Ordovician eunicid polychaetes of Estonia and surrounding areas: review oftheir distribution and diversification

Scolecodonts, the jaws of polychaete worms, are common and diverse palynomo rphs in the Ordovician rocks of Estonia and surrounding areas. Some 120 app aratus-based species representing about 40 genera have been recorded thus f ar. Relatively long stratigraphical ranges of the majority of species refle ct a low rate of evolution of jawed polychaetes. However, some individual s pecies, as well as structural changes in the assemblages, appear to be usef ul for stratigraphical purposes. Environmental events like those in the mid dle Caradoc and late Ashgill had some impact on polychaete faunas, but less than on several other groups. In order to study the spatial distribution o f eunicids, faunas of particular intervals of the Ordovician were investiga ted. Quantitative analysis revealed that polychaete assemblages with a very consistent qualitative and quantitative composition were widespread over l ong distances within the belts of similar facies conditions in the Baltosca ndian Palaeobasin. On the other hand, it appears that species of Ordovician jawed polychaetes were strongly influenced by particular facies, and accor dingly well differentiated along the palaeobasin gradient. The decrease in diversity and abundance towards the deeper-water part of the palaeobasin in dicates that the majority of Ordovician eunicids preferred relatively shall ow-water conditions. The increase in differentiation of environments is acc ompanied by an increase in differentiation of polychaete assemblages. (C) 2 000 Elsevier Science B.V. All rights reserved.