OPHIUROID ADAPTATIONS TO ANTARCTIC WATERS LINKED TO TAPHONOMIC LOSS, POOR FOSSIL RECORD

Ophiuroids are major components of Antarctic benthic communities but are rare in bottom grab samples and in cores of Cenozoic deposits from the Ross Sea. To test the hypothesis that there is a taphonomic filter that inhibits preservation of ophiuroid ossicles in Antarctic marine sediments, taphonomic experiments were conducted on specimens of the widespread and abundant Antarctic ophiuroid Ophionotus victoriae (Ov) from New Harbor, Antarctica, and of commercially available temperate and tropical species (TTS). Sacrificed ophiuroids were disaggregated in a bleach solution. TTS disaggregated over hours, whereas Ov remained coherent for several days. Examination with a stereo microscope and SEM reveals that Ov has abundant collagenous material binding the ossicles whereas TTS do not. Disaggregated ossicles were tumbled for up to 100 hours. SEM images taken before and after tumbling document that Ov ossicles are more damaged by tumbling than ossicles of TTS. The stereom structure of Ov ossicles is more open and more delicate, and thus more easily broken than that of TTS.

Skeletal differences between Ov and TTS are interpreted as adaptations to cold vs. warmer water. Calcium carbonate solubility increases with decreasing water temperature, so secretion and maintenance of calcium carbonate by Ov is energetically costly in the sub zero (Celsius) water; reduction of calcium carbonate mass is advantageous. This reduction is manifested by large voids and delicate struts that characterize the stereom of Ov. Collagenous material requires high levels of dissolved oxygen. Amount of dissolved oxygen increases with decreasing water temperature, so oxygen can be used profligately by Antarctic benthos, including by Ov, to produce large quantities of collagenous material that protects ossicles from dissolution. Oxygen supply in warm waters may be stretched thin by basic metabolic processes, leaving little extra for high-oxygen consumptive collagen production by TTS, accounting for the sparse collagenous material. Although producing large amounts of collagenous material that protects delicate ossicles is an effective strategy for Ov in Antarctic conditions, it is deleterious for the fossil record of Ov. Once the collagenous material is removed, as it will be geologically quickly, Ov's delicate ossicles are readily destroyed.