THE PALYNOLOGICAL ASSEMBLAGES FROM COURTLAND CLAY PIT, MINNESOTA

The Courtland Clay Pit, in southern Minnesota, is a mid-Cretaceous locality with pollen-rich sediments that provide palynological assemblages that can be used to analyze plant diversity during that time. Samples were collected from the lower massive dark gray clay and the upper sandy light gray clay and two distinct palynological assemblages were found. The palynological assemblage from the lower dark gray clay is characterized by Ceratosporites sp., Aequitriradites spinulosus, Classopollis sp., Foveotricolporites rhombohedralis, and Tricolpites vulgaris. There are 24 types of fern spores, 18 types of gymnosperm pollen, and 12 types of angiosperm pollen. Moreover, one type of algal spore is very abundant in this assemblage (about 15-30%) and rare dinoflagellate cysts are present. The palynological assemblage from the upper light clay is characterized by Laevigatosporites sp., Microfoveolatosporis pseudoreticulatus, Pristinuspollenites sp, and Foveotricolpites sp. There are 18 types of fern spores, 12 types of gymnosperm pollen and 6 types of angiosperm pollen. The algal spore acount for only 2% and dinoflagellate cysts are absent. The common elements which exist in both assemblages are Deltoidospora sp. Cyathidites australis, C. minor, Triporoletes reticulatus, Klukisporites sp., Verrucosisporites sp., Rugubivesiculites sp., Pityosporites sp., Araucariacites sp. and Taxodiaceaepollenites sp. There is no common angiosperm pollen in these two assemblages. Taxodiaceaepollenites sp. is very abundant (16%-43% of gymnosperm pollen) in the lower dark clay assemblage, but it accounts for only 2% in the upper light gray clay assemblage. Obviously, the plant diversity from the lower dark gray clay is greater than that of the upper light gray clay. This situation may reflect the importance of the control of sedimentary environments upon plant diversity recovered from the pollen and spore record rather than any aspect of plant evolution during the mid-Cretaceous.