A MODERN, SOFT-BOTTOM, SHALLOW-WATER, TROPICAL CRINOID FAUNA, WITH A COMPARISON BETWEEN LIVING COMATULA ROTALARIA AND FOSSIL UINTACRINUS SOCIALIS (ECHINODERMATA: CRINOIDEA)

Current understanding of modern, shallow-water, tropical crinoid ecology derives almost entirely from assemblages associated with coral reefs and adjacent hard bottoms. Only Amphimetra and Heterometra have been observed regularly on unconsolidated low-energy bottoms. A recent preliminary survey reveals that about a dozen species (all unstalked comatulids) occur on a gentle sandy slope (15-18 m depth) west of Watson’s Bay, Lizard I., Great Barrier Reef, Australia. Dominant macroorganisms are chlorophyte algae (e.g., Caulerpa spp., Halimeda spp., Udotea sp., Penicillus sp.), seagrasses (Halophila spp.) and small free-living scleractinian corals (see Fisk, 1983, Mar. Biol. 74:287-294). Several of the crinoids (e.g., Comatella nigra) also occur on reefs. Several others appear restricted to unconsolidated substrates but cling to sponges or algae (e.g., Amphimetra tessellata, Zygometra microdiscus and Z. elegans), or hide beneath rubble or sponges (Heterometra crenulata). A. tessellata arranges its arms in a radial fan or funnel, while Zygometra spp. form parabolic filtration fans similar to those of Pontiometra andersoni on reefs and stalked Isocrinidae in deep water. Comatula cf. purpurea was uniformly observed irregularly coiled but visible beneath sprawling branching sponges. Comatula rotalaria, which lacks anchoring cirri and bears ~20 arms up to 0.3 m long when mature, rests directly on the sediment with its central calyx elevated by flexing 5-7 of its long interior arms to form a shallow U or V; a few other long arms flex slightly but do not touch the bottom, while the shorter exterior arms orient more or less upward as a central tuft. C. rotalaria offers a potential model for substrate adaptations and arm postures in Late Cretaceous Uintacrinus socialis, which, though it bears fewer longer (>1 m) arms, also lacks cirri and exhibits similar brachial morphology. U. socialis may have oriented its arms vertically despite their great length. Its arm bases are 8 mm across; anterior arms of modern confamilial Alloeocomatella pectinifera, held vertically for feeding, are <2.5 mm across the base and reach 0.5 m long. A similar, elevated-calyx posture occurs in a few reef-dwelling species without cirri (e.g., Comanthus alternans, Comaster nobilis), but these are extremely bushy taxa with >100 much shorter arms.