--> ABSTRACT: Short-Term Community Transition and "r Selection" in Shallow Marine Embayment Fauna from Pennsylvanian of North-Central Texas, by Alta S. Cate; #91030 (2010)

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Short-Term Community Transition and "r Selection" in Shallow Marine Embayment Fauna from Pennsylvanian of North-Central Texas

Alta S. Cate

Community and size-frequency analyses were obtained for macro-fauna from a thin fossiliferous interval within an otherwise barren shallow marine embayment facies in the East Mountain Shale (Strawn Group). Vertically contiguous sampling of this discrete unit allowed reconstruction of short-term community transition and detection of shifts in the population structure of two gastropod species (Glabrocingulum G. grayvillensis, Straparollus A. catilloides). These biological phenomena could be related to environmental shifts brought about by deltaic progradation.

A marine reworked bioclastic sandstone at the base of the sampled interval provided coarse substrate that was rapidly colonized by attached suspension-feeding organisms (bryozoans and crinoids). As the influx of fine-grained sediment brought about the return to a muddy substrate, these forms were replaced by the "mud-loving" gastropods Glabrocingulum, Donaldina, and several pseudozygopleurids. In the upper part of the interval, increasingly harsh environmental conditions were induced by the progradation of a major distributary, and infaunal pelcypods became the dominant faunal element.

Exponential population growth rate, micromorphism, and details of survivorship curves suggest that populations of the deposit feeder Glabrocingulum, at this locality, may have been "r-strategists" for which initially rapid population growth rates prevailing during early colonization were checked by a density-dependent limiting factor such as trophic resource. The suspension feeder Straparollus exhibited a lower, linear population growth form in which high mortality rates, incurred by increase in suspended inorganics, regulated population growth. Although information from other localities is at present sketchy, populations of these species probably consisted of smaller numbers of larger individuals in more stable environments.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.