San Diego Natural History Museum--Your Nature Connection[BRCC San Diego Natural History Museum: Paleontology]

Whale Fossil found in Chula Vista
Thomas Deméré, Ph. D., SDNHM Curator of Paleontology

More photos of this fossil (100K!)

On 30 March 1998, paleontologists from the San Diego Natural History Museum excavated the partial skull of a fossil baleen whale (Cetacea: Mysticeti) from ancient marine sandstones in southwestern San Diego County, California. The sandstone layer that contained the fossil is part of the San Diego Formation, a sedimentary rock unit that was deposited in this area during the later part of the Pliocene Epoch, approximately 2 to 3 million years ago.

[After six weeks of work, the underside is nearly finished.] The fossil specimen was unearthed by grading equipment at the Otay Ranch, Village 1 residential development site in eastern Chula Vista, California. Paleontologists Richard Cerutti, John Pfanner, and Pat Sena discovered the partial skull as they were following in the wake of a bulldozer. When the bulldozer struck the buried fossil, the top of the braincase was removed. It was the smear of fossil bone that attracted the attention of the paleontologists. Initial examination of the half buried skull revealed that it was generally intact and worth collecting.

Collection of the specimen followed conventional field methods for large fossil vertebrates. The skull was first assessed as to the degree of damage. Broken or weakened pieces of bone were stabilized using special hardeners (e.g., glyptal or cyanoacrylate glue). The perimeter of the specimen was then defined by probing around it using hand tools (dull kitchen knives, putty knives, and ice picks). Initial excavation was then begun around the perimeter using picks and shovels. When the excavation reached the point where the specimen was isolated in a matrix-supported pedestal from the rest of the outcrop, the pedestal was then slightly undercut at its base to form an overhanging lip. Damp newsprint was then placed on the upper surface of the block. A solution of 20 minute Plaster-of-Paris was then mixed and 5-10 inch-wide strips of burlap cloth soaked in the solution. The strips were then lifted out of the plaster solution and laid across the block to dry.

Because of the size of the specimen, three layers of plaster-soaked burlap "bandages" were formed on the block. During the plaster work this jacket was also reinforced with metal stakes. Once the plaster hardened, the remainder of the supporting pedestal was undercut and the plaster jacket turned over. Hand digging tools were then used to remove any excess matrix from the bottom (now top) of the block. When all layers of plaster were completely hardened the completed plaster jacket was then labeled with a field number and north arrow, removed from the field, and transported to the Museum.

Once the plaster-jacketed specimen was in the Museum the next step was to expose the fossil by removing as much of the enclosing sandstone matrix as possible. Fortunately, the sandstone that the specimen was buried in is a damp, friable sand that is very easy to dig. As the matrix is removed from the fossil we are discovering that the bone is quite brittle in places. These areas are cleaned as much as possible with small knives and paint brushes and then hardened with cyanoacrylate glue. So far we have uncovered the base of the braincase and exposed the glenoid fossae (where the lower jaws articulate with the skull), the occipital condyles (where the atlas vertebra articulates with the skull), the external ear canals, and the middle ear bones. The specimen is upside-down in the plaster jacket, which means that it was right-side-up when we found it in the field and that 3 million years ago it was laying right-side-up on the sea floor while it was being buried by sand.

We have not yet determined the species of baleen whale that this skull belongs to, and in fact the specimen may represent a species new to science. Certain anatomical details of the specimen are puzzling and provide a strong incentive for us to complete the preparation of the specimen and begin its scientific study. We will keep you posted.

Photo: Skull, still in its plaster jacket, being cleaned in the Paleontology Department.
Credit: Judy Gibson 1998