FROM THE MIRE TO THE FLOODPLAIN AND BACK: THE PALEOECOLOGICAL HISTORY OF FOUR PENNSYLVANIAN ALETHOPTERIS

Equatorial mire (peat swamp) communities remained stable through successive Pennsylvanian glacial-interglacial cycles, despite evidence suggesting that mires disappeared during part of each cycle. Floodplains could have sheltered mire species during dry intervals. However, Pennsylvanian mires, viewed as evolutionary refugia, and floodplains, viewed as centers of innovation, apparently shared few species. Preservational differences complicate the identification of mire species in floodplain assemblages. The mire macroflora is permineralized, permitting detailed anatomical and three-dimensional shape analyses. The floodplain macroflora consists of adpressions (impressions and carbon films) in concretions and siliciclastic sediments, which give the two-dimensional shape of foliage, venation, and sometimes, the anatomy of the epidermis. Four permineralized Alethopteris, assigned to adpression species or species groups, link Pennsylvanian mires and floodplains. A. ambigua and A. lesquereuxi appeared first in mires, were replaced in mires by other Alethopteris species, yet persisted on floodplains. A. sullivantii appeared nearly simultaneously in both habitats and persisted on floodplains after disappearing from mires. The permineralization window closed before the Berryville Alethopteris disappeared from mires. Our results challenge the view of Pennsylvanian mires as evolutionary refugia and edaphic islands, sharing few species with floodplain communities. If mires were evolutionary refugia, species should persist in mires after disappearing from floodplains. Mire alethopterids show the opposite pattern. The high floral diversity of floodplains relative to mires, and the rarity of mire species in floodplain assemblages contribute to the view of mires as edaphic islands. However, floodplains may have higher floral diversity because they encompass a wider spectrum of environments than mires. If mires were edaphic islands, we would expect to find unique seed-fern foliage species in mires. Instead, most mire seed-fern foliage can be identified with an adpression species or species group (9 of 10 species in 4 genera: Alethopteris, Neuropteris, Laveineopteris, and Reticulopteris). Floodplains were the likely refuge of mire species when mires disappeared.