PUSH ME – PULL YOU: EXPERIMENTAL BIOMECHANICS OF IMMOBILE SUSPENSION FEEDERS ON SOFT SUBSTRATES

Immobile suspension feeders on soft substrates (ISOSS; Thayer 1979) although rare in modern marine habitats, were relatively common in the Paleozoic. Numerous Paleozoic taxa have been interpreted as dwelling on soft unconsolidated sediments and possessing morphologic features that either anchor them to the sea floor (e.g., crinoid holdfasts) or prevent them from sinking in (strophomenid brachiopods). Thayer (1975) reviewed the morphologic adaptations for forms living on soft-muddy bottoms and provided a quantitative expression of the static stresses involved. The same quantitative expression can also be used to describe the forces involved in anchoring. With the exception of Leighton and Savarese (1996), Thayer’s model has not been experimentally examined.

Tests were conducted in order to quantify the actual forces involved in penetrating or being pulled out of soft substrates. For holdfasts, soldered brass models were constructed based on actual specimens, as well as models that represent variability in key parameters believed to control their ability to resist dislodgement. Plaster casts were made of the “grapnel” holdfast Ancyrocrinus. Sediment penetration was analyzed for a range of actual strophomenid specimens differing in size and geniculation. Forces were measured using a digital force gauge mounted on a motorized test stand. Substrates used include fine quartz sand, pure kaolin mud and coarse carbonate sand. For each holdfast design and substrate combination, repeated trials were done of the effects of varying depth of burial and speed of vertical and horizontal pull on the maximum force for removal. For the strophomenids, forces of sediment penetration were measured in both convex up and convex down orientations.

Results for Ancyrocrinus suggest a minimal ability to passively anchor in soft substrates. For the strophomenids, shells in the “convex up” position require greater force to penetrate the sediment than in a “convex down” position; this difference increases with the geniculation of the shell. These results have direct implications for our understanding of the life habits of these crinoids and brachiopods.