MCINTOSH’S RULE AND HOMOLOGY OF THE CRINOID CUP BASE

Dicyclic crinoids have two plate circlets below the arm-bearing radials, basals and infrabasals. Monocyclic crinoids have only a single circlet of plates below the radials, ambiguously interpreted as either basals or infrabasals. Contributing to this uncertainty is the plasticity of echinoderms, which are capable of fusing, resorbing, and adding novel ossicles. For example, the larvae of many extant crinoids are dicyclic, but the infrabasals are later resorbed. Such crinoids are termed “pseudomonocyclic”, a condition that may develop from the loss of any one of the three cup-plate circlets.

Another difference between dicyclic and monocyclic crinoids is that the former have lumen lobes that are aligned with the arms and misaligned with the stem lobes. The opposite is true of monocyclic crinoids. These differences in symmetry, in part generated by a 36° offset of primary plate circlets, characterize the “Law of Wachsmuth and Springer” (LWS). Violations of the LWS may be used to test homology, as in the confirmed prediction of infrabasal loss among extant crinoids. A different violation is encapsulated by “McIntosh’s rule”: dicyclic crinoids that lose basals may exhibit midcup irregularity. In other words, individuals may follow the LWS in some rays but not others. This is observed among aberrant specimens of the dicyclic Bactrocrinites that are missing basals.

Since the late 19^th century, the cup base plates of most dicyclic crinoids have been interpreted as infrabasals, whereas those of monocyclic crinoids have been interpreted as basals. This is true despite consistent orientation of the stems and cup bases of dicyclic and monocyclic crinoids. The relationship between the radials and stem-cup junction is less stable; the radials and ambulacral system may be “interradially” disposed in crinoids that have lost basals. This observation forces a reevaluation of what defines a “radial” orientation in crinoids, traditionally fixed by the position of the arms. I argue “radial” should be instead defined from the perspective of the more conservative stem-cup junction, which is governed by the orientation of the radially disposed convoluted organ. Doing so provides a more parsimonious explanation for the LWS and suggests the cup-base circlets of most monocyclic crinoids from the Paleozoic may be infrabasals, not basals.