. Bulletin. Natural history. ORDOVICIAN APPALACHIAN ECOLOGY 37 to sedimentation rate. Maps of species numbers and diversity indicate that most Re- cent bryozoans are able to tolerate only very moderate to low rates of deposition, and therefore they are abundant only in quiet-water environments away from the delta fronts and inaccessible to sediment-laden currents. The common surface of attachment is a very slightly mobile sand-silt (Lagaaij and Gautier, 1965, p. 52). There is no reason to expect the Upper Ordovician forms to have been more tolerant of high sedimentation rates than modem forms.
. Bulletin. Natural history. ORDOVICIAN APPALACHIAN ECOLOGY 37 to sedimentation rate. Maps of species numbers and diversity indicate that most Re- cent bryozoans are able to tolerate only very moderate to low rates of deposition, and therefore they are abundant only in quiet-water environments away from the delta fronts and inaccessible to sediment-laden currents. The common surface of attachment is a very slightly mobile sand-silt (Lagaaij and Gautier, 1965, p. 52). There is no reason to expect the Upper Ordovician forms to have been more tolerant of high sedimentation rates than modem forms. Recent bryozoans adopt an encrusting form when turbulence increases and the substratum becomes more mobile. Similarly, the Ordovician encrusting bryozoans are found in a coarser, better sorted sandstone. In some stratigraphic sections, increase in the silt/mud ratio is accompanied by a change from the ramose to the encrusting habit within the same genus (Fig. 30). Generic diversity as related to sediment influx along the central Appalachian Late Ordovician shoreline is shown in Figure 19. Bryozoans are generally lacking in south- central Pennsylvania, Maryland and northern Virginia, the most probable source of BRYOZOAN DIVERSITY. FIG. 19. A generalized outline of trepostomatous bryozoan diversity (genera) in the central Appalachian Upper Ordovician. Of particular note is that the low generic diversity in Pennsyl- vania, northern Virginia and eastern West Virginia may be directly related to the main source of terrigenous clastic influx during the Late Ordovician. Numerals refer to number of genera identified at each locality where abundance of the indi- vidual genus was greater than 2 on the relative abundance Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Peabody Museum of Natural History. New Have
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