Cephalopods from reef limestone of the Vasalemma Formation, northern Estonia (latest Sandbian, Upper Ordovician) and the establishment of a local warm-water fauna (original) (raw)

The cephalopods of the reef limestones of the Vasalemma Formation, northern Estonia, are highly diverse and comprise a number of 22 species, of 10 families and seven orders in a sample of +300 specimens. Most of the specimens were collected from shell concentrations in synsedimentary cavities and are interpreted parautochthonous as washed in from nearby palaeo-habitats. Nearly all of the shells are fragmented and nearly 15% are partially encrusted by epibionts. The assemblage is dominated by small (mostly less than 30 mm wide) straight shelled actinocerids and orthocerids, additionally coiled tarphycerid are common. The higher taxonomic composition of the Vasalemma cephalopod assemblage with a dominance of actinocerids and an absence of endocerids is in agreement with an interpretation of the depositional environment under a warm water (tropical or subtropical) shallow subtidal regime. At species level the assemblage is highly endemic, but the generic composition allows for a statistical comparison with other faunas. A cluster analysis of contemporary assemblages reveals a high degree of similarity with late Sandbian cephalopod faunas of epicontinental Laurentia. The palaeogeographical distribution pattern is similar than in brachiopods, which supports earlier interpretations of these clusters as mainly controlled by water temperature and depositional depth. Several of the Vasalemma genera became conspicuous elements of epicontinental Laurentia during the Katian, which emphasizes that immigration toward Laurentia was an important factor in Late Ordovician diversity dynamics. Of the described taxa, the following are new: