End-Permian Isoetalean spore tetrads
The end-Permian crisis caused a dramatic re-organization of terrestrial ecosystems. Palynological data from Greenland indicate a step-wise change from closed conifer dominated woodland to stable open shrubland mainly composed of seedferns and lycopsids. The collapse of the terrestrial ecosystem can be witnessed by the rapid conversion from by gymnosperm pollen dominated assemblages to ones dominated by lycopsid microspores. Strange enough a large amount of these spores in this time interval occur in tetrads (fig. 1). This phenomenon is not restricted to Greenland. It has been recognized in Permian-Triassic sequences all over the world. These spores are assignable to the microspore form-genera Densoisporites, Lundbladispora, Uvaesporites and megaspore Otynisporites. TEM-analysis of preserved sporoderm ultrastructure shows that individual spores are juxtaposed or connected to each other by interlocking of the para-exospores at either the interradial contact areas or equatorial regions. The wall organization of Densoisporites (fig. 2), Lundbladispora, and Otynisporites (fig. 3) confirms an isoetalean (Pleuromeiaceae) affinity. The Uvaesporites wall structure is far more complex than so far recognized in extant Selaginellales and extant and fossil Isoetales. Uvaesporites might be related to a distinct lycopsid lineage, with characteristics of rhizomorphic lycopsids and the Selaginalles.
Looy C.V., Collinson M.E, Van Konijnenburg-Van Cittert J.H.A. and Visscher, H., 2005. The ultrastructure and botanical affinity of end-Permian spore tetrads. International Journal of Plant Sciences, 166: 875-887.