During the time interval represented by the 8 meters (26 feet) of strata at the Caleb Quarry, certainly tens to a few hundred thousands of years, there were minor climatic and sea level oscillations that produced cycles of sedimentation. Overall, the Lewiston B, C, and D-E submembers record a prolonged cycle, perhaps 400,000 years, involving fairly abrupt deepening (B to C) and gradual shallowing of the epicontinental sea with a water depth change of perhaps 20 to 30 meters. The cause of such sea level oscillation is uncertain but may reflect glacially induced rise (during melting of ice) and fall. The rise and fall of sea level was paralleled by migration of benthic seafloor organisms, which apparently tracked their favored environments.
Thus, the fossil beds near Middleport give significant and, at times, very clear insights into life and depositional environments on a muddy sea floor during a key time of transition, when the very first life forms – tiny psilophyte plants and myriapod (millipedes, centipedes) animals, were just emerging on land and just prior to a time when jawed fishes became an important part of the marine ecosystem. Here, hosts of typical middle Paleozoic marine animals lived on a subtropical muddy seafloor and died, sometimes dramatically during the backwashes of major Tropical storms, thriving locally and retreating in response to changing climates and major and minor changes in relative water depth.
Perhaps the broadest story these fossils tell is one of relative stability in the face of substantial environmental change. As with many Paleozoic assemblages, there is little evidence for evolution of the many dozens of species of organisms during the hundreds of thousands of years recorded in the Rochester Shale. A few species may have gone extinct during the course of this time and a very few may have immigrated into the local area, but the majority of the species remained in relative stasis: a stable Silurian "dynasty.” Rather than changing, these organisms primarily responded to environmental change by tracking their preferred environments. More rapid restructuring of communities and evolution may eventually have accompanied the changes that were associated with deposition of the Lockport Group, but, in part, this may record an artifact of poor preservation in these dolomitized rocks. As more data emerges, it is apparent that a majority of Rochester Shale species probably persisted longer until the greatly increased salinity and/or shallowing associated with a transition to the Salina Group ultimately exterminated a majority of the long ranging species and ushered in the new "dynasties" of the later Silurian and the Devonian Period.
Thus, the fossil beds near Middleport give significant and, at times, very clear insights into life and depositional environments on a muddy sea floor during a key time of transition, when the very first life forms – tiny psilophyte plants and myriapod (millipedes, centipedes) animals, were just emerging on land and just prior to a time when jawed fishes became an important part of the marine ecosystem. Here, hosts of typical middle Paleozoic marine animals lived on a subtropical muddy seafloor and died, sometimes dramatically during the backwashes of major Tropical storms, thriving locally and retreating in response to changing climates and major and minor changes in relative water depth.
Perhaps the broadest story these fossils tell is one of relative stability in the face of substantial environmental change. As with many Paleozoic assemblages, there is little evidence for evolution of the many dozens of species of organisms during the hundreds of thousands of years recorded in the Rochester Shale. A few species may have gone extinct during the course of this time and a very few may have immigrated into the local area, but the majority of the species remained in relative stasis: a stable Silurian "dynasty.” Rather than changing, these organisms primarily responded to environmental change by tracking their preferred environments. More rapid restructuring of communities and evolution may eventually have accompanied the changes that were associated with deposition of the Lockport Group, but, in part, this may record an artifact of poor preservation in these dolomitized rocks. As more data emerges, it is apparent that a majority of Rochester Shale species probably persisted longer until the greatly increased salinity and/or shallowing associated with a transition to the Salina Group ultimately exterminated a majority of the long ranging species and ushered in the new "dynasties" of the later Silurian and the Devonian Period.
page 24