In situ spores of marattialean ferns from the Triassic in Central and Northern Europe

The Middle to Upper Triassic successions of Europe have yielded several rich and well-preserved palaeofloras in which fertile fronds of marattialean ferns with conspicuous sporangia are common components. Here we describe the in situ spores of several marattialean fern species from these floras belonging to the genera Asterotheca, Merianopteris, Danaeopsis, and Symopteris with a focus on intra- and interspecific variability. Knowing this spore variability is important for the interpretation of dispersed spores and may provide insights into phylogenetic relationships and ecological influences. The spores of the various Asterotheca species are generally similar among each other, but Ladinian specimens are distinct as they have a punctate exine, whereas those from the Carnian are laevigate or microverrucate, rarely verrucate. Merianopteris augusta, long regarded as a junior synonym of A. merianii, differs markedly in the size and ornamentation of its spores. The spores of the various Danaeopsis and Symopteris species are generally circular, trilete, smooth-walled or discreetly ornamented, and mostly correspond to Todisporites or Punctatisporites. However, the rays of the trilete mark are often of unequal length. The various species mostly differ in the average size of the spores, but with notable outliers, potentially pointing to macromorphologically indistinguishable but biologically distinct lineages. In samples with sufficient yield, abnormally small, dense, and often dark spores that we interpret as abortive can be observed. These occur with frequency in every plant, usually in low frequencies, but with considerably high frequencies in a few individuals, which may indicate pathological conditions or natural hybridisation.

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