Department of Paleontology staff members Tom Deméré and Kesler Randall measure the length of the tusk discovered at the 16th and Market project site. Photo by Chris Plouffe.
Fossil Discoveries in Downtown San Diego
By N. Scott Rugh, Collections Manager, Fossil Invertebrates
During the summer of 2007, field staff from the PaleoServices Department of the San Diego Natural History Museum monitored daily the excavation for a new building in downtown San Diego at the corner of 16th and Market Streets for signs of fossils. The owner of the project, Father Joe Carroll of Saint Vincent de Paul Management, Inc., was building a low-income family complex: the Saint Vincent de Paul Village high-rise. Most of July and August passed, and no fossils were seen. A local environmental consulting company, SCS Engineers, was also contracted by the owner of the property, and oversaw the removal of 12,000 tons of contaminated soil (hazardous and non-hazardous waste) from the project site. Toward the end of the excavation, on August 23, 2007, SCS staff member Bob Gutzler stopped by to check on the progress.
The excavation had just reached the final grade, when one of the grade checkers asked Bob about a small patch of white material exposed near the northwest corner of the pit. Fortunately, before working at SCS, Gutzler had been a long-time staff member of the Paleontology Department at the San Diego Natural History Museum and recognized the small patch of white material as part of a large tusk. The fossil was later identified as the tusk of a mammoth. For the next several days, about half of the staff of the Paleontology Department worked diligently in the 85-degree heat of late summer to get the eight-foot-long tusk jacketed with plaster and removed from the 30-foot-deep pit.
Species of mammoths, Mammuthus spp., existed from the early Pliocene (4.8 million years before present) to the Holocene (4000 years before present). If mammoths were alive today, they would look familiar to most of us. The bone and tooth structure of mammoth fossils are very similar to those of modern-day elephants. Mammoths and elephants belong to the same family, Elephantidae, of the Order Proboscidea. Like elephants, mammoths had vertically laminated, flat-topped molars adapted for chewing tough grasses. They ranged in size at the shoulder from about 10 feet—slightly larger than an Asian Elephant—to 16 feet, taller than an African Elephant. The most obvious external characteristic of a mammoth is its long, curving tusks. Another type of Proboscidean, the mastodont, Mammut spp., of the extinct family Mammutidae, existed prior to and during the time of the mammoths. Unlike mammoths, the mastodonts had molar teeth with large cusps, resembling large human teeth. These teeth were adapted to crushing leaves and branches. Fossil bones and teeth of mastodonts have also been found in San Diego County. However, do not confuse the two—mammoths had long, curved tusks and flat-topped molars, whereas mastodonts had teeth with distinct, high cusps, and tusks much less curved than those of even modern elephants.
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.
The top layer is a fine-grained, light gray sandstone containing great numbers of a long, graceful fossil marine snail shell, Turritella gonostoma. Other species less common in this bed include the large bivalve, the Giant Egg Cockle, Laevicardiuim elatum. This upper “Turritella Bed” is not often uncovered, probably because it is at a higher position in the sedimentary layers, and it may have been lost in several areas by ancient erosion or historical excavation and subsequent filling as downtown San Diego was developed. 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.
Read Part II of this article in next month’s Field Notes.
A nicely preserved pair of the extinct bay scallop, Argopecten abietis,
found in the northeast corner of the Thomas Jefferson School of Law excavation.
SAN DIEGO NATURAL HISTORY: FIELD NOTES, September 2009