Browsing by Author "Bennington, J. Bret"

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    Community persistence and the pattern of community variability over time: a test using fossil assemblages from four marine transgressions in the Breathitt Formation (Middle Pennsylvanian) of Eastern Kentucky
    Bennington, J. Bret (Virginia Tech, 1995-11-06)
    Four intervals of Middle Pennsylvanian marine strata in the Breathitt Formation of eastern Kentucky, the Elkins Fork, Kendrick, Magoffin, and Stoney Fork, were measured, sampled for fossils, and described. Each marine interval was shown to be the product of a single episode of transgression and regression that led to the establishment of a variety of marine habitats in the central Appalachian basin. These habitats included marginal marine, nearshore normal marine, nearshore stressed, offshore normal marine, and offshore dysaerobic environments. Fossil collections were made and species abundance distributions were obtained for each paleontological sample. Multivariate statistical analyses such as cluster analysis and principle components analysis were used to classify fossil assemblages sampled into a variety of paleocommunity types, each associated with a particular lithological facies. Paleocommunity types that were found to occur in more than one of the four marine intervals were resampled at selected localities, with replicate samples taken at each locality. These additional samples were used to test for the presence of the same statistically defmed paleocommunity at different localities within the same marine units and to test for the recurrence of statistically defmed paleocommunities between marine units. Results of the analyses suggest that recurrence of similar fossil assemblages (paleocommunity types) is common in the Breathitt marine units whenever there is recurrence of comparable marine facies. However, recurrence of the same statistically defined paleocommunity is rare, with significant variation in the abundances of member species appearing each time a particular paleocommunity type recurs. This suggests that the recurrence of distinct fossil assemblages over intervals of geologic time can be explained by the repeated reinvasion of suitable habitats by the individual members of a persistent species pool, without recourse to ecological mechanisms needed for preserving or perpetuating particular community structures.
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    Paleocommunities of the Yorktown Formation (Pliocene) of Virginia
    Daley, Gwen Marie (Virginia Tech, 1999-08-06)
    The fossiliferous Yorktown Formation (Pliocene) of Virginia was used as a natural laboratory for testing predictions of ecological and evolutionary theories. Specifically, coordinated stasis and ecological locking models have testable elements that can be analyzed using data from the Yorktown Formation. The ecological locking model requires that species within an ecosystem have strong interactions in order to stabilize morphologies of multiple lineages over millions of years. Species intereactions that are strong enough to do this should also be strong enough to be a major ordering force on the composition of paleocommunities. Single and replicate samples were taken from 30 cm stratigraphic intervals within the Rushmere and Morgart's Beach Members at several localities. A total of 142 samples were collected from 5 localites, which yielded 29,000 specimens belonging to 140 species of bivalves, gastropods, and other taxonomic groups. Principle components analysis, ANOVA, MANOVA, and other analyses were used to test the occurence and recurrence of local paleocommunities, paleocommunities, and paleocommunity types. Three paleocommunity types which occured under specific paleoenvironmental conditions were defined: rubbly bottom, transitional, and muddy bottom. Within a single locality samples from the same paleocommunity type yielded very similar faunal compositions, based on the relative abundance of the contained species. However, samples from the same paleocommunity type but different localities displayed low similarity values. This is consistant with local paleoenvironmental control of paleocommunity composition being more important than strong species interactions. The pattern predicted by the model of ecologic locking is absent from these Yorktown paleocommunities. A guild analysis was performed on the data to test whether the same types of organisms recurred in a predictable fashion under similar paleoenvironmental conditions. While the guild structure of the rubbly bottom paleocommunity type did recur at several localities, the guild structure of the other paleocommunity types varied greatly from place to place.