PENNSYLVANIAN BRACHIOPODS FROM EASTERN GREAT BASIN, NEVADA (U.S.A): EFFECTS OF VARIATIONS ON PALEOECOLOGICAL CONTROLS RELATED TO GLOBAL AND LOCAL ENVIRONMENTAL STRESSES

Carboniferous brachiopods seem to represent an important source of information of biotic response to paleogeographic and climatic change during the Carboniferous Period (Dietl and Kelley, 2001; Kelley and Raymond, 1991; Raymond et al., 1990). However, it is unknown how these changes modify brachiopod faunas. To address this question, preliminary studies on Pennsylvanian brachiopods from the eastern Great Basin, Nevada (U.S.A), have been conducted. These brachiopods were selected because of their stratigraphical context within cyclic depositional sequences, the origin of which may be related to eustatic change and Milankovitch variation linked to fluctuations in ice volume on Gondwana. In addition, these faunas are well preserved by a process of silicification and are usually present as in situ communities.

Preliminary observations have shown a general trend in the distribution of brachiopods within the cycles. Spire-bearing brachiopods represent the major fraction of biomass at the top of each cycle, in the shallowest part, and productids are dominant in deeper facies. This general distribution suggests that water depth and temperature might be viewed as primary paleoecological factors and these vary with eustatic and climatic changes related to the glaciation on Gondwana. To explain other modifications at community level as well as morphological modifications of individual species, secondary paleoecological variables are considered to play an important role. Among these variables, presence of competitors, type of substrate, and food supply seem to be the most important. The presence of rugose corals has a negative effect decreasing the number of brachiopods and promoting populations dominated by juvenile specimens. The nature of the substrate can also determine the nature of brachiopod populations. A good example is the species Heteralosia slocomi that can be found attached to the coral Chaetetes or to silty substrates. The number of this species increase as well as the range of sizes when this coral is available for attachment. Additionally, the dominance of specific types of brachiopods in different parts of the cycles can be explained through fluctuations in food supply and their ability to process more or less amount of it.