Paper No. 8
Presentation Time: 10:05 AM
FLUME STUDIES USING 1:1 SCALE MODELS TO DETERMINE FEEDING POSTURE AND MODE OF ATACHMENT IN LOWER AND BASAL MIDDLE CAMBRIAN GOGIIDS FROM GUIZHOU PROVINCE, CHINA
With the onset of the Cambrian agronomic revolution various types of echinoderm algal mat attachers were proposed in the literature. Most in fact, especially in deeper quieter water genera, seem to have attached directly to siliciclastic substrate or biodetritus by bioglueing (?extensible collagen). Feeding postures of abundant gogiids from Lower and basal Middle Cambrian, Guizhou Province, China were subject to interpretations based on morphology. Genera with long stalks (eg. Sinoeocrinus) were interpreted to have heeled over in the current from the thin flexible distal end of the stalk, with the brachioles streaming in a loose essentially horizontal bundle down current from the theca. To test these and other assumptions, 1:1 scale models (holdfast, stalk and theca) were carved from soft rubber and brachioles were modeled from braided fishing line. Models with both straight and spiraled brachioles were tested in a flume with water velocity ranging from 8.5 to 10 cm/sec. Models did not heel over, even with thin stalks. Brachioles, straight and spiraled, extended vertically from essentially vertical thecae and curved distally downstream. Disrupted flow around straight brachioles (Sinoeocrinus) kept them somewhat evenly spaced. Coiled brachioles (Guizhoueocrinus, Globoeocrinus) are initially straight and angle outwards so that each defines a sector over the theca; this spacing keeps the brachioles free and from intertwining distally. Biogluing seems to be more correctly interpreted. Superglue was used as a gluing agent for the models and was marginally successful. Success was limited because we were able to only partly reconstruct the early Phanerozoic, low water content, non-bioturbated, seafloor. The somewhat viscous glue did not penetrate the illite substrate very well and a large thinly glued surface area was necessary for stability. Genera with pancake-like holdfasts tend to have larger thecae and spiraled brachioles; both characters contributed significant hydrodynamic drag. In life, it is probable that bioglue had low viscosity, penetrated sediment easily and was able to agglutinate a large three dimensional anchoring body of sediment without (as commonly observed) disrupting bedding. Because the bioglue was probably susceptible to bacterial degradation, its renewal was a biological necessity.