IMPLICATIONS OF A NEW WATER FERN FROM THE CRETACEOUS DAKOTA FORMATION OF WOODBURY COUNTY, IOWA, U.S.A

A new fern megaspore recovered from exposures of the mid-Cretaceous Dakota Formation in northwest Iowa indicates a greater abundance and richness of ferns than previously documented. Bulk samples of a dark brown-gray, poorly consolidated clay were collected from the Sioux City Brick Quarry in Woodbury County, Iowa (N 42̊ 24.517, W 096̊ 21.267). Litho- and palynostratigraphy indicate that this unit falls within the Woodbury Member of the Dakota Formation, dated as late Albian - early Cenomanian (±99.6 Ma). The sediments were deposited in a large, fluvial-deltaic system along the eastern margin of the Western Interior Seaway of North America. Charcoalified debris was extracted using standard paleobotanical processing techniques and a 63 µm sieve. Exploratory study of the resulting residue under reflected light microscopy has produced >50 specimens of the new fern megaspore. The new megaspore consists of a main spore body (mean equatorial diameter = 253.04 µm, sd = 27.14, n = 23) whose smooth surface is decorated by tube like appendages (mean length = 75.61 µm, sd = 20.88; mean number of appendages = 15.22, sd = 3.42, n = 23). The spore body is crowned with an acrolamellae (a ridged, tapered, flamelike structure) with a mean height of 272.17 µm (sd = 37.52, n = 23). The new megaspore’s morphology suggests a close affinity to the extinct genus Arcellites, a member of the heterosporous water fern family, Marsileaceae. The genus Arcellites currently consists of sixteen species, in which the two species most similar to Sioux City Brick specimens (A. disciformis and A. hexapartitus) are differentiated primarily by characteristics of the megaspore. To determine if the new megaspore is taxonomically distinct, we will thin section some specimens and use transmitted light and scanning electron microscopy to describe thickness and microstructure of the exoexine, exine, and intexine. When taken in context of a recent paleoecological assessment of the Woodbury Member’s palynoflora, the addition of an abundant Marsileaceaous megaspore tentatively identified as Arcellites supports the interpretation that ferns continued to represent an important component of mid-Cretaceous plant communities even as angiosperms diversified.