Gravitational settling of conodont elements: Implications for, paleoecologic interpretations of conodont assemblages

Gravitational settling experiments show that conodont elements settle with a wide spectrum of velocities, ranging from 0.5 to 5.5 cm/s in synthetic seawater. Significant differences in settling velocity exhibited by representatives of the three major shape classes of conodonts (pectiniform, coniform, and ramiform elements) are due to differences in grain mass and cross-sectional area exposed to fluid shear and normal forces during settling. Settling trajectory is a spiral or helical pattern that is less variable than settling velocity, but differs from models that characterize most skeletal carbonate grains. Conodont elements exhibit viscosity-dominated (Stokesian) settling and are hydraulically equivalent to phosphate spheres of coarse silt to fine sand size. Thus, even under conditions of modest net horizontal currents, lateral separation and sorting due to differences in size and shape of conodont elements belonging to a single animal will occur during gravitational settling. Differential sorting related to gravitational settling probably influenced the formation of most preserved conodont assemblages. The disparity between observed element types, forms, and sizes compared to known or proposed species apparatus compositions can be used both as an indication of taphonomic disruption and as a useful factor in interpreting sedimentary environment.