A lost world? Archaic crinoid-dominated assemblages on an Antarctic seamount

The previously unsurveyed Admiralty (67°S, 171°E) and Scott Island (67°22′S, 179°55′E) seamounts to the north of the Ross Sea were explored using cameras and physical sampling gear during the New Zealand IPY-CAML research voyage (TAN0802) in February 2008. A striking assemblage dominated by stalked crinoids and brachiopods was found at 580–600 m depths on isolated knolls at the northwest and southeast extremities of Admiralty Seamount. The seabed at these sites was littered with crinoid ossicles, and crinoid stalk bases were conspicuous on exposed rocks, suggesting that these assemblages have persisted for a considerable period of time. The crinoid sites were limited to the isolated knolls but large areas of the flanks of the main seamount were covered by dense populations of suspension-feeding ophiuroids. These assemblages are more similar in structure to those preserved in fossil strata from the Palaeocene and late Eocene than to any extant assemblages yet described from the Antarctic. In comparisons with faunal assemblages on Scott Island seamount, the abundance of stalked crinoids was strongly inversely correlated with the abundance of echinoid, asteroid, and lithodid crab predators and both asteroids and echinoids were photographed feeding on crinoids. These observations are consistent with the prevailing hypotheses that crinoid- and ophiuroid-dominated assemblages, which were widespread in the Palaeocene, were displaced by the radiation of mobile predators, and that conditions in isolated habitats in the Southern Ocean may have acted as refugia, allowing the persistence of archaic benthic assemblages.