CONTRACTILE CONNECTIVE TISSUE (CCT) IN THE STALK OF THE BOURGUETICRINID CRINOID, DEMOCRINUS: FUNCTIONAL, ECOLOGICAL, AND EVOLUTIONARY IMPLICATIONS
Democrinus has 5 short arms, a stalk characterized by synarthrial columnal articulations (with a central fulcrum), and lives on sandy/muddy substrates where low current velocities predominate. The combination of low current and an open filter result in weak lift and drag on the crown, limiting the ability to passively reorient into an optimal feeding posture. The presence of CCT in the stalk may allow Democrinus to actively re-orient in response to changing current directions. Synarthrial (fulcrum-bearing) columnal articulations found in adult Democrinus also characterize juvenile isocrinids and comatulids; because passive reorientation of the crown would be particularly ineffective in these juveniles due to their small size, CCT is likely to be involved. Since ontogenetic, fossil, and molecular data suggest that bourgueticrinids evolved from comatulids through paedomorphosis, CCT may characterize all extant crinoids (Articulata), and perhaps ancient crinoid clades. Many of the latter are thought to have lacked muscular articulations of any type, and in these the functional role of CCT may have been especially important.