Comparative morphology of Rhipidocystis Jaekel, 1900 and Cryptocrinites von Buch, 1840 (Eocrinoidea; Ordovician)
| Aasta | 1994 |
|---|---|
| Raamat | Echinoderm through Time. Proceedings of the eighth international echinoderm conference, Dijon, France, 6-10 September 1993 |
| Kirjastus | A.A. Balkema Publishers |
| Kirjastuse koht | Rotterdam |
| Leheküljed | 173-178 |
| Tüüp | artikkel kogumikus |
| Keel | inglise |
| ISBN | 9054105143 |
| Id | 1738 |
Abstrakt
Comparison of thecal plating of Rhipidocystis and Cryptocrinites shows that these genera have been closely related. Some differences in the plating were caused by the process of theca flattening in Rhipidocystis. The bend of the theca in both genera in the same plane of symmetry but in opposite directions as well as the different position of anus resulted from the process of heterochrony. The aim of this report is to determine the relationships of two genera of the Ordovician eocrinoids Cryptocrinites and Rhipidocystis on the basis of new morphological data. This particular problem is connected to a broader problem of the relationships between classes of echinoderms. Phylogenetic relationships between the numerous classes of echinoderms are in most cases very uncertain. A similar problem exists at a lower taxonomic level, and is particularly found within some extinct classes, especially the Eocrinoidea. Two small groups, usually placed in eocrinoids, though they lack epispires serve as a good example. The first group, the family Rhipidocystidae consist of the e.ocrinoids with flattened theca. The genus Rhipidocystis is typical of the group. The second group (6 genera distributed among 2 families) consists of eocrinoids with globular theca but without epispires. A typical representative is Cryptocrinites-the type genus of the family Cryptocrinitidae (Ubaghs, 1967). The morphology of the two groups seemed so different that Broadhead (1982) proposed to place the eocrinoids with flattened theca in an independent class, and to transfer Cryptocrinites and similar genera into the class Paracrinoidea. Here I intend to show that a very close relationship exists between Cryptocrinites and Rhipidocystis and correspondingly between the two groups to which they belong. In the interests of brevity comparisons will be largely confined to just these two genera, both of which are represented by abundant of well-preserved 173.