The presence of small, broken and abraded fragments of shells and bryozoans in many of the limestones of Lewiston E probably reflect multiple periods of reworking of old skeletal debris by storm waves and currents.
Organisms with multi-element skeletons, such as crinoids, trilobites and starfish are especially sensitive indicators of burial rate. In general, these types of skeletons are partially disarticulated within a few days after death unless buried. Some of these organisms, such as ophiuroids ("brittle stars") and asteroids (starfish or sea stars)
(Figures 218 - 236) are extremely delicate and can only be preserved if buried very rapidly, probably within just hours after their death or while alive. The latter is particularly interesting for these mobile organisms. Experiments have shown that ophiuroids can burrow out of layers of up to 10 cm of sediment accumulated in just a few hours. This is one reason that these organisms are so rare in the fossil record. Specimens in the Homocrinus beds were commonly found above the main buried seafloor level with crinoids and other fossils. They appear to have expired while attempting to disinter themselves from burial layers. In other cases, the organisms may have been killed prior to sediment accumulation.
Even many of the best-preserved crinoids in the Rochester Shale show evidence of very minor decay prior to burial (Figure 260a and 285). More resistant modules, such as segments or pieces of columns and calyxes/thecae of crinoids and cystoids, may resist decay for longer periods. Evidence from encrustation of calyxes of the common Silurian crinoid Eucalyptocrinites by bryozoans, other crinoid holdfasts, and worm tubes (Liddell and Brett, 1982) indicate that these cups could remain unburied on the seafloor for periods of many months, but ultimately they would collapses along the sutures of plates. Many partially articulated crinoids and cystoids are known from the Rochester Shale. These cups accumulated along with mainly articulated brachiopods and bryozoans. In some cases, the lower portion of a crinoid that was partially buried in sediment was well preserved, whereas the upper part was exposed and underwent decay and disarticulation (Brett and Baird, 1986).
Trilobites likewise, would probably disarticulate into cephala, pygidia and single segments if not buried quickly. In many other deposits where trilobites are well preserved, for example, the famed Devonian Harragan beds of Oklahoma or those of Morocco, they are found in semi-enrolled to enrolled configurations and/or occur in random orientations, including vertical and oblique positions within the sediments. These specimens appear to have been caught up in mudflows or density currents that flowed along the seabed (Brett et al. 2012). Conversely, in other types of obrution deposits, including most in the Rochester Shale, the trilobites are parallel to bedding and subhorizontal, very few are enrolled even among trilobite genera that are commonly enrolled, such as Calymene. Some show incipient decay or evidence of gentle orientation prior to burial. This suggests that the source of mortality was separated in time from the burial. Brett et al. (2012) suggest that these (type A) trilobite beds reflect mass mortality produced by other effects (such as temperature or salinity changes) associated with storm events prior to their burial. In these cases the muddy sediment may have been suspended for a period of time, as plumes of mud carried seaward by currents along boundary zones of different water density. However, following the dispersal of the muddy plume the fallout of muddy sediment evidently was rapid. Clays may have become aggregated as brackish water from flood runoff mixed with saline water producing chemical reactions that caused the clays to become flocculated into larger grains, which settle much more rapidly than clays. Scanning electron microscope (SEM) study of Rochester mudstone layers that rest upon buried well-preserved fossils show "flocs" or aggregate grains of clay flakes (O'Brien et al., 1994).
Several beds excavated at the Caleb Quarry display nearly perfect articulation of numerous organisms. At least 15 such horizons are recorded in the Silurian Experience book. This extraordinary preservation, recording rapidly smothered seafloor conditions or obrution Lagerstätten (Brett and Seilacher, 1991), provides "snapshots" of the skeletonized communities on the sea bottom and is not typical of most sedimentary deposits nor even most of the Rochester Shale. In two decades of study of the lower (A-B) and upper (D-E) units of the Lewiston Member, the author collected nearly 300 cystoids and almost 1000 specimens of Stephanocrinus, the two most common echinoderms.
