Fossil Dennstaedtiaceae and Hymenophyllaceae from the Early Eocene of the Pacific Northwest

Premise of research. Newly recognized compression fossils of fertile leptosporangiate ferns assignable to Dennstaedtia Bernh. (Dennstaedtiaceae) and Hymenophyllum Sm. (Hymenophyllaceae) are described from the latest early Eocene floras of the Okanogan Highlands, British Columbia, Canada, and northeastern Washington. Methodology. Specimens were photographed with reflected light microscopy and were studied morphologically in comparison with related fossil and extant forms. Focus-shift stacking was used to layer photos of some specimens. Some were photographed with polarized light. The holotype specimen of Dennstaedtia christophelii sp. nov. was also studied with environmental scanning electron microscopy. Pivotal results. We describe D. christophelii sp. nov. (Dennstaedtiaceae) and Hymenophyllum axsmithii sp. nov. (Hymenophyllaceae) on the basis of vegetative and fertile remains of fragmentary fronds. Specimens of D. christophelii are one-pinnate-pinnatifid to bipinnate and bear frequent marginal round sori in cuplike indusia on small crenulate to dissected pinnae. Sporangia each have a prominent annulus with approximately six to eight long cells visible. Hymenophyllum axsmithii has the type of lamina cutting, sorus position, alate rachis, sorus shape, and venation corresponding to Hymenophyllum, subgenus Sphaerocionium (Hymenophyllaceae). The numerous sporangia per sori each bear a prominent annulus, and 7-10 casts of spores are noted in some sporangia. Conclusions. These fossils document the occurrence of leptosporangiate fern diversity in the early Eocene Okanogan Highlands floras. Dennstaedtia christophelii represents one of several occurrences of Dennstaedtiaceae in the Eocene. Hymenophyllum axsmithii is the first unequivocal member of Hymenophyllaceae to be reported from these diverse plant and insect assemblages from lacustrine and swamp settings under warm temperate climates of the latest early Eocene.