First Known Feeding Trace of the Eocene Bottom-Dwelling Fish Notogoneus osculus and Its Paleontological Significance

Anthony J Martin,Gonzalo M Vazquez-Prokopec,Michael Page,Andrew Allen Farke

doi:10.1371/journal.pone.0010420

Abstract

BackgroundThe Green River Formation (early Eocene, about 42–53 Ma) at and near Fossil Butte National Monument in Wyoming, USA, is world famous for its exquisitely preserved freshwater teleost fish in the former Fossil Lake. Nonetheless, trace fossils attributed to fish interacting with the lake bottom are apparently rare, and have not been associated directly with any fish species. Here we interpret the first known feeding and swimming trace fossil of the teleost Notogoneus osculus Cope (Teleostei: Gonorynchidae), which is also represented as a body fossil in the same stratum.Methodology/Principal FindingsA standard description of the trace fossil, identified as Undichna cf. U. simplicatas, was augmented by high-resolution digital images and spatial and mathematical analyses, which allowed for detailed interpretations of the anatomy, swimming mode, feeding behavior, and body size of the tracemaker. Our analysis indicates that the tracemaker was about 45 cm long; used its caudal, anal, and pelvic fins (the posterior half of its body) to make the swimming traces; and used a ventrally oriented mouth to make overlapping feeding marks. We hypothesize that the tracemaker was an adult Notogoneus osculus.Conclusions/SignificanceOur results are the first to link a specific teleost tracemaker with a trace fossil from the Green River Formation, while also interpreting the size and relative age of the tracemaker. The normal feeding and swimming behaviors indicated by the trace fossil indicate temporarily oxygenated benthic conditions in the deepest part of Fossil Lake, counter to most paleoecological interpretations of this deposit. Lastly, our spatial and mathematical analyses significantly update and advance previous approaches to the study of teleost trace fossils.

Highlights

The Green River Formation, a semitropical lacustrine deposit in the western U.S, is worldfamous for its exquisitely preserved fossil-fish assemblage, in the area of Fossil Butte National Monument in Wyoming, USA [1], [2], [3], [4], [5]
Identification of the Tracemaker and Its Behavior Specimen FOBU-12718 is interpreted as a compound swimming and feeding trail made by a bottom-dwelling teleost, Notogoneus osculus, on the basis of interrelated qualitative and quantitative criteria, including the stratigraphic co-occurrence of the trace fossil with body fossils of N. osculus in the 18-inch Layer
The single, high-amplitude trail is ascribed to the caudal fin, which was formed as an incision of the sedimentary surface by the ventral of the fin as it moved the fish along the bottom

Summary

Introduction

The Green River Formation (early Eocene, about 42–53 Ma), a semitropical lacustrine deposit in the western U.S, is worldfamous for its exquisitely preserved fossil-fish assemblage, in the area of Fossil Butte National Monument in Wyoming, USA [1], [2], [3], [4], [5]. 23 species of fish are identified from Fossil Lake strata One of these fish species, Notogoneus osculus Cope (Teleostei: Gonorynchidae), is the only one restricted to a single fossiliferous bed (F-1 of 1), colloquially called the ‘‘18-inch Layer.’’ N. osculus, the type species for the genus, is notable for its ventrally oriented mouthparts, a rare anatomical trait among Green River Formation teleosts, supporting its interpretation as a bottomfeeder [1], [5]. The Green River Formation (early Eocene, about 42–53 Ma) at and near Fossil Butte National Monument in Wyoming, USA, is world famous for its exquisitely preserved freshwater teleost fish in the former Fossil Lake. We interpret the first known feeding and swimming trace fossil of the teleost Notogoneus osculus Cope (Teleostei: Gonorynchidae), which is represented as a body fossil in the same stratum

Methods

Results

Discussion

Conclusion