SPORE WALL ULTRASTRUCTURE OF PROTOSALVINIA

Protosalvinia is an enigmatic fossil which has been historically assigned to several major taxonomic groups. Stratigraphically, the fossil occurs in a narrow range of Upper Devonian sediments. Tetrads of spores are associated with shallow depressions on the surface of approximately 5% of the specimens collected from the Ohio Shale in Columbus, OH. Spores are approximately 250 μm in diameter and have a spore wall which is composed of at least two distinct layers. The outer layer is coarsely laminated in regions where adjacent spores are in contact. Individual laminar units are thinnest toward the inside and gradually thicker toward the surface of the spore. In non-contact regions, the outer layer is composed of globular units. The inner layer of the wall has little discernable structure except for the presence of a distinct suture beneath the proximal trilete mark. This firmly establishes the meiotic nature of these structures. Comparison with eggs and tetraspores of several extant phaeophycean algae shows little similarity.