Crinoids Aweigh: Experimental Biomechanics of Ancyrocrinus Holdfasts

Abstract

Immobile suspension feeders living on soft substrates, 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 prevented them from sinking (e.g., strophomenid brachiopods) or anchored them to the sea floor (e.g., crinoid holdfasts). The quantitative expression of the static stresses for forms living on soft, muddy bottoms developed by Thayer (1975) can be easily modified to describe the forces involved in anchoring. One of the more unusual putative anchoring structures is the ``grapnel{''} holdfast of the Devonian crinoid Ancyrocrinus. This form does not match the paradigm for most typical current anchoring structures, lacking recurved, pointed, and flattened lateral processes. Its form is suggestive, however, of the modern, nautical mushroom anchors deployed on muddy bottoms. Plaster casts of the Ancyrocrinus holdfast were used to quantify the actual forces involved in penetrating, being pulled out of, or dragged across soft substrates. The forces were measured using a digital force gauge mounted on a motorized test stand. Substrates used included fine quartz sand, pure kaolin mud, and coarse carbonate sand. The holdfasts readily penetrated soft mud, but encountered much greater resistance in fine sand. They did, however, readily penetrate the latter substrate when rocked, supporting the comparison with mushroom anchors. Holdfasts do not penetrate the sediment when dragged across it, suggesting a minimal ability to passively anchor in this way. Simple calculations of the sinking velocity of Ancyrocrinus suggest that when dislodged, they would have easily reimplanted in soft substrates.