CRAWLING IN STALKED CRINOIDS: IN SITU OBSERVATIONS, FUNCTIONAL MORPHOLOGY, AND IMPLICATIONS FOR PALEOZOIC TAXA
The video reveals N. decorus crawling over a distance of several meters at a speed of 4-5 cm/s. With the oral-aboral axis parallel to substrate, movement is in the oral direction with the stalk dragged behind the crown. All arms are curved aborally but only a few facing towards the substrate are involved in a sequence of power and recovery strokes. The power stroke (1) consists of an aborally-curved arm, extended with the aboral side towards the substrate and the tip flexed into the substrate, flexing aborally. During the recovery stroke (2), the tip of the arm is lifted off the substrate and the arm extends orally (straightens), while still retaining partial aboral flexure.
The above sequence is repeated by several arms, but in adjacent arms it is 180 degrees out of phase: as one arm completes the power stroke, the adjacent arm completes the recovery stroke. During crawling the stalk is held straight and the long axes of cirri at near right angles to the stalk; the stalk itself is not in contact with the substrate, instead it is supported by rows of extended cirri whose distal-most cirrals contact the substrate.
The crawling of N. decorus allows us to consider features compatible with this behavior in stalked crinoids: (1) The rather complex and rapid sequence of arm flexure and extension suggests that muscular arm articulations are a pre-requisite to crawling; (2) Non-permanent mode of attachment of the stalk to the substrate and an ability to reattach suggest that a cirriferous stalk with highly motile cirri is required. Not surprisingly, these features characterize the isocrinids and some of the stem articulates. It is therefore plausible that some Paleozoic stalked crinoids were capable of crawling; trace fossils and taphonomy may provide tools independent of functional morphology for identifying this behavior in fossils.