What can fossils tell us about the rock surrounding them? Fossil scallops in the Coquille River as a case study

During a benthic survey off the Coquille River, Oregon, in September 2013, ANAMAR was collecting samples of epifauna using a 12-foot otter trawl when suddenly the gear encountered unidentified rock.  The trawl net snagged and the cable instantly snapped, losing the gear on the seafloor in about 45 feet of water.  Although many attempts were made to recover the trawl using a grapple hook off the deck of the survey vessel (R/V Pacific Storm), the gear was too entangled on the seafloor to be brought up with that method.  Directly following completion of the benthic survey, an ANAMAR subcontractor returned to the site and recovered the trawl gear using SCUBA divers.  The trawl was still in good shape and the remaining trawl tows were completed for the survey.  In addition to finding the trawl gear, the divers also observed several fossil scallop shells embedded in the rock on the seafloor.  The fossil scallops were in excellent condition (see images below).  The divers were able to pry a few of the fossil shells loose for closer inspection and photography.

Fossil Scallops Coquille Pic1

Because the area where the survey took place is an ocean dredged material disposal site (ODMDS), information on the naturally occurring rocks found there is of interest to agencies tasked with managing the site (U.S. Army Corps of Engineers and the U.S. Environmental Protection Agency).  For this reason, and also out of personal interest, I began collaborating with paleontologists to determine the identity of the fossil scallops in the hopes of learning more about the rock they were found in.  I soon found my answer after contacting specialists at the Burke Museum of Natural History in Seattle, Washington.  Dr. Elizabeth Nesbitt, Curator of Paleontology, graciously identified the fossil scallops as either Patinopecten coosensis or P. oregonensis based on photos I sent her.  The flared portions of the shell adjacent to the hinge (called auricles) serve as key characteristics differentiating these two species.  These fossils lacked auricles so they could not be identified beyond these two species.  However, based on the fossils and the associated matrix, Dr. Nesbitt was able to identify the rock formation the fossils were found in!

The rocks and fossils are part of the Empire Formation which is better known from exposures about 20 miles south of the Coquille River, at Cape Blanco, Oregon.  The Empire Formation, composed mostly of sandstone, along with the fossils it contains, are as old as 12 million years (Miocene) but it is theorized to be closer to 8 to 5 million years (Miocene-Pliocene epoch boundary).  Since we know the identity of the rock as being part of the Empire Formation, we therefore know something about its composition.  In this case, the rocks that snagged the trawl gear must have been composed of sandstone and some siltstone.  This formation represents sands deposited in what was then a small marine basin, which now is only represented by Coos Bay.  It is probable that other rocks within the ODMDS are also fossiliferous sandstone/siltstone from the Empire Formation.

The above is an example of how fossils can help us infer the identity of the surrounding substrate.  In this case, the identity of the fossil scallops, along with the matrix attached to the fossils, were used to pinpoint the exact formation they represent.  Knowing the formation, we then were able to learn more about the composition and approximate geological age of surrounding rocks that represent the same formation.  All this information came from observing and collecting a handful of fossils incidental to recovering of some equipment from the seafloor!

Interestingly, the French word for scallop is Coquille.  Thus, the Coquille River, where the fossils were collected, was actually named after a scallop!


Ehlen, J.  1967.  Geology of state parks near Cape Arago, Coos County, Oregon.  The Ore Bin 29(4):61–82. 

Nesbitt, E.  Department of Paleontology, Burke Museum of Natural History, University of Washington, Seattle, WA.  Pers. comm. 12/06/13.

Portell, R.W.  Department of Invertebrate Paleontology, Florida Museum of Natural History, University of Florida, Gainesville, FL.  Pers. comm. 11/18/13.

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