the Prague Basin (Czech Republic) as a new Late Ordovician bivalve with pustules. Alena pustulosa nov. gen., nov. sp. appears to be an endobyssate bivalve based on its functional morphology. It is placed within Cyrtodontida (Pteriomorphia) and included in the diversified Modiolopsis draboviensis Association from the Letná Formation, which comprises eleven bivalve species. The bivalve association from the Letná Formation is formed predominantly by pteriomorphids (now six species out of eleven) and shows that pteriomorphian groups also radiated strongly in shallow-water sands and silts in mid-latitude areas. All main groups of Pteriomorphia are already known from the Early Ordovician and diversified rapidly during the Ordovician. The Gondwana and peri-Gondwana margins with two Early Ordovician radiation centers (Central Australia and Northwestern Argentina) played important roles in their early diversification and dispersion to other palaeocontinents. The function of the sculpture in bivalves is discussed with an emphasis on the Ordovician bivalves. The ornamentation in bivalves serves several purposes such as supporting the stable position of the shell in a substrate, helping in burrowing, strengthening of shell, and protecting against predators. In the Ordovician bivalves, the main function of the sculpture is to stabilise the shell in the substrate.