Abstract

The fossiliferous beds at the top of the Reedsville and Martinsburg Formations (Upper Ordovician) in the central Appalachians, including the classic Orthorhynchula Zone, provide one of the earliest known examples of a prolific nearshore, clastic-facies fauna that contains species of distinctly modern aspect. A study of the faunal associations and their geographic variation along a 600-mile-long Late Ordovician shoreline and shelf was undertaken.

About 35 abundant fossil species dominate the fauna; these are mostly trepostomatous bryozoans, brachiopods, gastropods, and bivalve mollusks, with some trilobites and crinoids. Three distinct faunal communities are recognized on the basis of species which consistently occur together. The communities are:

(1) The Sowerbyella-Onniella Community lived on muddy silt bottoms of the outer infralittoral from central Pennsylvania to north-central Virginia and has abundant strophomenid (Sowerbyella, Rafinesquina) and dalmanellid (Onniella) brachiopods, pleurotomariid gastropods, Loxoplocus (Lophospira) and Ruedemannia, nuculoid (Ctenodonta?, Praenucula) and actinodontoid (Lyrodesma) bivalves, crinoids, and lesser numbers of trilobites.

(2) The Orthorhynchula-Ambonychia Community lived on sand-silt bottoms of the inner infralittoral and intertidal from south-central Pennsylvania to south-central Virginia and has abundant rhynchonellid (Orthorhynchula) and linguloid (Lingula?) brachiopods, bellerophontid gastropods Plectonotus?, Bucania, modiomorphid (Modiolopsis, Ischyrodonta?), ambonychiid (Ambonychia), and nuculoid (Tancrediopsis) bivalves.

(3) Zygospira-Hebertella Community lived on mud and silty mud bottoms of the inner and outer infralittoral from southwestern Virginia to northern Tennessee and has abundant atrypid (Zygospira) and orthid (Hebertella) brachiopods, pterioid bivalves, Pterinea (Caritodens), murchisoniid gastropods, Murchisonia, and trepostomatous bryozoans, Hallopora, Dekayia, Monticulipora, Amplexopora, Batostomella.

This type of study of ancient marine communities provides an environmental framework in which the evolution of the major taxa and the evolution of community structure may be considered.

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