EOCENE-NEOGENE COOLING AND THE STRUCTURE OF MARINE BENTHIC COMMUNITIES IN ANTARCTICA

Benthic faunas currently living in shallow-water habitats (<100 m depth) in Antarctica display functional attributes and trophic relationships that are reminiscent of Paleozoic community structure. Clades of fast-moving, skeleton-breaking (durophagous), functionally modern predators, including teleosts, crabs, sharks, and skates and rays, are either absent or present at very low diversity. Instead, slow-moving invertebrates of a Paleozoic functional grade, particularly seastars and nemertean worms, are the primary occupants of higher trophic levels in shallow-water environments. The occurrence of dense ophiuroid and crinoid populations suggests that durophagous predation is limited in Antarctica today, as it was worldwide during the Paleozoic. This retrograde community structure has its evolutionary roots in the cooling trend that began in the late Eocene to early Oligocene. In the late Eocene La Meseta Formation at Seymour Island, Antarctic Peninsula, declining temperatures are associated with a sharp increase in the occurrence of dense ophiuroid and crinoid assemblages. The late Eocene ophiuroid and crinoid populations exhibit low frequencies of sublethal damage (regenerating arms), suggesting that predation pressure was severely limited. The disappearance of durophagous predators during and after the late Eocene was likely a product of complex causality rather a simple effect of declining temperature. Why modern-type predators remain scarce in Antarctica today is an important but unanswered question of marine biogeography.