An extinct grammitid fern genus from Dominican amber, with revision of Grammitis succinea

Abstract

We report the second record of a grammitid fern from Dominican amber. The fossil comprises a single fertile pinna with excellent preservation, thus allowing a clear view of unaltered morphological characters. Venation, indument, and sori match the protologue of the previously described species from Dominican amber, Grammitis succinea L. D. Gómez, so we assign our fossil to that species. Characters of both specimens of G. succinea are sufficient to place it within the grammitid clade of the Polypodiaceae. However, it does not agree with Grammitis as it is currently circumscribed. We explore possible relationships between the fossil and other extant grammitid genera through Principal Coordinates Ordination (PCOa) of 109 morphological characters and through phylogenetic analysis of combined morphological and plastid DNA sequence data. Character combinations found in the fossil are inconsistent with those of any extant genus and our PCOa analysis did not suggest a clear affinity with any extant genus. This uncertainty is reflected in the results of our phylogenetic analyses, which find multiple most-parsimonious positions for the fossil within the grammitid clade. Based on these results, we suggest that the fossil belongs to an early divergent lineage of grammitid ferns, and cannot be assigned to any extant genus. Consequently, we place both specimens in a newly described fossil genus, Polymniopteris, and make a new combination for G. succinea within it. Because the type of G. succinea cannot be located, we designate the present specimen a neotype.