FISHES FROM THE EOCENE GREEN RIVER FORMATION AROUND FOSSIL BUTTE, WYOMING: EFFECT OF SMALL VERSUS LARGE SAMPLE SIZE ON SPECIES RECOVERED AND PROPORTIONS OBSERVED

Fossil Butte in southwest Wyoming is world-renowned for its numerous Eocene fossil fishes. Investigators and quarriers working for over a century have excavated thousands of specimens now in museum and private collections. We accessed two additional suites, each resulting from one intense field-day by one professional paleontologist, and asked how well such short-term single-worker sampling would match the long-term multi-person results recorded in monographs like Grande '84 (Wyo Geol Surv Bull 63).

Collected well before establishment of the national monument here, one suite is from the western end of the Historic Quarries (H) up on the south face of Fossil Butte (center NW¼ NE¼ sec 6, T21N, R117W, Kemmerer 15' quad). The other, obtained during a GSA-organized trip, is from 4 mi (7 km) to the south, the Lewis Ranch Quarry (L; then leased commercially by J. Tynsky; SE¼ NE¼ NE¼ sec 30, same T-R-quad). Both suites are from the latest Early Eocene (50 MY BP) “18-inch layer” or F-1 bed, in the “middle unit” and high in the Fossil Butte Member, of the Green River Formation. The two localities differ slightly in fish size and lithology. The H fish average 5-6 cm in length, and are in cream-tan, soft, chalky, very thinly laminated, shaly limestone. The L fish are larger, 8-12 cm long, and in light-gray, hard, thin-bedded, slabby limestone.

Nearly 200 fossil fish were examined, identifiable from Grande's monograph. Six fish species were identified among our specimens. Two were abundant: Knightia eocaena (herring) and Diplomystus dentatus (archaic herring). Four were very rare: Eohiodon falcatus (moon-eye), Amyzon? sp. (sucker), Amphiplaga brachyptera (trout-perch), and Mioplosus labracoides (perch). One, though expected, was absent: Priscacara spp. (archaic bass).

The fish species identified, and their relative proportions, in our samples resembled those reported by Grande, a result encouraging to reconnaissance fieldwork in general, although our total number of specimens naturally was much smaller, and varied between suites. The principal differences were that we found fewer rare species than he recorded (an effect of our much smaller sample size), and that we found none of his common Priscacara spp.

Feeding habits suggested by previous workers for each of the Green River fish species can be integrated into a trophic/food web to show graphically the probable energy flow/pathways through the Fossil Lake paleocommunity.