A PECULIAR EARLY EOCENE, NEARSHORE, SHALLOW WATER SUSPENSION-FEEDING COMMUNITY FROM SEYMOUR ISLAND, ANTARCTICA

The Eocene La Meseta Formation at Seymour Island, Antarctic Peninsula, is comprised primarily of unconsolidated clastic sediments suggested to have been deposited in a repeatedly rejuvenated fault-bounded channelway. The La Meseta Formation has produced a diverse marine fauna, but as is typical of Cenozoic marine deposits, bivalves and gastropods dominate.

Dense autochthonous suspension-feeding ophiuroid and crinoid populations occur in and are limited to the uppermost interval of the La Meseta. Similar dense populations were comparatively common during the Paleozoic but disappeared from normal marine settings with diversification of durophagous predators during the Mesozoic marine revolution. Climatic cooling during late La Meseta time disrupted predator-prey relationships and permitted recurrence of the dense echinoderm populations. La Meseta species, however, are taxonomically readily distinguished from corresponding Paleozoic species.

Several workers have recorded a second predominantly suspension-feeding fauna from a number of sites in the lowermost part of the La Meseta. This earlier assemblage is dominated by diverse brachiopods and bryozoans. Unlike assemblages of the remainder of the La Meseta, mollusks are comparatively uncommon and their diversity is limited but oysters and pectinids are included. In addition to the lophophorates, the lowermost La Meseta, exposures have yielded serpulid worms, corals, shark teeth, and diverse echinoderms but no ophiuroids. Fossils are physically rather small as compared to the mollusks that dominate the higher intervals. Thus two La Meseta assemblages, one from the lowermost interval and the other from near the top, exhibit traits suggestive of Paleozoic communities; however, the two are distinct in taxonomic composition and apparent community structure. The two assemblages are thought to represent distinctive nearshore settings. Comparison of the lower faunas with those of higher intervals suggests that complex and incompletely understood mechanisms controlled community structure in the Southern Ocean during the Paleogene time of La Meseta deposition.