Systematics and evolutionary significance of some new cryptospores from the Cambrian of eastern Tennessee, USA

Abstract

The highly bioturbated mudstones of the uppermost Rome Formation and the Conasauga Group in eastern Tennessee contain an extensive palynoflora that consists primarily of nonmarine, spore-like microfossils, which are treated systematically as cryptospores because they present characters that are consistent with a charophytic origin. The following new taxa are proposed: Adinosporus voluminosus, Adinosporus bullatus, Adinosporus geminus, Spissuspora laevigata, and Vidalgea maculata. The lamellated wall ultrastructure of some of these cryptospores appears to be homologous to extant, crown group sphaerocarpalean liverworts and to the more basal genus, Haplomitrium. There is direct evidence that some of these cryptospores developed via endosporogenesis—entirely within the spore mother cell wall. The topology of enclosed spores indicates that the meiotic production of spore dyads represents the functional spore end-members, but the diaspore itself appears to be a spore packet corresponding to the contents of each original spore mother cell. Aeroterrestrial charophytes of this time period underwent sporogenesis via successive meiosis rather than simultaneous meiosis. Overall, these remains are consistent with Bower's antithetic origin of the plant sporophyte because they present a picture of extensive and varied spore development (i.e. sporogenesis) well in advance of the occurrence of vegetative sporophytes in the fossil record.