A mammoth tusk is discovered during an excavation for a new building
The discovery of the mammoth tusk at 16th and Market Streets was very exciting because never before had a fossil mammoth tusk been found in downtown San Diego. Prior to this discovery, mammoth fossil remains had been collected from 10 locations in San Diego County within the past 30 years, half of these within the last decade. Fossil tusks were only collected from four of these previous locations. Amazingly, another mammoth tusk was found just a few months later, increasing this total to five tusk discoveries at 11 collecting sites around the county. On December 10, 2007, Field Paleontologist Pat Sena of the Museum’s PaleoServices was monitoring the progress of the excavation of a bore hole at the SDG&E Silvergate Substation near the San Diego Bay harbor, just over a mile southeast of the corner of 16th and Market Streets.
A large boring auger had broken through the tusk 20 feet below street level near the bottom of the drilled hole. Sena collected the relatively small fragments and brought them to the Paleontology laboratory at the Museum where they were pieced back together.
Prior to the discoveries of fossil mammoth tusks at these two sites, other fossils had been discovered in downtown San Diego at numerous excavation projects. These fossils, found in great abundance, are shells occurring in sandy sedimentary layers, often a few feet thick. These fossil shell beds are positioned nearly horizontally, several feet beneath downtown San Diego. There are two main strata, or sedimentary sandstone layers, with minor intermediate beds containing less pronounced shell beds.
Several feet below the Turritella Bed is a “Pecten Bed” consisting of a medium-grained, loosely compacted, orange-brown sandstone containing abundant individual shells of two species of scallop shells, Argopecten abietis abietis and Oppenheimopecten vogdesi. Both species reached a fairly large size, up to three or four inches in diameter. The Pecten Bed, occurring more deeply than the Turritella Bed, almost always occurs in the part of the city where the Broadway Faunal Horizon shell beds are present. These shell beds were described by Department of Paleontology curator Dr. Tom Deméré (1981) after the beds were uncovered during excavation for Horton Plaza. He referred to the shell beds collectively as the “Broadway Faunal Horizon.”
These “Ice Age” deposits are from the Pleistocene Epoch with an estimated age of about 500,000 years old. Turritella gonostoma, Laevicardium elatum, and Oppenheimopecten vogdesi (as well as several other species occurring in these beds) are species that occur today along the west coast of Mexico, but are absent in southern California, indicating the ocean water of this time period was considerably warmer than it is today. Furthermore, all species found from these beds are species that are today typical of protected bays. Therefore, the shell beds of the Broadway Faunal Horizon provide evidence that in the late Pleistocene, while the mammoths lived on land, downtown San Diego consisted of a large, tropical bay.
At the 16th and Market project site, mammoth fossils were found in sediments where no shell beds were found. At the Silvergate Substation, the auger bit that retrieved the tusk fragments mixed sediments from different layers together as it bored through them. It was not possible to go into the hole to observe the sediments, but from street level, shell layers including pectens were observed above and below the tusk.
Other fossils were found downtown before the mammoth fossils were discovered. During the first few years of collecting fossils from the many project sites within the East Village area, thousands of fossil shell specimens were collected, most commonly from the Pecten Bed. However, very little was found in the way of vertebrate fossils, aside from occasional small shark teeth and small fossil elements from other fish species. Also, the fossil plates of a barnacle species that only lives on sea turtles provided evidence that sea turtles were present in the prehistoric Pleistocene bay as well. On March 29, 2004, during excavation for the Metrome housing project, a partial camel jaw was found by Excavation Operator Jose Morales. In addition, other small vertebrate fossils, including shark, bird, rodent bones and teeth, and an Antilocaprid (pronghorn family) partial tooth were found by PaleoServices field collectors within the Broadway Faunal Horizon beds.
Three weeks later, when Pat Sena informed the Paleontology Department that grading had uncovered the bones of a large whale, it was almost impossible to believe. In the same southeast corner of the project site, about 10 feet below the spot where the mammoth bones had been excavated and collected, another extremely important vertebrate discovery was made. According to Dr. Tom Deméré, the bones of a Gray Whale (Eschrichtius robustus) or a very closely related ancestor were uncovered—including a lower jaw bone, skull fragments, shoulder blade, and other diagnostic bones. The Paleontology Department has collected fossil bones of a number of other whales from Pliocene-age deposits (about 3.5 million years old) in San Diego County. However, the fossil whale found at the Thomas Jefferson School of Law is the first Pleistocene-age whale found in San Diego County (about 0.6 million years old), and only one of a few whale fossils from the Pleistocene in southern California. Whereas the mammoth was carried into the environment where the shell bed existed (probably with river sediments), the whale was deposited on the bottom of a deep bay and was directly incorporated into the sediments of that environment, tens of thousands years older than the bay environment that the mammoth remains were carried into.
With each of these fossil discoveries, almost every staff member of the Paleontology Department worked in the field to unearth the bones, and prepare them for transport to the Museum warehouse where they were temporarily stored before being relocated to the Museum. With each new fossil discovery, several days were required to plaster jacket and transport the very large fossils from the project site, yet this was relatively quick and efficient in relation to the total work schedule for the project. Even though the fossil bones of the mammoth and whale have been removed from the Thomas Jefferson School of Law job site, a great deal of work still needs to be done in the laboratory to complete the preparation of the fossils, and this work could last the better part of a year. When completed, the fossil discoveries made at the Thomas Jefferson School of Law will be a fantastic addition to the San Diego Natural History Museum’s Paleontology collections.