Floristic composition and comparison of middle Eocene to late Eocene and Oligocene floras in North America

Abstract

In comparison to the early and middle Eocene, the late Eocene and particularly the Oligocene floral record is sparse in North America. Changing tectonic, environmental and climatic conditions during these times resulted in the development of fewer depositional systems favorable for fossil preservation. Floras are known from the Southeast, the Pacific Northwest and the Rocky Mountains. Each area has a distinct geological history that shaped both the vegetation adjacent to sites of deposition as well as the depositional environments themselves. The floristic change from middle to late Eocene, and then to Oligocene reflects a changing paleoclimate from the thermal maximum to cooler and drier conditions in the late Paleogene. In the present paper, major middle, and then late Eocene and finally Oligocene floras of North America are summarized, with an emphasis on their regional geology, depositional setting, paleoclimate and significant floral elements. The North American occurrences of coryphoid palms (Sabal) and cycads are reviewed in relationship to their biogeographic history. Finally, we suggest several directions for future research that will further illuminate the floristic changes from middle, to late Eocene and Oligocene that occurred in North America.•

A review of paleobotanical studies of the Early Eocene Okanagan (Okanogan) Highlands floras of British Columbia, Canada, and Washington, USA

“…1). Within this regional uplift is a series of grabens and half-grabens that preserve fossiliferous lacustrine shale and coal deposits throughout its geographic extent Archibald et al 2011;DeVore and Pigg 2010). Interest in the plant fossils found in this region extends back to geological exploration of the Pacific Northwest of both Canada and the United States in the 1870s Archibald et al 2011).…”

“…Coastal assemblages of similar age include many tropical to subtropical elements that grew under warm climates (Rouse 1962;Mustard and Rouse 1994). These include the localities at Appian Way on Vancouver Island (Mindell et al 2009) and Burrard Inlet in British Columbia (Rouse 1962;Mustard and Rouse 1994) and the Chuckanut Formation (Mustoe and Gannaway 1997;Breedlovestrout et al 2013), the Swauk Formation (DeVore and Pigg 2010), and the Puget Group in Washington (Wolfe 1968;Burnham 1990Burnham , 1994. In contrast, the Okanagan Highlands floras are characterized by plant assemblages that include a dominant temperate component (Rouse 1962;Johnson 1996;Greenwood et al 2005).…”

The first Cenozoic spinicaudatans from North America

“…The described flora contains 94 species belonging to 66 genera, including both dicots and conifers, representing a mixed forest biome (Wing, 1987;Graham, 1999;DeVore and Pigg, 2010). In addition to the spinicaudatans, the invertebrate component of the fauna includes insects (including caddisfly larval cases), freshwater gastropods tentatively assigned to Planorbidae, the bivalve Sphaerium sp., and several species of ostracodes (Becker, 1969;Lewis, 1972).…”

“…The Medicine Lodge Formation and associated sedimentary units within the Beaverhead Basin include interbedded thinto-thick shales and sandstones, including a basal conglomerate and a localized coal layer near the top, with noticeable vertical and lateral variations (Scholten et al, 1955;Becker, 1969;DeVore and Pigg, 2010;Lielke et al, 2012). These facies represent fluvial, lacustrine, fan-delta, and paludal facies (M'Gonigle and Darymple, 1996) that were deposited within a structural basin bounded by the hanging wall of a Paleogene low-angle fault on the west and the low-angle normal fault to the east (Scholten et al, 1955;Becker, 1969;DeVore and Pigg, 2010;Lielke et al, 2012).…”

“…These facies represent fluvial, lacustrine, fan-delta, and paludal facies (M'Gonigle and Darymple, 1996) that were deposited within a structural basin bounded by the hanging wall of a Paleogene low-angle fault on the west and the low-angle normal fault to the east (Scholten et al, 1955;Becker, 1969;DeVore and Pigg, 2010;Lielke et al, 2012). Based on paleobotanical data (e.g., Scholten et al, 1955;Becker, 1969;DeVore and Pigg, 2010;Lielke et al, 2012), the basin was estimated to have an elevation of~1600 m above sea level. Becker (1969) described three florules from Paleogene basinlake deposits in Beaverhead County, Montana: the Christensen, Horse Prairie, and Medicine Lodge florules.…”