SCHORN, HOWARD E.* AND DIANE M. ERWIN. Museum of Paleontology, University of California, Berkeley CA 94720-4780. - The impression record history and ecological diversification of Pseudotsuga Carriere (Pinaceae) in western North America during the later half of the Cenozoic.
Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] is the state
tree of Oregon. It is appropriate that the geologically oldest
macrofossil record of the genus is also from Oregon; the ~32 Ma Rujada
site southeast of Eugene. Revision of the North American impression
fossils indicate that only 7 of the 27 (26%) literature citations are
correctly assigned to Pseudotsuga. A previously unrecognized
specimen from Cartwright Ranch, Idaho is added. These eight
occurrences are all within the present geographic range of extant
Pseudotsuga in western North America. The genus is known from
Miocene and younger horizons of Japan. Records from Europe are
considered equivocal. The North American impression fossils can be
segregated into three forms on the basis of the size of the ovulate
cone and/or winged seed; large, intermediate and small. Based on
climatic analyses of the associated dicotyledonous paleofloras, these
three forms occurred in three different climatic settings. It is
inferred that these occurrences are expressions of ecologically driven
evolutionary trends. The earliest record, a large winged seed form
near extant P. macrocarpa from the early Oligocene of Oregon,
is inferred to be the least derived Pseudotsuga. It was
adapted to a mean annual temperature centered on ~13 °C, an
intermediate mean annual range of temperature centered on ~22 °C, with
~150 mm precipitation distributed through a seven plus month growing
season; this condition is typical for the bulk of the least modified
species of extant Northern Hemisphere conifers. Through time,
Pseudotsuga adapted to dryer, and both cooler or warmer,
climates. The large seed form adapted to dryer-warmer. An
intermediate-sized form from the late middle Miocene was dryer-cooler.
The small-sized P. menziesii type is known from the middle
Miocene on in dryer, but both cooler and warmer settings. The history
of Pseudotsuga viewed in the present framework demonstrates a
dynamic adaptive history.
Key words: diversification, Douglas-fir, history, paleoecology, Pseudotsuga