Pennsylvanian-Age Cordaitalean Wood from Knoxville, Iowa

Abstract

The remains of fossilized cordaitalean trees were discovered in the middle of the 20th century in surface and underground mines in south-central Iowa, in the vicinity of the town of Knoxville. With some uncertainty, the source coal is assigned to the Laddsdale coal beds (Lower Cherokee Group), which are early Desmoniensian, equivalent to early Asturian and middle Moscovian age. The plant remains were preserved both as compressions and petrifactions. The compressions consisted of prostrate, nearly entire trees, found lying on the upper surface of the underclay paleosol, at its point of contact with the coal bed. The prostrate trunks, which often had crown branches and roots in attachment, were up to nearly 40 m in length, and were without preferred orientation, sometimes overlapping one another on the underclay surface. Permineralized material was preserved as both upright tree stumps and prostrate logs. The permineralized stumps were rooted in the underclay and extended upward a short distance into the coal bed. Permineralization provides access to anatomical features, which show the logs and stumps to consist of pycnoxylic wood with narrow mostly one-, sometimes partially two-cell wide, multi-cell high rays, and a broad, septate pith. The permineralizing mineral is predominantly calcium carbonate, and of sufficient density to permit cut surfaces to be brought to a high gloss polish, thus first attracting the attention of fossil collectors and later, in the early 1960s, that of professional paleobotanists. Some of these permineralized specimens are on display in museums around the United States. The cordaitaleans were an ecologically diverse group, ranging from wetlands, including peatlands, to seasonally dry habitats, including mountainous areas. Two related observations suggest that the Iowa cordaitaleans were among the drought tolerant forms, and not the wetland types that are abundant in Iowa coals, as known from coal balls and palynology. They formed a gallery forest on a paleosol surface that formed under seasonally dry conditions. The location of these remains abundantly at the contact of the seat earth and coal bed indicates unequivocally that they were growing on the soil; this is almost certainly to have been prior to the initiation of peat formation. Emphasizing this latter conclusion is the form of permineralization, which is with calcite. Due to its sensitivity to pH, the calcite must have been precipitated prior to peat formation – it is not to be expected once organic matter, and associated organic acids, began to accumulate in the environment. Permineralized stumps projecting into the coal bed also indicate that permineralization was completed prior to peat formation around the tree remains; in addition, the hardness of the calcite, indicated both by the high polish the specimens take and by the slowness with which they etch in hydrochloric acid, indicates resistance to those weak acids, permitting the permineralized wood to survive without dissolution once peat formation began. To our knowledge, the paleobotanical significance of these fossils was not pursued further once they became known to professional paleobotanists, and no record of them has appeared in the scientific literature prior to this report. The mode of fossil preservation discussed here appears to be confined to a small area in Iowa.