Pentamerus

Abstract: Organic microfossils (spore tetrads, acrit-archs, scolecodonts, chitinozoans) from six Late Ordovician through Early Silurian samples (Elkhorn Formation of the Richmond Group; lower and upper beds of Belfast Member of the Brassfield Formation) straddling the Ordovician-Silurian paraconformity provide evidence about water depth. The spore tetrads appear to have been derived from plants growing in environments no deeper than an intertidal, semiterrestrial or very shallow-marine if not a nonmarine position. Their concentration on both sides of the paraconformity indicates at least shallower depth conditions than are present in the overlying upper part of the Brassfield Formation with its Eocoelia and succeeding deeper water Pentamerus benthic marine life zone faunas. A progressive deepening of water in the Early Silurian away from the paraconformity is also indicated by the inverse relationship existing between abundant spores in the basal bed of the Belfast Member and abundant Acanthomorphitae acritarchs in the upper bed. The presence of Sphaeromorphitae acritarchs in all sixsamples indicates that those microplankton are restricted to very shallow water depths. Spores from the Elkhorn Formation may have been derived from plants living in Ordovician time or they have “leaked” into the Elkhorn from the Silurian. In the former event, these spores may represent the oldest vascular plant remains recognized in North America.

Abstract: Fluctuating water depths in platform seas covering central North America during late Llandovery time introduced shifts in the spectrum of coral-algal, pentameran, and stricklandian communities. Opportunistic orthotetacean brachiopods were common during these times of community replacement. A pavement constructed by their flat shells was sometimes used as the initial substrate for attachment by the spat of pentameran brachiopods. Both orthotetacean and pentameran shells served as the initial substrate for stricklandian brachiopods. With the decline of a pentameran or stricklandian population, disarticulated shells provided a substrate for repopulation. An assemblage of immature Pentamerus in life position indicates that the typical umbo-down posture was an early result of maximum packing. Development of more advantageously placed individuals severely restricted the growth of others trapped between the substrate and the beak regions of surrounding neighbors. After pentameran assemblages were buried in life position, scouring of the sea bottom frequently produced erosion surfaces which truncated their thin shells. These surfaces were sometimes penetrated by boring organisms, and were firm enough for brachiopod repopulation.

Abstract: The brachiopod Pentamerus oblongus is especially abundant in the Lower Silurian RytterAker Formation, which occurs widely throughout the Oslo region in southern Norway. Alternating thin limestones and shales typically occur in a shallowing-upward sequence ending in a massive grainstone often rich in tabulate corals and stromatoporoids. In the lower beds, colonization by pentamerid populations was enhanched by prior emplacement of an orthotetacean shell pavement over the clastic sea bed. Individual Pentamerus size decreases with increasing disturbance of populations or their fragmentation as lag deposits nearer the stratigraphic level of the massive grainstone. The frequency of specific storm events and the changing depth to the sea bed may be indexed according to the average size of pentamerid shells in a given population and the degree to which the population is disturbed. Preservation of prominent growth lines (assumed to be annual) suggests storms stirred the sea bed in deeper waters on an 8-10 year cycle. In shallower waters, storms more regularly disturbed the sea bed every 2-3 years. These data corroborate the interpretation that the Pentamerus community inhabited marine waters below fair-weather wave base, but still within reach of storm wave base.

Abstract: Over 1,000 m of Upper Ordovician to Lower Silurian mixed carbonate and clastic strata on Anticosti Island are nearly tectonically undisturbed, despite their proximity to the Northern Appalachians fronting Quebec's Gulf of St. Lawrence. Natural cliffs exposed along the coast and rivers in the eastern part of the island make a relatively conformable sequence belonging to the Ashgill and Llandovery Series. Fossil communities interpreted as depth-associated in life are especially repetitious in the Becscie, Gun River, Jupiter, and Chicotte Formations (Llandovery Series), and to a lesser degree in the Upper Vaureal and Ellis Bay Formations (Ashgill Series). Preliminary study of the pattern of changeovers in Eocoelia, Pentamerus and Stricklandia communities suggests that Anticosti seas deepened and shallowed three and a half times during the Early Silurian. High water peaks were reached during B1-B2, C1-C2 and C4-C5 times, with a final deepening trend beginning in late C5 time. Age determinations of these events are based on the occurrence of graptolites (with some new records from Anticosti) keyed to the standard graptolite zones, and species of the Eocoelia lineage are also useful for correlation. The profile of the Anticosti sea-level curve compares well with other curves reconstructed from the Lower Silurian of New York, Michigan, and Iowa. Widespread synchronism in sea-level changes on the North American platform is thus corroborated.