Functional morphology of the stem in the Lower Palaeozoic crinoid Macrostylocrinus Hall from Scotland

Abstract

Two species of the closely-related monobathrid crinoid from the Lower Palaeozoic of Scotland, namely Macrostylocrinus cirrifer Ramsbottom (Upper Ordovician, Katian) and Macrostylocrinus silurocirrifer Brower (Lower Silurian, Telychian), are similar in having elongate, unbranched radices proximally. These were not cirri, as suggested by their names, but were radices which were more or less inflexible, lacking contractile tissues. The function of these radices was uncertain. In the absence of contractile tissues, they could not have been for grasping other upright structures and crinoid do not need help to balance, their posture being maintained by mutable collagenous tissues. It is possible, but unlikely, that they may have acted to direct feeding currents towards the crown. Most probably, in an analogy to the post-Palaeozoic isocrinids, the stem acted like a ‘conveyor belt’, the proximal, radicular and upright part being carried away from the cup as further columnals are inserted, eventually forming a distal, recumbent attachment structure. The elongate radices would have stabilised the dististele, but, unlike isocrinids, the arms of Macrostylocrinus spp. were not adapted for crawling and thus escaping predators. Both M. silurocirrifer (type species) and M. cirrifer are included in Macrostylocrinus ( Scotimacrostylocrinus ) subgen. nov.