Paleoeriocoma (Gramineae, Stipeae) from the Miocene of Nebraska: Taxonomic and Phylogenetic Significance

Abstract

Anthoecia (lemmas and paleas) of the fossil grass Paleoeriocoma hitch- cockii Elias from the Late Miocene Ash Hollow Formation of Nebraska and certain living species of Oryzopsis are similar in micromorphological and macromorpholog- ical features. This evidence indicates that Paleoeriocoma hitchcockii stock is ancestral to species of Oryzopsis of the North American sections Eriocoma and Oryzopsis (=Euoryzopsis) and possibly some species of Stipa. The occurrence of Nassella and Paleoeriocoma together in the deposits indicates that the genus Nassella has been distinct from the genera Oryzopsis and some Stipa since at least the late Miocene. Attempts to trace the phylogeny of the grasses have been hampered by the limited or inadequately studied fossil record of this group (Thom- asson 1980b). With the exception of the classical studies of Elias (1942), the evolutionary history of the grasses has been interpreted primarily from modern evidence such as cytology, morphology, and geographic distribution (Avdulov 1931; Johnson 1945; Hartley 1958, 1973; Stebbins 1956, 1972, 1975; Tsvelev 1974). Recently, I have combined evidence from both fossil and modern sources in interpreting the phylogeny of some grasses (Thomasson 1978a, b, 1979a, 1980c). I have shown that the epidermal patterns of the lemma in the fossil and modern taxa of the Stipeae reflect evolutionary trends dating from the late Miocene (Thomasson 1978b). Living (1) and fossil (f) genera that I have studied