Giant island/pygmy mammoths: the late pleistocene prehistory of channel islands national park

Larry D. Agenbroad^1,2 and Don P. Morris³

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Post-1928 published research of these island mammoths was essentially non-existent until investigations by Phil Orr of the Santa Barbara Museum of Natural History (SBMNH) were published (Orr 1956a, b, c; 1959; 1960; 1967; 1968). Even then, the mammoths were of secondary importance to Orr, who concentrated on island archaeology. His collection of mammoth remains was in support of his interpretation that early island people ate the last of the island mammoths.

Louise Roth (1982; 1984; 1990; 1992; 1993; 1996) conducted a series of zoological studies on the island mammoths. It should be noted here that those studies were based primarily on museum collections housed at the Santa Barbara Museum of Natural History (Orr's collections) and the Los Angeles County Museum (collected by Stock, Furlong, and others). Access to Santa Rosa Island was restricted by the Vail and Vickers Cattle Company.

During the 1970's a large collection of M. exilis remains was accumulated by Boris Woolley, a member of the ranch family. This collection was donated to the Santa Barbara Museum of Natural History in 1995, by his widow, Margaret.

The National Park Service acquired San Miguel, Santa Rosa, Las Anacapas, and a portion of Santa Cruz in 1987. The establishment of Channel Islands National Park (CHIS) led to increased access to the islands, with concurrent research and researchers.
The 1994 Discovery

In June 1994 Tom Rockwell and a graduate student, Kevin Colson, from San Diego State University (SDSU) were examining elevated marine terraces and structural geology of Santa Rosa Island. At one locality on Carrington Point, Tom saw what appeared to be bones protruding from a steep, sandy, ice plant covered slope. Kevin examined the objects and verified they were bones, apparently representing the axial skeleton of a large (for Santa Rosa Island) land vertebrate. The location excluded large pinnipeds such as elephant seals.

Don Morris, CHIS archaeologist, contacted Agenbroad via telephone, asking if he would come confirm the tentative identification as an island mammoth. Jim Mead and Agenbroad flew to Oxnard from Hot Springs, South Dakota, and were transported to the island site. Examination of the exposed skeletal elements confirmed it was an articulated skeleton of Mammuthus exilis, and that it held the promise of being essentially complete.

My (Agenbroad) recommendation was that the specimen should be salvaged prior to the winter rains, as its location and exposure made it extremely vulnerable to loss by erosion. It was decided to excavate and salvage the skeleton in August 1994. Joined by Don Morris (CHIS), Tom Rockwell (SDSU), Louise Roth (Duke University), and my son Brett, we exposed, mapped, prepared and recovered more than 90% of a pygmy mammoth skeleton. There had been some pre-discovery erosional damage and loss.

The skeleton lay extended, on its left side, with the limbs extended toward the south (into the steep sand slope). Removal of the overburden exposed a nearly complete skeleton (Figure 2) of a mature, male, pygmy mammoth. Small bones were preserved, in life position. This indicated the specimen was in primary context (where the animal had died) rather than decomposing, disarticulating, and being scattered or redeposited. It appears that the mammoth lay down on the surface of the terrace, in the lee of a sand dune, and was buried by that dune, shortly after death.

The remains were air-lifted by helicopter to the ranch headquarters where they were put in containers and placed on a Park Service boat for transport to Ventura.

From Ventura, the skeleton was transported to the Mammoth Site of Hot Springs, South Dakota, for cleaning, preparation, preservation, and replication. A fiberglass replica can be viewed at the Channel Islands National Park visitor center, and also at the Santa Barbara Museum of Natural History. The original bones were returned to the Santa Barbara Museum of Natural History, the CHIS repository for paleontological remains.

Figure 1— A location map of Channel Islands National Park. The approximate boundary coincides with the shoreline of Pleistocene island, Santarosae.
Pleistocene Mammoths and the "Super" Island

Eustatic sea level lowering of ± 100 m, due to water tied up and stored as glacial ice and snow packs, changed the coast of Southern California. In particular,there was a seaward extension of the coastline and the presence of a large island offshore of the modern Santa Barbara_Ventura coast. That "super" island(Figure 1) was christened Santarosae by Phil Orr (1968).

