Cenomanian Angiosperm Leaf Megafossils, Dakota Formation, Rose Creek Locality, Jefferson County, Southeastern Nebraska
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The leaves described in this report comprise the first assemblage of early flowering plant leaf megafossils to be accorded formal systematic treatment using modern methods of foliar architecture and cuticular anatomy. The 20 species of dicotyledonous leaves are described from a new locality in the Dakota Formation of Nebraska, near Rose Creek (Rose Creek locality). Seventy percent of the species represent either subclass Magnoliidae or forms with a similar grade of foliar architecture. Among Magnoliidae, species are assignable to the orders Magnoliales and Laurales, and one species shows resemblance to Illiciales. Although three species have strong similarities to one extant family (Lauraceae, or the laurel family), most species of Magnoliidae or magnoliid-grade foliage possess generalized features or combine the characteristics of two or more extant families. One species possesses unique features of foliar architecture that represent an unsuccessful 'experimental" design. Two species of Rosidae are present in the flora, and these combine features of foliar architecture that today are restricted to either compound-leaved or simple-leaved families. The leaves examined in this study show little evidence for fragmentation prior to burial and are preserved in a sequence of rooted mudstones containing brackishwater bivalves (including one specimen in life position); thus, the leaves represent predominantly local, brackish-water vegetation. This indicates that flowering plants evolved the ability to tolerate greater-than-freshwater salinities by the Cenomanian. The vegetation represented by the Rose Creek leaf remains shows few similarities to modern mangrove swamps but instead is most analogous to brackish-water swamps that occur inland from mangrove swamps.