Paleoecologic, paleoclimatic, and evolutionary significance of the Oligocene Creede flora, Colorado

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Abstract
Application of multivariate statistical techniques, especially correspondence analysis, results in the recognition of four major communities for the Creede plant assemblages: fir-spruce forest, fir-pine forest, pine-juniper forest or woodland, and mountain mahogany chaparral. Physiognomy of the Creede assemblages indicates a mean annual temperature of <2.5°C. Paleoaltitudinal estimates based on this temperature estimate are inconsistent with physiographic and structural data of post-depositional uplift, unless the climatic effects of a closed basin are factored in; these effects imply that the Creede paleotemperature (and hence paleoaltitude) is applicable to the caldera rim. A variety of data indicate that summers were dry and most precipitation occurred as snow. The stratigraphic sequence of assemblages indicates that significant precipitational change occurred during deposition of the Creede plant-bearing beds. Chamaebatiaria, a shrub that now inhabits dry, open environments, had an ancestral taxon characteristic of the Creede fir-forest communities and strongly indicates a major shift in habitat for this lineage during the Neogene. A second genus, Luetkea, which is now herbaceous, is represented by a probable woody ancestral taxon that was common in the Creede forest communities. Consideration of the adaptive and morphologic histories of the Creede lineages suggests that physiologic adaptation may precede morphologic change. The Creede forest communities have no modern homologues.