Radial microornament in spiriferid brachiopods and paleogeographical implications

Abstract

Capillae are a major feature of the fine ornamentation in spiriferid brachiopods. Within the lower Devonian spiriferids, studied in this paper, two categories of organization can be distinguished: strictly radial orientation of the microstructures, which leads to the eospiriferid-type of organization, and pseudoradial orientation, which provides a delthyridid-type. Apart from this distinction, it can now be shown that the mode of development is quite different in each case. Thus, and allowing for differences in size, the capillae of eospiriferids are quite similar to the costae of costate spiriferids. On the other hand, the origin of the capillae is more complex in the delthyridids and as yet not fully known. Other factors (such as the rate of growth, the divergence angle of the capillae, etc.) have effects on the growth mechanisms and contribute to the existence of a large variety of microornaments. Furthermore. available data indicate that the divergence of the capillae is closely related to the paleogeographical distribution of the populations involved. If this observation proves valuable for the whole stock of spiriferids (s.l.), it opens new ground for paleozoogeographical investigations. □Spiriferid brachiopods, eospiriferids, delthyridids. microornament, Devonian, paleogeography.