Ultrastructure and Probable Botanical Affinity of the Enigmatic SporomorphFroelichsporites traverseifrom the Norian (Late Triassic) of North America

Abstract

Premise of research. Froelichsporites traversei is a prominent palynomorph in the Upper Triassic of North America that always occurs in tetrahedral permanent tetrads. It is an important regional biostratigraphic marker in the Norian of North America, and its abundance rises around 215 Ma, associated with a significant floral and faunal turnover. Its most striking morphological features are one well-developed distal pore (ulcus) on each grain and the annulus-like exine thickening around them. Previous works suggested that it was produced by spore-producing plants or Cheirolepidiaceae, but its botanical affinity is still unclear.Methodology. The wall ultrastructure of F. traversei was analyzed by TEM in order to reveal more information on the botanical affinity of the palynomorph.Pivotal results. The sporoderm consists of two layers and an inner faint discontinuous lamination. The outermost exine layer has a homogeneous texture (tectum), while the inner layer has a granular texture (infratectum). The laminae below the granular layer are not continuous but directly contiguous with the granules.Conclusions. The ultrastructure studies have ambiguous results, and the botanical affinity could not be revealed with certainty. It represents most likely a gymnosperm. The sporoderm layers indicate full development, and F. traversei was most likely dispersed as permanent tetrads at maturity. The ulcus might represent a germinal aperture or a rehydration pore similar to Cupressaceae pollen. The permanent tetrads may be related to polyembryony or polyploidy, and they probably provided an adaptive advantage to the parent plant.