Biostratinomy of Recent Crinoids (Echinodermata) at Lizard Island, Great Barrier Reef, Australia

Abstract

Skeletal sands on and around a fringing reef at Lizard Island, Great Barrier Reef, Australia, contain a small but recognizable component of skeletal ossicles of comatulid crinoids (feather stars), which inhabit the reef in great abundance and diversity. The distribution, abundance, and preservational condition of crinoidal ossicles were investigated as an analogue for biostratinomic processes in ancient crinoid environments. In the study area, crinoidal material occurs as completely disarticulated ossicles representing all elements of the comatulid skeleton (calyx, brachials, pinnulars, and cirrals). The crinoidfraction is greatest at the base of the forereef escarpment (3-5% by grain count), declining rapidly along slopes below the escarpment (-1 %), in the backreef ( 40 spp.) of living crinoids. Fish predation is a major source of crinoid mortality, which may partly account for the high concentration of crinoidal debris at the base of the reef, where it would most likely fall after being dropped by fish. The scarcity of calyxes in the sediment may also be a result of their selective removal by predators. Lacking rapid burial, crinoidal arms, pinnules, and cirri become virtually completely disarticulated within days; only the calyx remains articulated for longer periods. The result is a time-averaged accumulation that contains abraded and broken ossicles as well as fresh material. SEM photographs and thin sections of abraded ossicles show no evidence of infilling of the stereomic pores (prefossilization). Post-mortem movement of crinoidal material is evidently downslope rather than laterally away from the reef, despite regular along-reef tidal currents approaching 50 cmlsec or greater. No evidence for buoyant transport of crinoid ossicles was found, and only the finer skeletal elements were winnowed. Reduced concentration of crinoidal material along offreef slopes reflects the lower densities of living crinoids there. The concentration of crinoidal debris is thus directly related to proximity to the living site. INTRODUCTION