VARIABILITY WITHIN THE INFRABASAL CIRCLET OF THE CLADID CRINOID GENUS CUPULOCRINUS (ECHINODERMATA) AND IMPLICATIONS FOR THE ORIGIN OF FLEXIBLE CRINOIDS

Flexible crinoids (subclass Flexibilia) are considered to have originated from the cladid crinoid genus Cupulocrinus or one of its immediate ancestors. A remarkably constant and clade-defining character of flexible crinoids is the possession of three plates in the infrabasal circlet of the calyx. This character was a significant morphological modification for the origin of the flexible clade, resulting from the reduction of the number of infrabasal plates from five plates in the ancestral cupulocrinid.

The discovery of two anomalous specimens of Cupulocrinus sp. from the Middle Ordovician of Kirkfield, Ontario with four, rather than five, infrabasal plates prompted further examination of the number of infrabasal plates in individuals within Cupulocrinus. The discovery locality is significant because Kirkfield deposits also produced the earliest known flexible crinoid, Protaxocrinus laevis (Billings).

An examination of Cupulocrinus specimens from the Field Museum of Natural History, the Royal Ontario Museum and the National Museum of Natural History has yielded additional specimens with four infrabasal plates, suggesting this may have been a relatively frequent condition within the genus. In many of the specimens, the reduction to four infrabasals was accomplished by the fusion of two plates to form a single, larger plate with a modified shape. The larger plate is not consistently located within the Carpenter ray scheme, and the variations occur in multiple species of Cupulocrinus from different localities.

Of 415 specimens of Cupulocrinus for which the number of infrabasal plates could be determined, 15 specimens, or 3.6%, displayed a deviation from five infrabasals, with 12 of these (2.9%) having four infrabasals. Although the number of infrabasals is typically thought to be constant within a species, this trait appears to have been variable in the immediate ancestor of flexible crinoids, in the time period just before the number of infrabasals became fixed at three for the Flexibilia.