Taxonomic implications of exospore structure in selected Mesozoic lycopsid megaspores

Abstract

The structure of the sporopollenin wall of fossil megaspores can be usefully applied to the determination of morphological similarities and differences between species and genera, which in turn can aid determinations of botanical relationships. Nine species belonging to seven genera of lycopsid megaspores from European Mesozoic successions that have been examined in thin section under a transmission electron microscope are discussed in this paper. Of these, seven are considered to be referable to the Selaginellales, namely Bacutriletes pragensisroman Echitriletes zemechensisroman Erlansonisporites sp. ‘pseudomoravica’ type, Horstisporites cenomanicensisroman Kerhartisporites kleiniiroman K. srebrecensisroman Ricinospora sp. and Verrutriletes maloninensis. The structure of the exospore of Aneuletes patera, an alete species of hitherto uncertain affinity, also suggests that this has selaginellalean origins. The construction of the wall of Echitriletes lanatus, however, resembles that of representatives of the Isoetales and implies derivation from a family within, or related to, one or other of these lycopsid orders that is now extinct. Along with aspects of their gross morphology, significant differences between the structure of the exospore of E. lanatus and E. zemechensis suggest that they do not belong in the same morphological genus. The value of the differentiating Kerhartisporites from Erlansonisporites is questioned.

Keywords:

• Jurassic
• Cretaceous
• Europe
• mesofossils
• morphology
• botanical relationships

Acknowledgements

I thank Rita Dutta (formerly IGES, Aberystwyth University) for obtaining the TEM micrographs and the late Ervin Knobloch for the Czech specimens examined. Both Ervin and Doug Nichols were not only colleagues but also good friends and are sadly missed. Although Doug and I did not collaborate on any papers, we struck up a friendship in 1980 during his participation in a field trip (of which I was one of the leaders) prior to the Fifth International Palynological Conference in Cambridge. We were in fairly regular contact subsequently, particularly after he agreed to help me in my capacity as Editor-in-Chief of the journal Cretaceous Research by becoming its North American Editor in 1992, a post he held until he took over as Editor-in-Chief in 2008.