A Tale of Two Excavations
by Scott Rugh, Paleontology Specialist
Our paleontologists kept their eyes open for fossils when the digging equipment reached the San Diego Formation, because fossil seashells and bones of extinct marine mammals are often found buried in the layers of this formation. For a long time there was no hint of fossils. Then on April 2, 1999, when the excavation was nearly finished, Richard Cerutti of the museum's Paleontology Department found fossil clams near the bottom. The workers broke out large chunks from the formation, and these blocks of sandstone were brought upstairs for preparation.Delicate Digging
The material was prepared by reducing the blocks with hammers and chisels and carefully working the fossils free with an X-acto knife. The fossils were molds and casts formed in the soft, fine-grained sandstone. No shell material was preserved, unlike the shell fossils recovered from an elevator shaft in the nearby Hall of Champions building in 1998. The casts were so soft that after they dried out they had to be coated with a layer of cyanoacrylate glue (super glue) in order to preserve them. Casts are formed over time as the original shell material is dissolved away by acidic ground waters. A shell cast retains the form of the outer surface of the original specimen, and the mold is the material surrounding it.A Comparison of Fossils from Two Sites
A couple dozen species of marine mollusks were preserved as casts. Most of these were bivalves (clams). A similar number of mollusk species was collected from the Hall of Champions site. In general, the species recovered from the museum excavation were different than those found at the Hall of Champions. Fossils of only nine species were found at both locations, and in most instances, those species common at one location were rare at the other. In general, more deep water species, such as the nut clam Nuculana taphria, scallops Patinopecten healeyi and Flabellipecten stearnsii, the Milky Venus Compsomyax subdiaphana, and the clam Thracia kanakoffi were found at the museum. In contrast, more shallow water species, such as the ark shell Anadara trilineata, the Gaping Clam Tresus nuttalli, the mussel Mytilus stearnbergi, and an oyster Ostrea sp. were found at the Hall of Champions site.
The general difference in the species of marine mollusks observed between the San Diego Natural History Museum and the Hall of Champions excavations may be surprising, since the two locations are close (about 1/4 mile apart) and at similar surface elevation (a little over 200 feet above sea level), but this difference can be explained by the orientation of the stratigraphic layers of the San Diego Formation under the surface. These layers dip gently down into the earth's crust towards the south so that a bed exposed at the surface at the Natural History Museum actually corresponds to a level well below the surface at the Hall of Champions. In other words, the fossil beds exposed at the Museum are older and occur at a lower stratigraphic level in the San Diego Formation than those at the Hall of Champions.
Paleontologists have suggested that fossils from the lower layers correspond to a deeper marine environment than those from higher levels. And this is exactly what was demonstrated by the fossil mollusks recovered from the two Balboa Park fossil locations.