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The oldest conifer compressions and permineralized remains from North America, which are assignable to Walchia Sternberg, are found in strata of Westphalian C and D ages in the central Colorado trough of Colorado, the Nemaha highlands of Oklahoma, and the central Appalachian basin. These early conifer occurrences are consistent with dry conditions in Colorado and less dry or wet-dry, better drained, more oxidizing upland conditions in the central Appalachian basin, possibly tectonically controlled, which may have been a prelude to a widespread climatic change in Stephanian or Permian time in North America. -Authors
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Earliest conifers of North America: upland and/or paleoclimatic indicators?