Seeds of the late Tertiary prairie grasses are fairly common throughout the most arid parts of the Prairie States. The most common among these fossil seeds show close relation to the most typical modern prairie grass: the spear-grass or Stipa, which has world-wide distribution. The oldest known ancestor of this grass was found in the Harrison formation of lower Miocene in western Nebraska.
Another prairie grass, a millet (Panicum), and one of its kin (Setaria) appeared here only in the Pliocene, when the prairie was also invaded by numerous borage herbs, ancestors of modern Lithospermum, Krynitzkia, and Lappula, which are now also very common among the forbs of the plains.
The preservation of the protective covers of the fossil seeds is so nearly perfect that some fine morphologic details, previously undescribed for Stipeae, were for the first time detected on the fossil Stipidium and subsequently on some species of its living descendant, Stipa.
Comparative study of the fossil and living forms reveals evolutionary trends of the seeds of the prairie grasses. The rather small and generalized Miocene ancestor gave rise to greatly diversified Pliocene and Recent species. The seeds of these include small and very large, very slender, and very stout forms, all of them variously adapted for protection against drought and for more efficient dispersal.
Abundance, good preservation, and rapid ecologic and evolutionary changes make grass seeds the best index fossils for subdivision of the continental late Tertiary rocks.