Abstract Bivalves first appeared in the Early Cambrian and were virtually cosmopolitan. These very small and insignificant molluscs were probably surface crawlers on the microbial mat floors of the Cambrian sea. They evolved little further in the Mid Cambrian and by the Late Cambrian had apparently disappeared from the fossil record. Their re-appearance in the Early Ordovician coincided with a major diversification in which all the principal bivalve clades evolved, but the class was confined to Gondwana; their habitat was now principally infaunal in siliciclastic sediments. In the Mid Ordovician a few forms reached Baltica and the eastern Laurentian margin, but it was the Late Ordovician before bivalves once again became cosmopolitan. This geographical dispersal allowed bivalves to colonize the low-latitude carbonate platforms and led to the development of diverse epifaunal faunas, although most remained as infaunal forms in siliciclastic sediments. The end-Ordovician regression occasioned by the Hirnantian glaciation caused major extinction of those epifaunal forms restricted to the carbonate platforms. The Silurian faunas were cosmopolitan and the major evolutionary event was caused by the appearance of a Gondwanan cephalopod limestone facies that provided sites for epibyssate praecardiidinid bivalves (=Nepiomorphia) that evolved rapidly and were able to withstand short periods of anoxia.

Abstract In an overview of the Silurian of Central Europe, it is important to realize that during this period the study area was spread more widely over the globe than nowadays because at least two oceans were present in the area which have since disappeared. Several palaeocontinents such as Baltica or Gondwana, smaller palaeo-plates such as Avalonia and Far Eastern Avalonia, and Peri-Gondwana terranes such as Perunica, were separated by the Tornquist Sea and the Rheic Ocean. These palaeocontinents were brought together in the present-day configuration by closing of the oceans and the subsequent orogenic collisions, respectively termed the Caledonian and Variscan orogenies. Plate movements before and during the Alpine orogeny also brought pieces of northern Gondwana into the study area. These Proto-Alps are now included in the basement of the Alps and are observable in several tectonic windows (e.g. Carnic Alps).