Ultrastructure of lower Paleozoic dyads from southern Ohio II: Dyadospora murusattenuata, functional and evolutionary considerations

Abstract

Fossil cryptospore dyads consistent with published descriptions of Dyadospora murusattenuata fall into two general categories when examined with the TEM. Type I dyads are enclosed, and have a wall consisting of (1) an outer homogeneous layer that is present on non-contact surfaces, but absent in large part over the contact surface, (2) a middle layer composed of plate-shaped units, and (3) an innermost granular layer. The latter two layers are continuous over each of the dyad members. Type II dyads show a range of ultrastructural types, differing primarily with respect to the extent of a region of spongy wall material that occurs at the edge of the contact face. Thin regions and breaks present in the walls of many dyads and pseudodyads may represent primitive sutures. Ultrastructural analysis suggests the existence of at least two separate lineages of plants in the Late Ordovician and Early Silurian, but the phylogenetic relationship between these groups and to more recent land plants remains uncertain.