ERWIN, DIANE M.* AND HOWARD E. SCHORN. Museum of Paleontology, University of California, Berkeley, CA 94720-4780. - FLORISTIC REVISION OF THE EOCENE THUNDER MOUNTAIN MEGAFLORA OF IDAHO.
The middle Eocene (46-45 Ma) Thunder Mountain flora of central Idaho
represents a rare, well-dated upland mixed conifer forest from North
America. As such, it has some of the earliest occurrences and
associations of Tertiary conifers and angiosperms in the US western
interior. Despite being newly published, the Thunder Mountain flora
contains numerous misidentifications, taxonomic and nomenclatural
problems that continue to misrepresent and obscure the fossil record
of many taxa. Preserved as compression/impressions the fossils are
from two sites: the upper Road Locality (lacrustrine shale) and the
stratigraphically lower Dewey Mine site (carbonized stream-borne
debris in coarse to finer-grained sandstone). Study shows the type of
Larix leonardii is a more likely a spruce, two axes identified
as L. leonardii are fern rhizomes, and the new species Pinus
baileyi, Abies deweyensis, and Chamaecyparis edwardsii are
fragmentary and at best identifiable to genus. Potamogeton is
an insect wing, the Dewey Mine Populus is five overlapping
conifer needles, Mahonia deweyensis is indistinguishable from
specimens referred to M. reticulata and M. simplex, and
Salix shows no natural margin with venation too poorly
preserved to describe. Spiraea idahoensis at Dewey Mine has a
toothed margin in the distal half of the leaf, the lower half entire,
yet the margin of the holotype of S. idahoensis is fully
toothed. Typha is a piece of bark and the Nymphaeites
root scar is an iron-stained sedimentary structure. The megafossil
occurrences of Pseudotsuga and Cephalotaxus are
equivocal. Although Thujopsis is not present in the Thunder
Mountain megaflora, there is a new undescribed cupressoid plant that
morphologically resembles the southern hemisphere Libocedrus in
the Road florule. We present these and other taxonomic changes that
more accurately reflect the floristic composition of the Thunder
Mountain flora, stressing accurate identifications are critical for
understanding the evolutionary and biogeographic histories of taxa.
Key words: Eocene, Idaho, paleobotany, taxonomic revision, Thunder Mountain