MID-CRETACEOUS CONIFERS OF THE DAKOTA FORMATION, KANSAS

The plant assemblages of the Dakota Formation are an well known subject of paleobotanical research since the late 19th Century, when the first paleontological studies were undertaken by Lesquereux (1874a, b, 1883, 1892). During the 20th Century, the accent was put on the systematics, taxonomy, phytostratigraphy or paleoecology of angiosperms, while the pteriodphytes and gymnosperms were treated almost always as secondary subjects. This is why the knowledge on the Dakota conifers is still incomplete and the conifer assemblages need a new approach. This paper focuses mainly on the results of systematic and taxonomic studies undertaken on fossil conifers collected from the Dakota Formation. The fossil material consists of compressions, impressions and various ironstone casts. Some of the detail preserved is excellent and the fossils preserved as compressions in clay can be prepared for cuticular analysis. Almost all types of conifer organs are recorded, such as trunks, branches, leaves, which represent the most of the material, and cones, both female, with or without seeds, and male. The Family Cupressaceae, to which we include species assigned formerly to the Family Taxodiaceae, is the most diverse group of conifers, are represented by species belonging especially to the genera Sequoia, Widdringtonia and cf. Moriconia. The Family Pinaceae has few species of Pinus and Abietites while the Family Cheirolepidiaceae is the least represented, having only one genus, Brachyphyllum, (a form genus of uncertain affinities). Incertae sedis conifers are numerous as well, with species belonging to Podozamites, Dammarophyllum and conifer leaves formerly assigned to Phyllocladus. The dominance of Cupressaceae and Pinaceae over any other gymnosperms and the gradual extinction of the Cheirolepidiaceae is related to the floristic changes that occurred during the Albian, Cenomanian and Turonian time intervals in North America. While this change in the assemblages of conifers was taking place it is conceivable that this floristic change had a profound effect from the evolution and diversity boom of angiosperms recorded at this time.