A PENNSYLVANIAN PLANT ASSEMBLAGE RAPIDLY BURIED IN TIDAL SANDS, PELLA, IOWA

Assemblages of fossil plants preserved under conditions of rapid burial and limited transport are key components in understanding the taxonomic diversity and structure of ancient vegetation. Here we report an example of such an assemblage, latest Atokan in age, preserved above either the Black Oak or Kilbourn coal bed a short distance above the Mississippian-Pennsylvanian unconformity surface near Pella, Iowa. The fossil assemblage is encased in a thickly bedded, very fine-grained quartz arenite that overlies 4-5 m of sandstone with millimeter-scale tidal rhythmites. There are no indications of erosion or periods of non-deposition within the rhythmite sequence indicating abundant accommodation space. This deposit formed when sands were rapidly emplaced above a coal bed during marine transgression, probably associated with sea-level rise following the melting of polar ice. Rapid burial of the vegetation is evidenced by numerous in situ tree trunks of giant lycopsids, up to 2 m in height, rooted in the top of the coal. The main flora occurs approximately 3 m above the basal tidalites and appears to represent the upper parts of the plants buried during the initial period of sedimentation. Included in the flora is a partial crown of a Lepidodendron sp. (“scale tree”), an exceptional specimen and one of only two known to be in museum collections. The associated compression flora includes many large trunks of lycopsids and parts of other plants, such as vines that may have been in the tree crowns, many with leaves still attached. The flora exhibits an unusually high diversity of lycopsids, including one or more species of Lepidodendron, Sigillaria, Diaphorodendron, and Asolanus. Seed fern foliage is also abundant and dominated by Eusphenopteris, a probable vine. Other, less common pteridosperms or ferns include Sphenopteris, Neuropteris, and Alethopteris. Pecopteris is rarely encountered. Sphenopsids are common elements, including Calamites, Asterophyllites and Sphenophyllum.