SIGNS OF LIFE ON AN 'ANOXIC' LAKE BOTTOM: FISH AND INVERTEBRATE FEEDING TRACES IN DEEP-WATER FACIES OF THE GREEN RIVER FORMATION (EARLY EOCENE), WYOMING USA

Trace fossils often supply snapshots of animal behavior confirming or contradicting previous interpretations based on body fossils alone. Feeding behaviors in particular are greatly clarified by trace fossils, especially when linked with specific tracemakers that interacted with their original ecosystems. One example of this was a combined swimming-feeding trace (Undichna cf. U. simplicatas) attributed to an adult teleost Notogoneus osculus Cope (Teleostei: Gonorynchidae). The specimen, recovered from a Fossil Lake deposit in the Green River Formation (early Eocene) near Fossil Butte National Monument (Wyoming), is composed of four undulating fin trails and a medial set of mouth marks. This trace fossil supports a long-held hypothesis that Notogoneus was adapted for bottom feeding, an interpretation based on its downwardly pointing mouth and exclusive occurrence in a laminated kerogen-rich micrite bed (the “18-Inch Layer”). High amounts of kerogen, a lack of burrows, and excellent preservation of fish body fossils in the 18-Inch Layer imply deep-water (~20 m) and anoxic conditions for Fossil Lake at time of its deposition. However, the swimming-feeding trace of an adult Notogoneus suggests it was oxygenated in that instance. Hence the discovery of a second trace fossil of Undichna, also credited to Notogoneus, and from the same stratum and same area as the previous specimen, supplies further evidence of at least temporary bottom oxygenation. This specimen, however, was made by a smaller (subadult) Notogoneus, indicating that immature bottom-feeding fish were not excluded from this bottom environment, either. Moreover, the trace fossil shares the same bedding plane with numerous small meandering trails, identifiable as the invertebrate trace fossil Cochlinchus. These traces of abundant epibenthic invertebrates (e.g., aquatic nematodes) feeding on the lake bottom also point toward at least occasionally oxygenated conditions in the deepest part of Fossil Lake.