POLAR OPHIUROID TAPHONOMY: DISINTEGRATION AND OSSICLE DESTRUCTION OF OPHIONOTUS VICTORIAE IN EXPLORERS COVE, ANTARCTICA

Ophiuroids (Phylum Echinodermata; brittle stars) are abundant benthic animals on the Antarctic continental shelf. Therefore, it is surprising that their ossicles, internal calcium carbonate skeletal elements held together by muscle and by mutable cartilaginous tissue (MCT), have not been reported from the hundreds of meters shelf deposits recovered by the CRP-3, ANDRILL 1B, and MSSTS-1 drill cores. Given the low rate of sedimentation, particularly under semi-permanent sea ice, and low water temperature (<-1^oC), one explanation for the absence of ossicles is that they dissolve before being buried by sediment. Related hypotheses are that: a) Antarctic ophiurods fortify their ossicles with less calcium carbonate than those living temperate and tropical areas, resulting in ossicles with more open stereom and fragile struts, rendering them susceptible to breakage and dissolution; and b) Antarctic ophiuroid ossicles are surrounded by more MCT than their temperate or tropical counterparts, protecting the ossicles from dissolution; high oxygen content of the extremely cold water allows enhanced production of the oxygen-expensive MCT.

These hypotheses to explain the discrepancy between the modern ophiuroid abundance and the paucity of their ossicles in the Cenozoic sedimentary record are being tested by laboratory experiments and in situ experiments on Ophionotus victoriae in Explorers Cove (EC), Antarctica, as well as by assessment of the abundance of ossicles in shallow cores from EC. Preliminary observations indicate that ossicles are absent from 10 cm long cores, in spite of the fact that O. victoriae is the third most abundant epifaunal animal in the area. Sacrificed ophiuroids currently are suspended in mesh bags above the substrate and buried in sediment at 8 locations in Explorers Cove and surrounding areas; they will be retrieved in 2010. The extent of the degradation of the muscle tissue and MCT, and the preservational state of the calcite ossicles will be assessed to elucidate the extent to which taphonomic processes of disintegration and dissolution control the translation of the living benthic community into the fossil record. Previous work suggests that both the taphonomic processes and the survivorship of living O. victoriae may be temperature dependent and thus sensitive to climate change.