Ontogeny of the crinoid Eucalyptocrinites

The calyx of a Paleozoic camerate crinoid is composed of a mosaic of large, polygonal plates. Ontogenetic size increase results from accretionary additions of calcite to thecal plates. When new calcite is added to the lateral edge of a plate, a new layer is also added over the external surface, obscuring the earlier growth stages of the plate. However, ontogenetic development can be studied by measurement of a growth series. This is illustrated by regression analysis of growth series of two Silurian species of the genus Eucalyptocrinites: E. crassus (Hall) and E. tuberculatus (Miller & Dyer). Growth of the principal plates is isometric; most growth parameters have a very similar mode of development in both species; only a few parameters show any specific difference. A table of correlation coefficients for 51 growth parameters demonstrates that the entire development of the calyx was highly coordinated throughout life, with coefficients almost always equal to or larger than 0.90. Integrated expansion of the cup plates provided a larger cavity for the viscera. The stem increased in diameter to support the increasingly larger theca, as did the diameter of the root system. Growth of the vaulted plates in the upper part of the theca provided a protective recess for the arms, which probably contributed to the evolutionary success of this animal on a widespread basis during the Silurian and Devonian. Based upon growth studies, the following synonymies are suggested: E. constrictus Hall, E. ellipticus Miller, E. ovalis Hall, and E. subglobosus Miller are synonyms of E. crassus; E. clrodi Miller is a synonym of E. tuberculatus Miller & Dyer.