However, this may in part reflect a bias because these small forms are known primarily from washings of disaggregated shales. To date, similar washings have not been obtained from the upper Lewiston. Other distinctive differences that do not simply reflect bias include the absence of Anastrophia and especially of calceocrinid crinoids in the upper Lewiston. Three genera of such "slipper" crinoids are present and moderately common in the Lewiston B, Calceocrinus chrysalis, Charactocrinus pustulosus, and Eohalysiocrinus typus. None of these crinoids have been found in the D or E units. The reasons for these differences are not known. However, in general, the similarities between the two units are striking and it appears that the same or very similar environment and biota migrated back into the region of present-day Niagara Escarpment outcrop belt from shallower areas to the north, where the organisms had retreated during the deepening recorded in unit C.
In addition to the broad B-C-D-E deepening-shallowing cycle, smaller, meter-scale cycles are also present in the Lewiston B and D successions. This is well illustrated in the data of the Caleb quarry (Figure 5). Each cycle commences with a thin bed or series of stacked beds of skeletal debris or pavement of shells (e.g. units 13, 18-19, 25-26, in part) these beds may also show a slight enrichment in bryozoans and cystoids and other echinoderms. These shelly beds have moderately elevated diversity. They are overlain by thicker intervals of sparsely fossiliferous mudstone, typically with a few thin calcisiltites (e.g., 1-17, 21-23, 26). Similar half-meter scale cycles of bryozoan rich and sparse mudstone occur in the Lewiston B. Together, these couplets of shelly and sparse sediments appear to reflect alternations of sediment starved conditions during which thin shell beds built up. Enhanced shell production and diversity may have been promoted by taphonomic feedback in which the development of shelly pavements permitted colonization of additional forms that required hard substrate for settlement. The cause of these periods of low influx of muddy sediments is not known with certainty; they may record periods of climatic drying during which less sediment was washed off from mountainous source areas. Alternatively, these may be times of minor sea level rise. As noted above, base level rise can cause sediment to be sequestered in nearshore areas. These smaller scale cycles were superimposed upon the general deepening or shallow trends of the larger Lewiston cycle.
Summary and Concluding Remarks
The extraordinary fossil assemblages excavated from the Caleb Quarry near Middleport, NY serve to highlight the richness of middle Silurian invertebrate animal life. These spectacularly preserved fossil assemblages occur in the middle Lewiston Member, famed Rochester Shale formation, the first formally named rock unit in North America and a widespread unit of gray mudstones and thin limestones of middle Silurian age (early Wenlock Epoch; Sheinwoodian Age), roughly 432 million years old. These exposures reveal rare glimpses of communities that inhabited a muddy Silurian seafloor beneath about 20 to 50 meters of normal marine water in the southern Subtropics, perhaps 20 degrees south of the paleoequator of Laurentia (ancestral North America). The organisms, including rare small corals and enigmatic conulariids, both probably tentacle bearing polyps, filter feeding brachiopods and bryozoans, elevated suspension feeding crinoids and cystoids, scavenging or predatory trilobites and predatory swimming cephalopods, lived together in loosely structured communities, interacting to varying extents (Figure 6).
In fact, these benthic organisms formed a general gradient of associations of decreasing diversity southward along a gently deepening seafloor or ramp. Shallower water areas to the north were inhabited by thickets of ramose (twig-like) and fenestrate (lacy ) bryozoans with diverse associated brachiopods, less common attached bivalves, and stalked echinoderms, including especially small crinoids, cystoids and the uniquely abundant coronoid echinoderm Stephanocrinus. The trilobites were relatively diverse and dominated by Bumastus. Gastropods, including the platyceratids scavenged for food, crawling on the seafloor, or in some cases, attaching themselves over the anal vents of crinoid and cystoid hosts for free meals.
In addition to the broad B-C-D-E deepening-shallowing cycle, smaller, meter-scale cycles are also present in the Lewiston B and D successions. This is well illustrated in the data of the Caleb quarry (Figure 5). Each cycle commences with a thin bed or series of stacked beds of skeletal debris or pavement of shells (e.g. units 13, 18-19, 25-26, in part) these beds may also show a slight enrichment in bryozoans and cystoids and other echinoderms. These shelly beds have moderately elevated diversity. They are overlain by thicker intervals of sparsely fossiliferous mudstone, typically with a few thin calcisiltites (e.g., 1-17, 21-23, 26). Similar half-meter scale cycles of bryozoan rich and sparse mudstone occur in the Lewiston B. Together, these couplets of shelly and sparse sediments appear to reflect alternations of sediment starved conditions during which thin shell beds built up. Enhanced shell production and diversity may have been promoted by taphonomic feedback in which the development of shelly pavements permitted colonization of additional forms that required hard substrate for settlement. The cause of these periods of low influx of muddy sediments is not known with certainty; they may record periods of climatic drying during which less sediment was washed off from mountainous source areas. Alternatively, these may be times of minor sea level rise. As noted above, base level rise can cause sediment to be sequestered in nearshore areas. These smaller scale cycles were superimposed upon the general deepening or shallow trends of the larger Lewiston cycle.
Summary and Concluding Remarks
The extraordinary fossil assemblages excavated from the Caleb Quarry near Middleport, NY serve to highlight the richness of middle Silurian invertebrate animal life. These spectacularly preserved fossil assemblages occur in the middle Lewiston Member, famed Rochester Shale formation, the first formally named rock unit in North America and a widespread unit of gray mudstones and thin limestones of middle Silurian age (early Wenlock Epoch; Sheinwoodian Age), roughly 432 million years old. These exposures reveal rare glimpses of communities that inhabited a muddy Silurian seafloor beneath about 20 to 50 meters of normal marine water in the southern Subtropics, perhaps 20 degrees south of the paleoequator of Laurentia (ancestral North America). The organisms, including rare small corals and enigmatic conulariids, both probably tentacle bearing polyps, filter feeding brachiopods and bryozoans, elevated suspension feeding crinoids and cystoids, scavenging or predatory trilobites and predatory swimming cephalopods, lived together in loosely structured communities, interacting to varying extents (Figure 6).
In fact, these benthic organisms formed a general gradient of associations of decreasing diversity southward along a gently deepening seafloor or ramp. Shallower water areas to the north were inhabited by thickets of ramose (twig-like) and fenestrate (lacy ) bryozoans with diverse associated brachiopods, less common attached bivalves, and stalked echinoderms, including especially small crinoids, cystoids and the uniquely abundant coronoid echinoderm Stephanocrinus. The trilobites were relatively diverse and dominated by Bumastus. Gastropods, including the platyceratids scavenged for food, crawling on the seafloor, or in some cases, attaching themselves over the anal vents of crinoid and cystoid hosts for free meals.
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