PALEOBIOLOGY OF THE TRIASSIC "FLAT CLAM" HALOBIA IN OXYGEN-DEFICIENT MARINE FACIES

Species of the enigmatic Late Triassic "flat clam" Halobia often occur in organic-rich black shale facies indicative of oxygen-poor basinal marine environments. Although the living habit(s) of halobiids remain unresolved, they undoubtedly played a role in the cosmopolitan distribution and narrow stratigraphic ranges of many halobiids making them excellent biostratigraphic indices of the Late Triassic. Although Halobia are frequently abundant in monotypic shell beds which clearly represent allocthonous and probably time-averaged accumulations, they occasionally occur as relatively isolated individuals in organic rich mud or in thinner, autochthonous shell beds. Life strategies of Halobia are best inferred by functional morphologic and taphonomic evidence which suggests special adaptations for life in low-oxygen settings. Halobia's very thin shell (< 50 µ) may have aided in oxygen diffusion across an essentially flat shell that is optimized for maximum surface area. In addition, narrow valve widths in Halobia enclose a compressed body (including the gills) that, together with a sedentary life habit, suggests relatively low respiration rates and therefore a low-oxygen requirement. However, the dorso-anterior triangular field (including the anterior auricle) likely associated with passage of a byssus in mature individuals rather than a "sulfur pump" of other authors. Morphologic evidence suggests a planktotrophic larval stage followed in maturity by an epibyssate life habit. Halobia are thought to be byssally attached on any firm substrate, including other halobiid individuals or perhaps sea-weed. Although a pseudoplanktonic life-strategy cannot be ruled out, halobiids were likely opportunists able to rapidly colonize benthic substrates in dysoxic settings.