ANATOMICALLY PRESERVED ISOETES FROM THE PALEOCENE OF NORTH DAKOTA: UTILITY IN ASCERTAINING PALEOHABITAT

The quillwort, Isoetes L. is a typically small, often aquatic-to-semiaquatic pteridophyte that has a long lineage of ancient relatives spanning much of the plant fossil record. The oldest unequivocal record for the genus is Isoetites rolandii Ash and Pigg from the Jurassic of North America which exhibits well defined lacunate leaves. Tertiary members are best represented in the Paleocene and Early Eocene of North America. Several occurrences were recorded from the Rocky Mountains by Roland Brown in the 1960s, with others known from Joffre Bridge (Paleocene, Alberta), Golden Valley (Early Eocene, North Dakota), and Dennison Gap (Early Eocene, Wyoming). These fossils are known from shales and provide general features of plant morphology. One of the most obvious vegetative features is the shape of the sporophylls with their characteristic pillow-like air chambers or lacunae that often appear as small boxes on the surface of the leaf. These lacunae mark a synapomorphy that unites all extant members of Isoetes. Recently discovered silicified Isoetes remains in the Late Paleocene Beicegel Creek flora of western North Dakota provide a significant touchstone for better understanding the anatomy of these vegetative features. Comparison of anatomical remains with compression fossils is valuable in looking at several important morphological features including: 1) size and shape of corm; 2) attachment and branching of roots; 3) morphological features of leaves; and 4) megaspore features. In particular we would like to address the utility of morphological features of the materal for ascertaining potential habitat preferences (aquatic, semiaquatic, terrestrial) of isoetalean taxa.