New observations and synthesis of paleogene heterosporous water ferns

Abstract

Premise of research. Reproductive structures of modern genera of heterosporous water ferns (Marsileaceae and Salviniaceae) are widespread and abundant in plant mesofossil assemblages from the Paleogene. For Salviniaceae, whole fertile fossil plants give a good understanding of morphology. These fossils can be applied in paleoenvironmental analysis and to study water fern origin, evolution, and diversification. Methodology. New specimens were examined by SEM and TEM. Synchrotron x-ray tomographic microscopy (SRXTM) is evaluated as a nondestructive tool for investigating Azolla Lam. morphology. Pivotal results. Azolla anglica Martin and Salvinia cobhamii Martin (earliest Eocene, United Kingdom) are fully characterized using SEM and TEM. SRXTM enables digital rendering of the float system in Azolla, but individual floats are difficult to distinguish. Modern water fern genera characterize the Paleogene, but extinct sister taxa characterize the Cretaceous. Literature review documents that water ferns are intolerant of salinity over 5 psu. Conclusions. The oldest fully documented Salvinia Séguier sori and spores occur in earliest Eocene deposits at Cobham, United Kingdom, probably linked to warm climates. An unusual co-occurrence of Salvinia with Azolla is preserved at this site. The Azolla species differs from those present in the same region during other Eocene warm-climate intervals. SRXTM offers potential to retrieve taxonomically useful information on internal structures of Azolla. There is a major turnover in water ferns (dominantly extinct to almost entirely modern genera) across the Cretaceous-Paleogene transition. The utility of water ferns as indicator taxa is exemplified by recognition of freshwater ocean surfaces and widespread continental wetlands during the latest Early to earliest Middle Eocene in and around the Arctic and Nordic Seas.