A RE-EVALUATION OF THE TAXODIACEOUS TAXA PRESENT IN A LATE PALEOCENE DEPOSIT IN CENTRAL NORTH DAKOTA: A SCANNING ELECTRON ANALYSIS OF MORPHOLOGY AND ANATOMY

The Sentinel Butte Formation near Almont, North Dakota contains a rich and diverse fossil flora assemblage of the Late Paleocene. Exceptionally well-preserved plant material from this location provides a great opportunity to study the time interval between the Cretaceous-Tertiary extinctions and the Paleocene-Eocene climatic transitions. The fossils are found in a hard, brown, fine-grained siliceous shale exposed only in excavation pits in mounds on agricultural land. Although the flora has been well documented for its angiosperm content, the specific composition of gymonosperms, particularly conifer taxa, is ambiguous and warrants review. Previous studies have noted the presence of Taxodiaceous needles but workers disagree over which genus is represented or whether more than one may be present.

During the summer of 2004, new specimens of conifer remains were collected at the Almont fossil locality. Hand samples were removed from the bedrock using a pry bar and pick axe. At least six morphologically distinct shoot types were identified in a collection of 48 specimens, recovered over the course of three days. The fossils and reference specimens of modern Metasequoia and Taxodium were studied under a scanning electron microscope to note similarities and differences between morphotaxa. Needle shape, cell structure and stomatal patterns were used as distinguishing characteristics.

Results indicate the presence of more than one conifer taxon present in the Almont fossil flora based on dissimilar morphological and anatomical features of the analyzed needles. At least one fossil specimen has ribbon-like midrib cell alignment similar to modern Taxodium, though elongate ripple-margined epidermal cell structure and parallel lengthwise orientation of oval stomata are like that of modern Metasequoia, implying a closer affinity to this genus. Such evidence suggests that the recent assignment of the needles to the genus Parataxodium may not be valid for all taxa represented at this site. These findings could have implications towards the current understanding of the paleoenvironment of the area, as well as, the relationships between restricted floras of the Great Plains of the United States during the Late Paleocene.