Danian (Paleocene) Molluscan Paleoecology in the Aftermath of the Cretaceous-Tertiary Extinctions, East-Central Texas

Danian shelf sediments from the Brazos River region of east-central Texas span approximately the first two million years after the Cretaceous-Tertiary (K-T) extinctions with only minor changes in lithofacies. Study of these virtually continuous stratigraphic sections reveals the nature and timing of the molluscan faunal rebound in the wake of the K- T extinction event. Diverse Late Maastrichtian molluscan faunas were dominated by epifaunal suspension-feeders, primarily oysters. Immediately above the K-T extinction horizon, low-diversity molluscan assemblages were characterized by a bloom in the abundance of the pelagic herbivore family Litiopidae, which were rapidly replaced by a low diversity assemblage dominated by a deposit-feeding bivalve of the family Nuculanidae (Upshaw, 1989; Hansen and Upshaw, 1990). This low diversity assemblage persisted for approximately 300,000 years after the K-T extinctions and may have been caused by the low productivity of the hypothesized Strangelove ocean of Hsu (1986). Diversity rapidly increased to nearly pre-extinction levels within the Globigerina pseudohulloides zone (Plb), roughly 300,000 years after the K-T extinctions. Coinciding with this diversity increase was a change in the composition of the fauna from a roughly even mixture of Cretaceous, Paleocene, and undescribed species to a fauna composed primarily of Paleocene species. The relative proportions of deposit-feeders and suspension-feeders also return to roughly Late Cretaceous levels within this biozone. Above the base of the G. pseudobulloides zone (Plb), the molluscan assemblages were characterized by increased stability. Diversity and trophic proportions of these assemblages persisted at nearly pre-extinction levels for the remainder of the studied interval, a period of at least 1.6 million years. In contrast to the large amount of faunal turnover apparent in the earliest Paleocene, molluscan assemblages within and above the G. pseudobulloides (Plb) biozone are characterized by increased species longevity in the studied sections. Some faunal constituents were affected more permanently by the K-T extinctions. Epifaunal soft-bottom suspension-feeders such as oysters were a prominent component in local Late Cretaceous faunas, but they never regained their former levels of diversity and abundance in the studied Paleocene stratigraphic sections. Infaunal burrowing carnivorore-scavengers such as Naticidae and various opistobranchs became much more abundant in the Paleocene than they were in the Cretaceous