2009 Portland GSA Annual Meeting (18-21 October 2009)

Paper No. 8
Presentation Time: 9:00 AM-6:00 PM


WEHNER, Matthew and RAYMOND, Anne, Dept. of Geology & Geophysics, Texas A&M University, College Station, TX 77843-3115, matthewwehner@yahoo.com

Alethopteris ambigua Lesquereux, the foliage of a medullosan seed fern, is well documented in Pennsylvanian adpression assemblages from North America and Europe. This species is so similar to A. lesquereuxi that some workers suggested placing the two in synonymy. We have identified permineralized alethopterid foliage from the latest Atokan - earliest Desmoinesian Williamson No. 3 Mine in south-central Iowa as Alethopteris ambigua sensu White (1899) and Cridland (1968). Permineralized Alethopteris ambigua has narrower and more sclerotic pinnules (leaflets of compound seed-fern leaves) than Alethopteris lesquereuxi, indicating that both species should be retained. The permineralized alethopterids from North American mires (peat swamps) form a biostratigraphic sequence that can be used to date Pennsylvanian permineralized peat and to indicate paleoenvironments or paleoclimate. The earliest North American alethopterid is an undescribed species from the Hamlin Coal (latest Morrowan) with a wide, thin, parenchymatous pinnule, suggesting it grew in a continuously wet climate under a closed canopy. This form is so fragile that it may be rare in adpression assemblages. Alethopteris ambigua, from latest Atokan and earliest Desmoinesian coals, has a thick, narrow pinnule and copious sclerenchyma, suggesting growth in a seasonally dry mire or open-canopy forest. A. sullivantii, which has broad pinnules, occurs rarely in latest Atokan-earliest Desmoinesian mires. In the mid-to-late Desmoinesian, A. lesquereuxi, an alethopterid with relatively narrow pinnules, replaced A. ambigua in peat swamps. The parenchymatous pinnule of A. lesquereuxi suggests a relatively wet climate or closed-canopy forest. A. sullivantii, the species with broad pinnules, commonly co-occurs with A. lesquereuxi in mid-to-late Desmoinesian mires. In the Missourian, sclerotic pinnules assigned to the A. serli group replaced A. lesquereuxi and A. sullivantii in peat swamps, indicating the return of seasonally dry climates or perhaps, an open-canopy forest.