COMMUNITY DIVERSITY IN A PENNSYLVANIAN PHYLLOID ALGAL MOUND FROM TEXAS

Environments such as reefs and build-ups have been modified by organisms throughout the Phanerozoic. High biodiversity has often been associated with many biologically mediated environments, especially reefs. Other types of biologically mediated environments, such as phylloid algal mounds, can provide insight on the relationship between the species that had great effect on the environment (i.e., phylloid algae) and biodiversity.

Much of the Missourian Winchell Formation of Texas is comprised of phylloid algal mounds. One such mound near Perrin, north-central Texas was analyzed using a point-count method along bedding planes and within beds in the outcrop. Point-counting at 2-cm horizontal intervals along the outcrop has uncovered a significant decrease in both biodiversity and fossil abundance through the history of the mound. Crinoid debris is abundant throughout the mound and only slightly decreases in absolute abundance upsection, whereas brachiopods and bryozoans, abundant and diverse in the lower half of the mound, disappear fairly rapidly towards the top of the mound. Other fossils, such as echinoid plates and bivalves, are rare and occur only near the bottom of the mound, whereas rare small rugose corals and fusulinids are found in thin shales as isolated elements.

These diversity and abundance patterns suggest a change in height of suspension-feeder tiering during the growth of the mound. Both low-level and high-level suspension feeders are present during the early stages of the phylloid algal mound and the fauna is very diverse. This pattern changes upsection to a much less diverse fauna dominated by high-level suspension feeders, principally crinoids. The ratio of points containing faunal elements to phylloid algae and micrite during point counting also decreases upsection, suggesting that although the fauna is strongly dominated by crinoids, biomass of preservable animals may have also declined. This faunal pattern suggests a long-term change from high diversity and multi-level tiering in the community to decreased diversity and loss of low-level tiers, which occurred simultaneously with the development of the phylloid algal mound.