Abstract
Trepostomatous bryozoans are abundant in the upper Reedsville Formation (Ordovician) in the central Appalachians from Pennsylvania to Tennessee. A study of their faunal associations and zoogeographic distribution has emphasized the close relation between sedimentary environments and bryozoan morphology, abundance, and diversity. Reconstruction of the Late Ordovician environmental setting outlines a broad, gently sloping shelf with a prominent clastic wedge or deltaic complex in south-central Pennsylvania. Coarse sand and silt were common on the shelf in the north, and graded southward into silty mud and mud. The external morphology of the central Appalachian trepostomes shows little change along the north-south shoreline. The zoarial types usually are branching, either subcylindrical or flattened plates; such forms are most common today in quiet sublittoral waters. The Ordovician trepostomes are abundant only north and south of the clastic wedge. Recent bryozoans usually are found in nonturbid environments on a slightly mobile surface of attachment; therefore, they are abundant only away from delta fronts and sediment-laden currents. Similarly, there is no reason to expect that the Upper Ordovician bryozoans reflect any lesser sensitivity to substratum mobility or to have been more tolerant of turbidity. Where taxonomic diversity is mapped in Recent environments, those bryozoan assemblages with the highest diversity are found only where the rates of deposition are moderate to low. Generic diversity along the central Appalachian Upper Ordovician shoreline shows a sharp gradient from low diversity in the north to high diversity in the south. Two genera are common in the north, each isolated from the other but both equally abundant on the flanks of the clastic wedge. In southwestern Virginia and northern Tennessee, well off the apron of the clastic wedge, as many as 5 to 7 genera may be abundant at any one locality. The distributional pattern of the Upper Ordovician trepostomes in the central Appalachians, especially the pronounced diversity gradient, appears to be directly related to areas of sediment influx and to rates of sedimentation. End_of_Article - Last_Page 521------------
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