This study examined size-frequency distributions for an extinct Cretaceous-age benthic foraminifera called Orbitolina Texana in order to determine the health of this foram community during the time of accumulation. Forams were collected from limestone outcrops of the Glen Rose Formation in central Texas. Based on paleontological and sedimentological evidence, Orbitolina Texana are interpreted to be a shallow water (<10 m) benthic organism that was most abundant in back-reef environments. Sizefrequency distributions were generated from the diameters of 4,245 Orbitolina Texana fossils. The results indicate that the Orbitolina Texana population is characterized by a Gaussian (normal) size distribution. Size-frequency distributions of fossilized foraminifera in the stratigraphic record are controlled by two principle variables; environmental controls that affect the biology of the organisms (e.g., environmental stressors, like food availability and ambient conditions) and sedimentological controls that affect how the organisms are distributed (e.g., hydrodynamic parameters, like waves and currents). Based on a sedimentological characterization of the host limestone including lack of high-energy sedimentary structures, high mud content and back-reef position on the depositional profile, hydrodynamic parameters are interpreted to not have exerted a significant effect on the distribution of the forams preserved in the rock record. As such, the biological controls are interpreted to be the dominate control on the foram sizefrequency distribution. Therefore, the Gaussian (normal) size distribution suggests that the mortality rate is independent of size/age, which is classified as Type II survivorship. As this result is consistent with similar taxon, the results indicate that this Orbitolina Texana population was a healthy biologic community, despite the low biologic diversity observed in the Glen Rose Formation.
Size-Frequency Distribution of Orbitolina Texana Foraminifera.
Undergraduate Review, 10, 79-84.
Available at: http://vc.bridgew.edu/undergrad_rev/vol10/iss1/18
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