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Late Cenozoic Grasses

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Late Cenozoic Grasses and Other Angiosperms from Kansas, Nebraska, and Colorado: Biostratigraphy and Relationships to Living Taxa

By Joseph R. Thomasson

Originally published in 1979 as Kansas Geological Survey Bulletin 218. This is, in general, the original text as published. The information has not been updated.


Fossil angiosperm anthoecia, achenes, nutlets, and endocarps (collectively but incorrectly called "seeds" in the literature) were collected from late Tertiary deposits of Kansas, Nebraska, and Colorado, but especially at 32 sites in Ellis County, Kansas. Fossil reproductive and some vegetative material of the following taxa were recovered: Gramineae: Berriochloa (=Stipidium), Graminophyllum, Nassella, and Panicum; Cyperaceae: Eleofimbris gen. nov.; Ulmaceae: Celtis; Boraginaceae: Biorbia, Cryptantha, Eliasiana gen. nov., Prolappula gen. nov., and Prolithospermum. More than 25% of the taxa identified at the species level had not been previously described.

Fossils were compared to living forms with special emphasis on genera of the grass Tribe Stipeae. Species of the living genera Oryzopsis, Nassella, Piptochaetium, and Stipa were examined. Studies consisted of scanning electron and light microscopy of the epidermal features of the lemma of the living grasses and their fossil ancestors. The results permit the postulation of basic phylogenetic trends in the Stipeae and a more accurate interpretation of the relationships among modern and fossil Stipeae.

Deposits from which the fossils have been collected consist mainly of fluvial silts, sandstones, and conglomerates. Sediments also present in the study area, but from which very few fossils have come, include freshwater limestones, clays, and volcanic ashes. Traditionally, the majority of these deposits have been assigned to the Ogallala Formation, but present studies leave this open to question.

Diagnostic occurrences of fossils are used in the local correlation of deposits. A new assemblage zone is proposed to replace, in part, previously established "floral" zones. The new assemblage zone is characterized by the borage Prolithospermum johnstonii and the grass Nassella pohlii. Local correlations are also based on distinctive rock units.

The age of the main body of the studied deposits is considered to be late Miocene-early Pliocene. This was determined from mammalian faunas that were discovered during this study, as well as those previously reported in the literature.

Paleoclimates and paleoecology inferred from the faunas and floras indicate the existence of subhumid and subtropical savannas or savanna parklands, with temperatures rarely, if ever, dropping to 0° C or exceeding 38° C. No evidence has been found for the existence of the treeless grasslands, semiarid conditions, and extremes of temperature that currently characterize the region.

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Kansas Geological Survey, Geology
Placed on web May 1, 2009; originally published September 1979.
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