Late Cretaceous angiosperm leaves from the Courtland clay pit, Minnesota, USA
Wang, Hongshan; Dilcher, David L.
published: Sep 22, 2009
This study reports on the results of an examination of about 1, 000 impressions of fossil plants from the Upper Cretaceous (Cenomanian) strata at the Courtland clay pit, southwestern Minnesota, USA. The plant bearing strata are thought to be equivalent to the Dakota Formation along the east side of the Western Interior Seaway. We have been able to identify 20 angiosperm leaf species in the collection of which fifteen are assignable to the modern orders Laurales, Magnoliales, and Proteales. One species is assigned to the modern family Platanaceae and one other species may be related to Trochodendraceae. Five new genera, i. e., Crepetia gen. nov., Gooleria gen. nov., Jarzenia gen. nov., Manchesterii gen. nov., and Setterholmia gen. nov. have been established in this report. A total of 15 new species and new combinations have been recognized during the research. They include: Crassidenticulum cracendentis sp. nov., Crepetia minudentis sp. nov., Dicotylophyllum carlsonii sp. nov., Dicotylophyllum coughlantii sp. nov., Dicotylophyllum leptovenum sp. nov., Gooleria crasseprima sp. nov., Jarzenia kanbrasota sp. nov., Liriophyllum siemii sp. nov., Rogersia dakotensis sp. nov., Credneria cyclophylla (Heer) comb. nov., and Dicotylophyllum tulipiferum (Heer) comb. nov., Manchesterii macrophylla (Lesquereux) comb. nov., Setterholmia deleta (Lesquereux) comb. nov., Setterholmia rotundifolia (Lesquereux) comb. nov., and Wolfiophyllum pfaffianum (Heer) comb. nov. Comparisons with the Cretaceous angiosperm leaf floras from Braun Ranch, Kansas, and Rose Creek, Nebraska indicates that (1) there is only a small amount of angiosperm species overlap between any two localities and (2) angiosperms dominate the three leaf assemblages while the gymnosperms and ferns dominate the palynofloras. Rapid environmental changes through time and the warm middle Cretaceous (Late Albian ± Early Cenomanian) paleoclimate may have played an important role in affecting this diversity pattern. The high diversity of angiosperm leaves at each locality illustrates the success and local abundance of angiosperms in coastal areas that were displacing gymnosperms and ferns during the Early and Late Cretaceous. Comparisons with the angiosperm leaf assemblages from the Dakota Formation at other localities and the Potomac Group indicates that the Courtland assemblage probably represents one of the more advanced leaf assemblages from the Dakota Formation along the east side of the Western Interior Seaway and equivalent units, even though some species still retain one or more primitive leaf architectural features. This research represents the second of a series of detailed studies of the Dakota flora from localities in Kansas, Nebraska, Iowa, and Minnesota by the authors of this report and others. The results of these works provide important new information clarifying the early diversity and evolution of the angiosperms during the middle of the Cretaceous (Late Albian - Early Cenomanian), a critical time during which angiosperms began a rapid adaptive radiation.