MAZON CREEK-TYPE FOSSILS FROM THE PENNSYLVANIAN OF THE ARKANSAS RIVER VALLEY

The Mazon Creek area of northeastern Illinois is famous for its fossil-bearing siderite concretions (Pennsylvanian: Westphalian), with exceptional preservation of many soft-bodied and delicate animals and plants. The Mazon Creek biota includes two basic assemblages: the Braidwood assemblage, dominated by terrestrial plants and non-marine animals; and the Essex assemblage of aquatic organisms from varying salinity levels. Similar fossil-bearing siderite concretions occur in the Pennsylvanian of other states, such as Oklahoma and Missouri. We present a preliminary report on fossil-bearing concretions in the Pennsylvanian coal-bearing strata of the western Arkansas River valley. Most of the fossils are plants, including probable seed ferns; the predominant invertebrates are clams. The assemblage is much more like that of the Braidwood assemblage of Mazon Creek than the concretionary biota described from coeval rocks of eastern Oklahoma, which contains several Essex-type taxa. These fossils should prove useful in paleoecological reconstructions of the southern margin of Laurentia in the Pennsylvanian.