Strömberg, Caroline A.E. .
From subtropical forests to savanna: phytolith evidence concerning Tertiary vegetation change and grass evolution in the North American continental interior.
Despite intense study for over a century, the evolution of grasses and grasslands has remained a controversial topic, with competing scenarios based on various sources of data (faunas, paleosols, stable isotopes, macrofossil floras and palynofloras). This study used phytoliths (plant opal) preserved in Tertiary deposits from the continental interior of North America (Great Plains) to test hypotheses of vegetation change in association with the spread of grass-dominated habitats. A total of 99 Eocene to Miocene phytolith assemblages from Montana, Nebraska, Wyoming, and Colorado, forming a latitudinal transect through the Great Plains, were extracted from sediment and studied. A comprehensive reference collection of phytoliths from modern plants was described and analyzed quantitatively, forming the basis for interpretation.
Phytolith analysis showed that Eocene-Oligocene vegetation in the North American continental interior consisted of closed forests or woodlands with an understory of primarily bambusoid grasses (central Great Plains) or other potentially closed-habitat grasses (northern Great Plains). The variation in composition of the grass communities and in palm abundance indicated that a latitudinal and/or altitudinal gradient had developed in the continental interior by the late Eocene. Savanna-like habitats, dominated by open-habitat grasses (pooids, PACCAD grasses), spread in the earliest Miocene (>22 Ma) in the central Great Plains and somewhat later in the northern Great Plains. This is markedly earlier than what had been inferred from faunal (late early Miocene) and paleobotanical (middle Miocene) data.
Diagnostic phytoliths could further be used to suggest minimum dates for major cladogenetic events within Poaceae, which had previously been dated to 25-15 Ma (GPWG, 2001). Thus, the presence of Chusquea-type phytoliths in late Eocene vegetation and rare pooids and PACCAD grasses in the early Oligocene implied that the major clades of grasses, the PACCAD group, Pooideae, and the clade encompassing Bambusoideae and Ehrhartoideae, had diverged by 35 Ma.
1 - Swedish Museum of Natural History, Department of Palaeobotany, Svante Arrhenius väg 7, P.O. Box 50007, Stockholm, SE-104 05, Sweden
Presentation Type: Paper
Location: Maybird (Cliff Lodge)
Date: Wednesday, August 4th, 2004
Time: 8:45 AM