ULTRASTRUCTURE OF CAMBRIAN CRYPTOSPORES SUPPORT MULTILAMINATE WALLS AS THE PRIMITIVE CONDITION IN THE PLANT SPORODERM

The continued study of palynomorphs and organic fragments from the Cambrian of Laurentia has generated several new lines of evidence in support of an early origin to the land plants. Filaments, which occur in mats, possess oblique cross walls - a character state that is synapormorphic with respect to extant bryophytes. Small cuticular fragments associated with spore masses are unlike any algal structures known today. Multi-laminated, primary spore walls more closely resemble the sporoderm of extant liverworts than any known algae. The morphology and topology of Cambrian cryptospores is not a perfect match to those of younger, unquestioned embryophyte affinity. However, the range of morphological variation expressed in these microfossils clearly overlaps with that seen in Ordovician and Silurian assemblages. We examined the sporoderm ultrastructure of cryptospores from an assemblage recovered from the Bright Angel Shale (Glossopleura biozone) at Thunder River Falls in the Grand Canyon. Primary spore walls may contain from one to three distinct internal laminations, or, they may be a fused, multilaminate type. This latter form is distinctly reminiscent of both fossil liverworts and extant Riccia. Having a single sample with this range of expressed variation in wall ultrastructure provides a direct link between the single lamellae typical of resistant walls in algal zygospores with the multilaminate type of embryophyte sporoderm in liverworts. Fossil evidence now directly supports a suggestion put forth by Blackmore and Barnes in 1987, that the multilaminate sporoderm is the plesiomorphic state for the land plants.