Persistent ultrastructural features in microspores of heterosporous lycophytes

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Abstract
Ultrastructural analysis of the microspores from the Pennsylvanian lycophyte cone Selaginellites crassicinctus reveals a structural organization which is comparable to modern representatives of Isoetes and Selaginella. The most prominent similarities include the presence of a para-exospore—a layer external to and largely free from the exospore, but with a similar ontogeny and staining characteristics—and a proximal multilamellated region situated at the innermost region of the exospore in the center of each interradial area. A more rigorous comparison of specific structural features indicates a higher degree of similarity with Selaginella, particularly S. selaginoides. These results are somewhat anomalous in light of the suggested affinity of Paurodendron, the axis which presumably bore this cone, with the Isoetales. These results stand in support of the existence of a recently proposed group of Carboniferous lycophytes which are structurally intermediate between the rhizomorphic and herbaceous types.