Most researchers considered Santarosae to be the extension of the Santa Monica mountains into the Pacific ocean, creating a land bridge (Fairbanks, 1897;Stock and Furlong, 1928; Chaney and Mason, 1930; Stock, 1935, 1943; Valentine and Lipps, 1967; von Bloeker, 1967; Weaver and Doerner, 1967; Hooijer, 1976; Madden, 1977; Azzaroli, 1981). Mammoths were postulated to have crossed this land bridge from the mainland, to ultimately be marooned on the island, with the rise in sea level from melt water of the terminating Pleistocene glaciation.

If such a land bridge ever existed, it was submerged by late Pleistocene time (pre-mammoth). The presence of a deep water strait of 4_6 km width has been demonstrated by Johnson (1978) and Wenner and Johnson (1980). Late Pleistocene mainland mammoths (Mammuthus columbi) were the original island mammoths (Johnson, 1981; Madden, 1977, 1981; Roth, 1992; Agenbroad, 1998). This meant the island colonization by mammoths was accomplished by Columbian mammoths swimming to Santarosae. The sea breezes carrying the scent of vegetation from the island to the mainland was the apparent impetus for such a venture. This would be greatly enhanced by environmental stress of the coastal mainland pasturage, due to wild fires, or severe drought.

Once established on Santarosae, the mammoth population faced selective pressures which resulted in body size reduction. These pressures included shrinking territory (island submergence by eustatic sea level rise; reducing Santarosae by as much as 76%); overcrowding by increased population and decreased territory; resource stress caused by overcrowding and shrinking land mass; and by natural stresses such as lightning-strike fires and/or drought intervals. These forces became selective for smaller individuals, ultimately producing Mammuthus exilis, and the phylogenetic elimination of Mammuthus columbi from the islands. It is possible there were several colonizations of the island (islands) by Columbian mammoths, however there is no fossil evidence of pygmy mammoths on the coastal mainland (i.e. no reverse migration(s).

With sea level rise, there was an increasing width of deep-water strait which, in effect marooned the island mammoths.

The 1994 skeleton is the most complete Mammuthus exilis skeleton ever discovered. Recent information regarding the Wrangel Island mammoths (Mammuthus primigenius) (Vartanyan et al., 1993) as no longer considered to be dwarf forms (Tikhonov, 1997), places the distinction of the only pygmy, island dwelling mammoths as Mammuthus exilis.

Mammoth elements collected since CHIS became established, plus an intensive pedestrian survey and selective collection initiated in 1996 has nearly doubled the mammoth material in the SBMNH. That, with the Boris Woolley collection, has greatly increased the osteological collection. Preliminary comparisons of the pygmy mammoth bones and Columbian mammoth bones from the Mammoth Site of Hot Springs, South Dakota have been initiated.

The post-1994 survey of the islands has produced more than 150 localities (a locality being defined as mammoth remains not associated with the last locality). Erosion during winter storms exposes new remains while destroying others. We have observed material being destroyed within six months of exposure. Our procedure has been to collect those specimens threatened by erosion, while leaving more stable specimens in situ.

Chronology of the islands and their mammoth deposits is depauperate. Prior to 1994, there were only 15 published radiocarbon dates pertaining to island mammoths. Eleven of those dates were branded "equivocal" by Wenner et al. (1991). Their contention was two fold: 1) there is (was) no fire-produced charcoal on the islands, that the dated "charcoal" was due to groundwater carbonization; and 2) all mammoth remains were secondary (i.e. redeposited) so any dates of associated material were of no value.

Interior bone derived from the right femur of the 1994 skeleton was dated, using the accelerator-mass spectrometer method. Tom Stafford, then of the University of Colorado, derived collagen from the sample, which produced an AMS 14C date of 12,840 ± 410 (CAMS-24429). That date, derived from an in situ skeleton in primary deposition refutes many of the objections proposed by Wenner et al. (1991). In addition, five more dates for CHIS mammoth remains have been produced on associated material (Agenbroad 1999).

Additional radiocarbon dates are essential to understand the time of extinction, rate of dwarfing, possible environmental stresses and the potential for contemporaneity with the earliest humans on the islands. Recent archaeological investigations (Erlandson et al. 1996; 1997) have increased the antiquity of humans on the islands to greater than 11,000 years. Perhaps Orr (1968) was correct; maybe the last mammoths met the first people to arrive on the islands. A tight chronologic framework of the most recent mammoth remains will be crucial to evaluate that possibility.

The CHIS mammoth research is indebted to the early researchers, plus the support of the National Park Service, the Santa Barbara Museum of Natural History, Northern Arizona University, the American Philosophical Society, and the generosity of James Jensen of Denver, Colorado.

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