THE COMPLETE STEM AND HOLDFAST OF THE EARLY ORDOVICIAN CRINOID TITANOCRINUS AND ITS DEVELOPMENT

Titanocrinus is the largest, most primitive, and one of the oldest crinoids known from the Early Ordovician, but only the proximal stem was known from the type specimens. A large, long, Titanocrinus medial and distal stem plus holdfast totaling 49.3 cm, and a small, apparently juvenile calyx 2.75 cm long with a complete stem and holdfast 17.5 cm long, now supply additional information. Holdfast exteriors in both specimens are irregularly multiplated, as are the distal-most stems that are 0.5 cm and 2.2 cm long. Plates in this region are robust and thicker than wide. Other isolated holdfasts attributable to Titanocrinus show a skirt-like peripheral rim for holdfast attachment. The juvenile holdfast is domal and Podolithus-like, expanding abruptly at the stem attachment, whereas the larger adult specimen has a steeply conical holdfast with no inflection at the stem attachment. Insertion of five wedges of slowly widening meres gradually reduces the multiplated regions to two plates, then one plate that pinches out, over the next 19.1 cm of distal stem. Above that, pentameres alternate for some distance, then become opposite, forming alternating thicker and thinner pentameric columnals over the remaining 19.7 cm of medial stem. Proximal stems in the type specimens show gradual enlargement and thinning of somewhat irregular pentameric columnals just below the calyx.

Several Late Ordovician crinoids (e.g. Hybocrinus, Cincinnaticrinus, Quinquecaudex, Reteocrinus, Cleiocrinus?) show parts of this same stem morphology sequence, representing a developmental progression for early crinoids during their Ordovician radiation. Most other Late Ordovician crinoids have only holomeric columnals throughout. This early stratigraphic occurrence supports general evolution in crinoids from an irregular mosaic stalk, to a pentameric stem, and then to a holomeric stem in later taxa. Hybocrinus changes its stem from a mosaic to aligned pentameres, whereas Titanocrinus incorporates plate wedges and offset pentameres as intermediates.