MORPHOLOGY AND FUNCTION OF THE SPIRALING PIVOT POINTS IN THE PROXIMAL STEM OF GLYPTOCYSTITOID RHOMBIFERAN ECHINODERMS

In most clades of fossil echinoderms, the stem raises the theca or calyx above the sediment water interface and tethers the organism to the substrate. Most taxa are sessile and affixed to the substrate by a holdfast or root system. Early glyptocystitoid rhombiferans differ from this model by detaching from the holdfast early in ontogeny and living freely as motile epifauna. This change in life mode is recorded in the construction of the stem that bears a short and flexible proximal region and relatively rigid distal region. Proximally, the stem bears thin, large diameter and large lumened columnals that alternate between inner and outer sets. These sets transition distally where they become more homogeneous, smaller in diameter, extremely thick and narrow-lumened. Proximal stems are regularly preserved in a strongly curved posture, suggesting that this stem region is extremely flexible and habitually bent. The presence of paired spiraling pivot points bisecting the articulating surface of the columnals, allowing adjacent columnals to rock slightly in a see-saw manner, supports this interpretation. These pivot points are offset by about 10˚on opposite sides of the columnals spiraling down the proximal stem. They become less pronounced distally and are absent from the distal stem. Models of rhombiferan life positions suggest three possible postures for stem carriage. Positioning the stem as a curve in a plane with linear placement of the distal stem is unstable and models topple laterally. A similar posture, but with a distal S-bend, is slightly more stable but still easily topples. Finally, a proximal open helix bend with an open arc curve of the distal stem is highly stable. This position places the theca close to the center of the distal arc, providing a platform for the theca and feeding appendages. Articulated columnals with both linearly and spirally positioned pivot points show that the former is consistent with the the first two postures, whereas the latter is consistent with the open coiled model.