Paper No. 12
Presentation Time: 11:00 AM
MACROEVOLUTION OF SHALLOW MARINE BENTHOS IN THE NORIAN (LATE TRIASSIC)
TACKETT, Lydia S., Earth Sciences, University of Southern California, 3651 Trousdale Parkway, ZHS 233, Los Angeles, CA 90089-0740 and BOTTJER, David, Department of Earth Sciences, University of Southern California, Zumberge Hall 117, Los Angeles, CA 90254, Tackett@usc.edu
The onset of important macroevolutionary events in shallow benthic ecosystems may have occurred at some point in the Triassic, including the so-called Paleozoic-Modern Fauna transition and the Mesozoic Marine Revolution. Theories associated with these phenomena make a number of paleoecological predictions that may be simultaneously tracked through sequential fossil collections to illustrate some subtle aspects of the influence in different groups through geologic time. The Norian stage is exceptionally long (~228-204Ma), encompassing a substantial part of the entire Triassic period, with limited data on shallow marine benthic invertebrates. Collections for this study were taken as sequential bulk samples in Panthalassic and Tethyan
localities to address the presence and degree of change in the Norian faunas, which may be related to these macroevolutionary trends during this long stage.
Paleoecological analysis of the marine invertebrates of the Norian stage revealed communities that were not static with regard to diversity in both marine systems, although different macroecological developments could be identified between Panthalassa and Tethys. In the Tethyan realm of Lombardy, mollusks clearly dominated over any other shallow marine group, but there is a slow transition over the entire duration of the Norian from epifaunal to infaunal dominance, with a diverse semi-infaunal contingent throughout. In Panthalassic sections, this trend is not observed; rather, ecological groups appear to track the taxonomic proportions, with primarily epifaunal, suspension-feeding animals composing the bulk of most samples into the Rhaetian. Furthermore, evidence of increased predatory pressure is not apparent in the form of infaunalization, armorment, boreholes, or increased presence of predatory groups. These macroevolutionary trends may have had different onset times and rates of change in the two ocean systems, and indicate that in the Tethys they may have occurred exceedingly slowly or later than the Triassic.