Organisms with multi-element skeletons, such as crinoids, trilobites and starfish are especially sensitive indicators of burial rate. In general, these types of skeletons are partially disarticulated within a few days after death unless buried. Some of these organisms, such as ophiuroids ("brittle stars") and asteroids (starfish or sea stars)
(Figures 218 - 236) are extremely delicate and can only be preserved if buried very rapidly, probably within just hours after their death or while alive. The latter is particularly interesting for these mobile organisms. Experiments have shown that ophiuroids can burrow out of layers of up to 10 cm of sediment accumulated in just a few hours. This is one reason that these organisms are so rare in the fossil record. Specimens in the Homocrinus beds were commonly found above the main buried seafloor level with crinoids and other fossils. They appear to have expired while attempting to disinter themselves from burial layers. In other cases, the organisms may have been killed prior to sediment accumulation.
Even many of the best-preserved crinoids in the Rochester Shale show evidence of very minor decay prior to burial (Figure 260a and 285). More resistant modules, such as segments or pieces of columns and calyxes/thecae of crinoids and cystoids, may resist decay for longer periods. Evidence from encrustation of calyxes of the common Silurian crinoid Eucalyptocrinites by bryozoans, other crinoid holdfasts, and worm tubes (Liddell and Brett, 1982) indicate that these cups could remain unburied on the seafloor for periods of many months, but ultimately they would collapses along the sutures of plates. Many partially articulated crinoids and cystoids are known from the Rochester Shale. These cups accumulated along with mainly articulated brachiopods and bryozoans. In some cases, the lower portion of a crinoid that was partially buried in sediment was well preserved, whereas the upper part was exposed and underwent decay and disarticulation (Brett and Baird, 1986).
Trilobites likewise, would probably disarticulate into cephala, pygidia and single segments if not buried quickly. In many other deposits where trilobites are well preserved, for example, the famed Devonian Harragan beds of Oklahoma or those of Morocco, they are found in semi-enrolled to enrolled configurations and/or occur in random orientations, including vertical and oblique positions within the sediments. These specimens appear to have been caught up in mudflows or density currents that flowed along the seabed (Brett et al. 2012). Conversely, in other types of obrution deposits, including most in the Rochester Shale, the trilobites are parallel to bedding and subhorizontal, very few are enrolled even among trilobite genera that are commonly enrolled, such as Calymene. Some show incipient decay or evidence of gentle orientation prior to burial. This suggests that the source of mortality was separated in time from the burial. Brett et al. (2012) suggest that these (type A) trilobite beds reflect mass mortality produced by other effects (such as temperature or salinity changes) associated with storm events prior to their burial. In these cases the muddy sediment may have been suspended for a period of time, as plumes of mud carried seaward by currents along boundary zones of different water density. However, following the dispersal of the muddy plume the fallout of muddy sediment evidently was rapid. Clays may have become aggregated as brackish water from flood runoff mixed with saline water producing chemical reactions that caused the clays to become flocculated into larger grains, which settle much more rapidly than clays. Scanning electron microscope (SEM) study of Rochester mudstone layers that rest upon buried well-preserved fossils show "flocs" or aggregate grains of clay flakes (O'Brien et al., 1994).
Several beds excavated at the Caleb Quarry display nearly perfect articulation of numerous organisms. At least 15 such horizons are recorded in the Silurian Experience book. This extraordinary preservation, recording rapidly smothered seafloor conditions or obrution Lagerstätten (Brett and Seilacher, 1991), provides "snapshots" of the skeletonized communities on the sea bottom and is not typical of most sedimentary deposits nor even most of the Rochester Shale. In two decades of study of the lower (A-B) and upper (D-E) units of the Lewiston Member, the author collected nearly 300 cystoids and almost 1000 specimens of Stephanocrinus, the two most common echinoderms.
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