THE EVOLUTION OF BENTHIC ECOLOGY IN THE CENOZOIC OF ANTARCTICA

Assessing changes in the ecological composition of fossil communities, and how this affected the evolution of marine life, gives insight into how modern communities will react to environmental change. Modern Antarctic benthic invertebrate marine communities are described as having an archaic retrograde nature, dominated by epifaunal suspension feeding organisms. Studies have suggested that this evolved in the Eocene, with cooling decreasing durophagous predation. However, several lines of evidence do not corroborate this hypothesis, and it is unclear when this community structure arose.

The Cenozoic Era began with the Cretaceous-Paleogene (K-Pg) mass extinction, however, despite showing a level of taxonomic extinction equivalent to other areas in the world, Antarctica did not experience a permanent change in benthic community ecology. During the Eocene, there was a radiation of many taxa. Stalked crinoids, the main line of evidence for the original hypothesis of community structure arising at this time, are present due to asynchronous timing in the Marine Mesozoic Revolution in the Southern Hemisphere, rather than being related to the origin of the modern Antarctic community structure.

Evidence of the first glaciations in west Antarctica comes from King George Island in the South Shetland Islands. The Polonez Cove Formation and the Cape Melville Formation preserve marine sedimentary sequences from the Oligocene and Miocene. The presence of dropstones, diamictites and striated rocks confirm that they were deposited in a glacial environment. Both formations preserve abundant marine invertebrates. These represent Antarctica’s first glacial sea floor communities. The youngest unit, the Cape Melville Formation, does not preserve invertebrate communities with typical modern Antarctic structure. It is dominated by infaunal bivalves, with a significant proportion of durophagous decapods. Overall, we hypothesize that the evolution of the modern benthic invertebrate community structure occurred more recently than previously thought, due to factors such as further cooling and isolation of the continent leading to widespread glaciation, which resulted in a loss of shallow shelf habitats.