Miocene Fossil Grasses: Possible Adaptation in Reproductive Bracts (Lemma and Palea)

Abstract

Fossil reproductive bracts of Berriochloa, Nassella, and Paleoeriocoma of the tribe Stipeae, Archaeoleersia of the tribe Oryzeae, and Panicum of the tribe Paniceae (Gramineae) were collected from late Oligocene-Miocene strata in central North America, examined, and compared with modem taxa. One objective of these studies was to elucidate the evolution and possible adaptive significance of bract features. Berriochloa, the oldest known grass, appears in late Oligocene-early Miocene strata, where it has an indurate, cylindrical anthoecium (lemma and palea collectively) and an elongated, pointed callus. From early Miocene forms of Berriochloa two mid-late Miocene lineages evolved, one retaining a cylindrical anthoecium and the other developing a prominently inflated anthoecium. Both lineages retained the elongated, pointed callus. Paleoeriocoma, Nassella, Archaeoleersia, and Panicum first appear in the late Miocene with anthoecia similar to those of living taxa. The indurate anthoecium of the fossil and related living grasses probably evolved primarily as an adaptation to mammals and insects that use grasses as food. Related living taxa show adaptive mechanisms similar to those of the fossils, although some features such as the strongly interlocking bracts and blunt callus of some species of Piptochaetium are a post Miocene development. The Tertiary deposits of central North American Plains have produced rich and varied floras of considerable paleobotanical interest (Stansbury, 1852; Engelmann, 1876; Cockerell, 1914; Berry, 1928; Elias, 1931, 1932, 1934, 1935, 1942; Frye et al., 1956, 1978; Leonard, 1958; Leonard F Frye L Segal, 1965, 1966a, 1966b, 1966c; Skinner et al., 1968; Galbreath, 1974; Diffendal et al., 1982; Voorhies & Thomasson, 1979). Grasses in these floras are found as silicified reproductive bracts or as remains of leaves, stems, and roots and frequently exhibit detailed epidermal features that aid in determining phylogenetic relationships of fossils and their modem counterparts (Thomasson, 1976, 1977, 1978, 1979, 1980a, 1980b, 1984). Fossils described include reproductive bracts assigned to Archaeoleersia of the tribe Oryzeae, Berriochloa, Nassella, and Paleoeriocoma of the tribe Stipeae, and Panicum of the tribe Paniceae and leaf fragments assigned to the subfamilies Festucoideae and Arundinoideae. This paper presents results of my studies of the reproductive bracts (lemma and palea) or bract (lemma) enclosing the mature grain or caryopsis of fossil grasses and their related living taxa. It reviews the geologic and paleoecologic background, summarizes the morphologic features of the fossil and living grasses, examines evolutionary trends among the fossil grasses, and speculates on the adaptive significance of features seen in fossil and living grasses. GEOLOGIC AND PALEOECOLOGIC BACKGROUND The late Tertiary (Oligocene-Pliocene) strata of the central part of North America are a widespread group of continental sediments extending from North Dakota and Wyoming to Texas and New Mexico that were deposited principally in fluvial and aeolian environments. They consist of a large variety of sediments including clays, silts, sands, conglomerates, freshwater diatomites, and volcanic ashes and vary in thickness in individual sections from more than 200 m to 1 m. Although some early studies suggested rather simple models of deposition and biostratigraphy, more recent investigations have demonstrated their depositional and biostratigraphic complexity (Bart, 1975; Breyer, 1976, 1981; Skinner et al., 1968, 1977; Swinehart, 1979; Thomasson, 1979; Diffendal, 1980, 1982). Among the most richly fossiliferous Tertiary strata in the world, they contain assemblages of 1 This paper is one result of some studies that have been supported by the National Geographic Society (188379, 2197-80) and the National Science Foundation (DEB-7809150, DEB-800228 1, DEB-820468 1, CDP-8000918, and R1 1-8213915). I thank numerous colleagues and assistants who contributed their time and expertise in the field during these studies and N. Morin, P. Palmer, and anonymous reviewers for helpful suggestions that improved this paper. 2 Department of Biological Sciences, Fort Hays State University, Hays, Kansas 67601-4099. ANN. MISSOURI BOT. GARD. 72: 843-851. 1985. This content downloaded from 157.55.39.215 on Sun, 24 Jul 2016 04:34:41 UTC All use subject to http://about.jstor.org/terms 844 ANNALS OF THE MISSOURI BOTANICAL GARDEN [VOL. 72 TABLE 1. Stratigraphic relationships and ages of late Tertiary strata and study sites in the plains of central