A moment frozen in time – fossil fish trace solves paleontological puzzle

One of the fossil fish I found in the Green River Formation of Wyoming.


I had my doubts about whether we were going to reach the quarry. The Toyota Yaris my wife and I had rented for our excursion through Utah and Wyoming was not designed to handle the rough dirt roads which wound their way through the grassy hills of the Equality State, but eventually the outcrop of grey-and-yellow rocks came into view. It was part of the famous Green River Formation, an approximately 42-53 million year old slice of earth’s history known to be rich in fish fossils.

It did not take long to start finding what we were after. Almost immediately after we started splitting the shale left over in the quarry’s spoil pile we began to find the rust-colored skeletons of small fish which had long ago settled to the bottom of an Eocene lake (though, to be honest, that afternoon’s activities yielded far more fossil fish feces than fish). These quarries are so chock-full of these fossils, in fact, that fish quarried from them can be found at just about every gas station store in the area, yet despite the familiarity people have with this formation it still holds many secrets. In 2008, for example, scientists announced the discovery of a new fossil bat Onychonycteris finneyi from the Green River Formation, and a paper published this week in the journal PLoS One reports on another exceptional find from this formation.

The fossil fish Notogoneus osculus. From Martin et al. 2010.

For many people the word “fossil” translates to “really old bone”, but skeletal remains are only one type of fossil. Many organisms have also left behind signs of their activities in the form of footprints, burrows, or other impressions, and these specimens are one form of a broader category of what are called trace fossils. While they may not inspire the awe of an articulated skeleton they are especially interesting because they are records of fossilized behavior, and, in the case of the fossil described in the new PLoS One study, they can sometimes tell us about the environment in which extinct creatures lived.

Among the many fish species present in the Green River Formation deposits in the vicinity of Wyoming’s Fossil Butte National Monument is Notogoneus osculus, but this fish is unique for two reasons. It is restricted to a single deposit called the “18-inch layer”, and despite the fact that this bed has been interpreted as coming from the deep part of a lake with an anoxic bottom hostile to organisms, Notogoneus osculus has a downturned mouth which would have suited it to feeding off the lakebottom. This seeming mismatch between environment and adaptation has puzzled paleontologists, but the trace fossil FOBU-12718 suggests that this environment might not have always been as hostile as has been supposed.

The trace fossil FOBU-12718. The bottom photograph includes color-coded dots to denote which part of the fish made each part of the trace. From Martin et al. 2010.

Found by a private fossil collector and donated to Fossil Butte National Monument, FOBU-12718 superficially looks like a set of sinusoidal waves in the rock, with two running in parallel to each other and one of a different frequency in the middle. Paleontologists have found similar traces before, given the name Undichna, and this particular specimen appears to have been made by a swimming fish. The grooves along the outside were probably made by pectoral fins of the fish, with the higher-frequency wave made by the sweeps of its caudal fin, and upon close inspection a series of feeding marks running straight down the middle of these traces can be seen. Altogether the traces indicate the feeding of a fish with a downturned mouth, probably Notogoneus osculus given its occurrence in the layer, its size, and its anatomy – a bit of behavior preserved in stone. It is rare that scientists are able to specifically identify the creatures which have left trace fossils, so the ability of the authors of the new study to do so is quite exciting, indeed.

This discovery is more than just a paleontological curiosity, however. The pattern seen on the slab suggests that Notogoneus osculus grazed along the bottom of the lake in the way loaches and “sharks” do in home aquaria, and the existence of these traces means that there was at least one short period of time when the relatively deep bottom of this ancient lake was habitable by fish. Further studies will be needed to better understand the paleoecology of the lake, but based upon this new fossil evidence it appears that conditions along the bottom were not permanently inhospitable.

A restoration showing how Notogoneus osculus could have made trace fossil FOBU-12718. From Martin et al. 2010.

Even better, specimen FOBU-12718 is not the only fish-fin trace to have been found from these deposits. The authors promise to cover some of the others in future studies, and with any luck more will be recovered as excavations in the area continue. Now that paleontologists have a better idea of what to look for specimens which may have been overlooked or discarded can now be understood for what they are, and as these traces are collected and studied we will be better able to understand the lost world of the Green River Formation.

For more about this new discovery, see Andy Farke’s interview with Anthony Martin, the lead author of the paper, at the Open Source Paleontologist (Part 1, Part 2).

Martin, A., Vazquez-Prokopec, G., & Page, M. (2010). First Known Feeding Trace of the Eocene Bottom-Dwelling Fish Notogoneus osculus and Its Paleontological Significance PLoS ONE, 5 (5) DOI: 10.1371/journal.pone.0010420

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