Morphology and ultrastructure of Devonian spores: Samarisporites ( Cristatisporites) orcadensis (Richardson)

Abstract

Specimens of the dispersed spore Samarisporites ( Cristatisporites) orcadensis ( Richardson, 1965) have been isolated from Middle Devonian deposits of Cromarty, Scotland. This is the strata from which the type material was obtained ( Richardson, 1960) The specimens are extremely well preserved and of low thermal maturation. Comprehensive investigations using light, scanning electron and transmission electron microscopy provide detailed information on morphology, gross structure and wall ultrastructure. The spores are trilayered, with a bilayered inner body entirely enclosed within an outer layer. There is apparently no cameration between the layers comprising the spore wall. The outer layer is entirely homogeneous, and is extended forming a trilete mark on the proximal surface, a prominent ‘pseudozona’ in the equatorial region and distinctive distal ornament. The inner body is bilayered, with the inner (α) layer comprising closely packed, straight, parallel and continuous lamellae, and the outer (β) layer comprising similar but less closely spaced and more irregular lamellae. The morphological, structural and ultrastructural information is assessed in order to evaluate the taxonomic demarcation of this taxon, and also to shed light on spore wall formation, functional morphology and affinities/phylogeny. It is suggested that structural interpretation based on light microscope work is susceptible to misinterpretation, because the different layers of multilayered spores often possess different ultrastructure and hence differ in optical properties.