Clams of Champions:
Balboa Park excavation yields invertebrate fossils
by Scott Rugh, SDNHM Paleontology Specialist
On May 14, 1998, San Diego Natural History Museum paleontologist Hugh Wagner and I collected Pliocene-aged marine invertebrate fossils from a construction site in Balboa Park, not far from the Natural History Museum. These fossils are from the San Diego Formation, an approximately 2 to 3 million-year-old shell-rich stratum (bed) exposed at the bottom of a pit dug for an elevator shaft of the new Hall of Champions building.
The fossils were densely packed in the soft sandstone layer and consisted of whole and broken shells. Individual fossils were very fragile. However, by brushing the loose matrix off the exposed surface of the fossils and dripping cyanoacrylate glue into the cracks, we were able to preserve many specimens whole. After the glue dried, the soft sandy matrix was carefully cleaned away from each specimen with hand tools (e.g. butter knives). After a specimen was removed from the shell bed, it was gently wrapped in tissue paper and carefully stored in a large plastic bucket to be transported to the paleontology lab at the Museum.
At the Museum, the fossils were carefully cleaned of the remaining sandy matrix. Afterwards they were hardened with Vinac, an acetone-based polymer resin. Next they were identified and counted, and the specimens were organized in taxonomic order. The specimens were given identification numbers, including the site numbers, and entered into the collection data base on the computer. Finally, identification cards were printed out with the information for each specimen, and placed in a metal drawer and cabinet for storage.
With the exception of one mammal bone found by Hugh, the fossils we discovered were marine invertebrates, mostly mollusks. The molluscan assemblage was dominated by over 12 species of bivalves, including the large species Mitha xantusi, Dosinia ponderosa diegoana, Saxidomus nuttalli, and Protothaca tenerrima. These species were found both as single valves and as articulated pairs. Four species of gastropods and one species of scaphopod (tusk shell) were also found. Clusters of small neat holes in some of the clams provided evidence of boring sponges. Various species of microscopic foraminifers were also found in the fine sandstone removed from the insides of the larger clams. The single mammal bone fragment found in this bed is probably part of the jaw of a small dolphin.
Later in the summer of 1998 the Hall of Champions is planning further construction that will again cut into the deeper shell beds. More discoveries may be waiting there for the San Diego Natural History Museum.