DIVERSITY TRENDS AND SPECIES ABUNDANCE MODELS WITHIN THE EARLY CRETACEOUS (ALBIAN) WALNUT FORMATION, CENTRAL TEXAS

The Early Cretaceous (Albian) Walnut Formation crops out in the low, wide valleys of the Lampasas Cut Plain of central Texas. The formation is extremely fossiliferous, with faunas dominated by Molluscs and Echinoids. Fossils collection in the formation is relatively easy, as preservation for most specimens is as hard, internal molds naturally washing out of loose marls. New sampling localities are regularly revealed by home/business construction, a happy consequence of the fact that the Killeen-Temple-Fort Hood Metropolitan Statistical Area (MSA) is one of the fastest growing MSA’s in the entire United States. The other consequence of this unprecedented growth is that once revealed, these new localities have very short “lives” before they are covered over. In addition, many reliable, long-lived collecting stops are also lost to new construction. Over 5,000 individual fossils specimens from previous collections of the Walnut were recently supplemented by newly discovered localities within the K-T-FH MSA. Shallowest water faunas at the base of the formation are low evenness, low richness and dominated by the large gastropod, Peruviella dolium. These are replaced upwards in composite section by higher evenness, highest richness faunas dominated by the oyster bivalves Exogyra texana and Texigryphaea mucronata; many species of gastropods; the echinoid Heteraster texanum; and the ammonites Oxytropidoceras salasi and Engonoceras pierdenale. The upper, middle part of the formation contains the lowest evenness, lowest richness faunas of the T. mucronata biostromes. These beds are highly resistant to weathering and form prominent exposed benches throughout the Walnut valleys which are then replaced upwards by a very similar Mollusc-Echinoid fauna. Diversity trends for the entire formation were logged for the first time. Counts of the faunas were then compared with the geometric, log-series and log-normal species abundance models. For null hypotheses of no difference between actual data and the models, preliminary goodness of fit tests were significant only for the log-normal model, analogous to most large, mature communities today.