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