TEMPSKYA FERN FOSSILS FROM FLOYD COUNTY, IOWA CONFIRM A MID-CRETACEOUS (ALBIAN-CENOMANIAN) AGE FOR THE WINDROW FORMATION

The Windrow Formation consists of poorly sorted, coarse-grained conglomerates and sandstones with iron oxide cement that crop out discontinuously in the upper Mississippi Valley region of North America. Dating the Windrow has been problematic due in part to a lack of biostratigraphically informative fossils. Its stratigraphic position is poorly constrained, sitting unconformably on Paleozoic strata (Devonian or older) and overlain by early Pleistocene glacial drift. Similarities in lithologic composition and elevation have led to suggestions that the Windrow is equivalent to the non-marine Woodbury Member of the Dakota Formation that formed along the eastern margin of the Western Interior Seaway. One outlier of the Windrow Formation in Floyd County, Iowa (NE NW NE sec.18, T94N, R18W) includes a poorly bedded mudstone with iron oxide masses, some highly polished cobbles (possibly gastroliths), and silicified wood. Two silicified plant fossils have been identified as Tempskya Corda 1845, an extinct cosmopolitan genus of tree fern (Filicales, Tempskyaceae) used as index fossils for the Albian - Cenomanian. The material consists of two fragments of false-trunk, both elliptical in transverse section ±8cm x ±5cm with one ±16cm long and the other ±10cm long. Specimens were embedded in epoxy and thin-sectioned transversely and longitudinally for study under transmitted light microscopy. Tempskya’s false-trunk consists of an agglomerate of dichotomously branching, solenostelic stems in a mat of diarch adventitious roots. In the roots, xylem cells form the shape of a cross with a line of larger metaxylem arranged perpendicular to smaller protoxylem. The density of petiole bases in stems indicates short internodes between fronds. The presence of fecal pellets in the stem cortex suggests arthropod feeding damage. Twelve species of Tempskya have been described from North America and further morphological study will allow us to determine if the Iowa specimens represent a distinct species. However, identification of the Iowa specimens as Tempskya sp. confirms a mid-Cretaceous (Albian-Cenomanian) age for Windrow Formation, in addition to supporting ecological interpretations of the plant preferring a mild temperate, flood plain habitat with periodic moderate to high precipitation.