|Fossil Fishes of Fossil Lake, Green River
Formation (Eocene), Kemmerer, Wyoming
Fishes on this webpage come from the "18-inch" layer of Fossil Lake near Kemmerer, Wyoming. These
particular specimens come from a heavily weathered outcrop of the "18-inch layer" which results in
extremely soft and fragile matrix, where individual varves come free like pieces of loose paper.
Fossils from this layer typically require cleaning with fine, sharp instruments, often with the aid of a
pneumatic abrasive tool with dolomite media. In the case of these particular specimens, additional
methods were needed to prevent damaging the fossils.
Collection and Preparation of Specimens
Varves were removed layer by layer from 1 square meter quadrats (as part of a quantitative ecological
study) using a thin "steel". Ideally, fossils were discovered before the matrix covering them was
removed. The presence of a fossil fish is indicated by raised areas in the matrix corresponding to the
thicker elements of the fish skeleton (e.g skull, vertebrae) (Figure 1).
Figure 1. An unexposed fossil of Priscacara liops showing the protrusion of
the matrix prior to exposure of the bone. The red line demarcs the
approximate outline of the fish skeleton.
Once a specimen was discovered within the matrix, a cordless circular saw was used to cut through
the matrix about 1/4 inch deep in a rectangle surrounding the fossil (Figure 2).
Figure 2. A cordless circular saw with a masonary blade was used to cut into
the matrix in a rectangle around the fossil. Once the matrix outside of the cut
was removed to a depth of about 1/4 inch, the fossil could be carefully
removed with a steel and glued to a piece of locally available slate.
Once the matrix outside of the cut was removed to a depth of about 1/4 inch, the fossil could be
carefully removed with a steel. Because of the fragile nature of the matrix, the fossil-bearing slabs
were glued to a piece of locally available hard shale with white glue to prevent breakage and facilitate
transport to the laboratory. Additionally, diluted white glue was brushed over the top of the slab and
along the margins to prevent flaking of the varves (Figure 3). White glue is "reversible", i.e, by
applying warm water with a brush, it can be removed from the specimen in the laboratory.
Figure 3. Diluted (with water) white glue is brushed onto specimens, both those that were
accidentally exposed in the field and those that were still inbedded in matrix, to prevent
breakage and flaking.
The slabs were then labeled, allowed to dry, wrapped in bubble wrap, and placed in plastic totes for
transport to the laboratory. Once in the laboratory, the specimens were brushed with warm water and
the film of white glue was removed. The specimen was then placed under a binocular dissecting
microscope, and a needle attached to a wooden handle was used to begin picking away the matrix
(Figure 4), beginning at the cervical vertebrae, then the skull, and then the posterior half of the fish.
The needle must be extremely sharp to avoid the need to put too much pressure on the fossil and risk
damaging it. Additionally, water was applied to the matrix using a small paint brush. In this particular
outcrop, this procedure served to soften the matrix and help prevent damage to the fossil. Moreover,
during matrix removal, newly exposed fossil was simultaneously brushed with dilute white glue as
soon as it was exposed to hold the scales and bones in place. So the process involved wetting,
picking, removal of dust and pieces of matrix, and gluing, more or less simultaneously. An average
sized fish may take several hours to complete. Figures 5 - 7 show the incremental emergence of the
Figure 4. A sharp hypodermic needle, as seen under a dissecting scope, is used to carefully pick matrix away
from the fossil bone.
Figures 5 - 7. As matrix is carefully removed, the fossil Priscacara liops emerges. The brown material visible
between the bones is comprised of well-preserved scales. (These are thumbnails, click on them for a larger
The matrix and fossil from this particular site is too fragile for mechanical and pneumatic abrasive
devices. Scales and small bones are blown away. However, on the thicker bone elements such as the
skull and vertebrae a pneumatic abrasive tool has been used with success to remove matrix from
cavities in and between bones. Professional pneumatic abrasive tools can cost over $3,000.00. The
Paasche Air Eraser with remote media feed is suitable for cleaning (Figure 8). It costs about $125.00
and can operate off a portable Campbell Hausfeld compressor and be used with a countertop blasting
cabinet (Figure 9). Aluminum oxide is being used with some success, and I am also experimenting with
fine (USGS Seive # 200) dolomite powder.
Figure 8. Paasche Air Eraser with remote media feed.
Figure 9. Counter-top blasting cabinet.
Site under construction. Photos of fossil fish taxa coming soon.
Diplomystis dentatus (Mature)
Diplomystis dentatus (immature)