|GIANT ISLAND/PYGMY MAMMOTHS:
THE LATE PLEISTOCENE PREHISTORY OF
CHANNEL ISLANDS NATIONAL PARK
Larry D. Agenbroad1,2 and Don P. Morris3
1Department of Geology, Box 4099, Northern Arizona University, Flagstaff, AZ 86011
2The Santa Barbara Museum of Natural History, 2559 Presta del Sol, Santa Barbara, CA 93105
3Channel Islands National Park, 1901 Spinnaker Drive, Ventura, CA 93001
Abstract - The northern Channel Islands of California are included in Channel Islands National Park (CHIS). These modern islands are the remnant high ground of a late Pleistocene island named Santarosae. At some time during the Rancholabrean land mammal age Santarosae was colonized by mainland mammoths (Mammuthus columbi). With eustatic sea level rise due to the end of the Ice Age meltoff, as much as 76% of Santarosae was submerged. Mammoths met the challenge of diminished range and decreasing resources by size reduction, to less than 50% of the stature of Mammuthus columbi. The pygmy form (Mammuthus exilis) is known from San Miguel, Santa Rosa, and Santa Cruz islands. The Channel Islands contain the remains of the only island dwelling pygmy mammoths in the world.
The California Channel Islands (Figure 1) have been known to produce remains of small mammoths since a Coast and Geodetic survey in 1856. These remains were first reported in scientific literature by Stearns (1873). Fifty-five years passed until the first paleontological report (Stock and Furlong, 1928) was published, giving the new species designation (Elephas) Mammuthus exilis.
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).
Figure 2— The articulated 1994 skeleton of Mammuthus exilis from Santa Rosa Island. (Drawn by Susan Morris)
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.
Agenbroad, L. D., (in submission) Radiocarbon geochronology of Mammuthus exilis on the California Channel Islands: A review plus new 14C dates since 1994.
______, 1998. New pygmy mammoth (Mammuthus exilis) localities and radiocarbon dates from San Miguel, Santa Rosa, and Santa Cruz Islands, California. in Weigand, P., ed., Contributions to the Geology of the Northern Channel Islands, Southern California. Pacific Section of the American Association of
Petroleum Geologists, Bakersfield, California: 169_175.
Azzaroli, A., 1981. About pigmy mammoths of the Northern Channel Islands and other island faunas. Quaternary Research 16:423_5.
Chaney, R. W., and H. L. Mason, 1930. A Pleistocene flora from Santa Cruz Island, California. Carnegie Institute of Washington Publication 415:1_24.
Erlandson, J. M., D. J. Kennett, B. L. Ingram, D. A. Guthrie, D. P. Morris, M. A. Tveskov, G. J. West, and P. L. Walker, 1996. An archaeological and paleontological chronology for Daisy Cave (CA-SMI261), San Miguel Island, California. Radiocarbon 38:1_29.
______, M. A. Tveskov, D. J. Kennett, and B. L. Ingram, 1997. Further evidence for a terminal Pleistocene occupation of Daisy Cave, San Miguel Island, California. Current Research in the Pleistocene 13:161_168.
Fairbanks, H. W., 1897. Oscillations of the coast of California during the Pliocene and Pleistocene. American Geologist 20:213_245.
Hooijer, D. A., 1976. Observations on the pygmy mammoths of the Channel Islands, California. in Essays on Palaeontology in honor of Loris Shano Russell. C. S. Churcher (ed.) Athlon. Royal Ontario Museum, Ontario, Canada. pp. 220_225.
Johnson, D. L., 1978. The origin of island mammoths and the Quaternary land bridge history of the Northern Channel Islands, California. Quaternary Research 10:204_225.
______, 1981. More comments on the northern Channel Island mammoths. Quaternary Research 15:105_106.
Madden, C. T., 1977. Elephants of the Santa Barbara Channel Islands, southern California. Geological Society of America. Abstracts with Programs 9:458_459.
______, 1981. Letters to the editor: origin(s) of mammoths from Northern Channel Islands, California. Quaternary Research 15:101_104.
Orr, P. C., 1956a. Radiocarbon mammoths, and man on Santa Rosa Island. Geological Society of America Bulletin 67:1777.
______, 1956b. Radiocarbon dates from Santa Rosa Island. Bulletin of the Santa Barbara Museum of Natural History 2:1_10.
______, 1956c. Dwarf mammoth and man on Santa Rosa Island. University of Utah Anthropological Papers 26:75_81.
______, 1959. Santa Rosa Island dwarf mammoths. Museum Talk. Santa Barbara Museum of Natural History. pp. 25_29.
______, 1960. Radiocarbon dates from Santa Rosa Island II. Bulletin of the Santa Barbara Museum of Natural History 3:1_10.
______, 1967. Geochronology of Santa Rosa Island, California. In Philbrick, R. N. (ed.) Proceedings of the symposium on the biology of the California Islands, Santa Barbara Botanic Garden, Santa Barbara. pp. 317_325.
______, 1968. Prehistory of Santa Rosa Island. Santa Barbara Museum of Natural History. 253 p.
Roth, V. L., 1982. Dwarf mammoths from the Santa Barbara, California Channel Islands: size, shape, development, and evolution. Unpublished Ph.D. Dissertation. Yale University. University Microfilms. Ann Arbor.
______, 1984. How elephants grow: heterochrony and the calibration of developmental stages in some living and fossil species. Journal of Vertebrate Paleontology 4:126_145.
______, 1990. Insular dwarf elephants: a case study in body mass estimation and ecological inference. In Body size in mammalian paleobiology: estimation and biological implications. (eds. J. Damuth, and B. J. MacFadden), Cambridge University Press, Cambridge, p. 151-179.
______, 1992. Inferences from allometry and fossils: dwarfing of elephants on islands. In Oxford surveys in evolutionary biology. Volume 8 (eds., D. Futuyma, and J. Antonovics), Oxford University Press, New York, p. 259-288.
______, 1993. Dwarfism and variability in the Santa Rosa Island mammoth: an interspecific comparison of limb bone sizes and shapes in elephants. in Third California Islands symposium: recent advances in research on the California Islands (ed. F. G. Hochberg), pp. 433_342. Santa Barbara Museum of Natural History.
______, 1996. Pleistocene dwarf elephants from the California Islands. In The Proboscidea (eds., J. H. Shoshani, and P. Tassy), University of Oxford Press, Oxford, United Kingdom. pp. 249_253.
Stearns, R. E. C., 1873. [no title: paraphrase of comments attributed to Stearns in minutes of Academy's regular meeting] Proceedings of the California Academy of Sciences 5:152.
Stock C., 1935. Exiled elephant of the Channel Islands, California. Scientific Monthly 41:205_214.
______, 1943. Foxes and elephants of the Channel Islands. Los Angeles County Museum Quarterly 3:6_9.
______, and E. L. Furlong, 1928. The Pleistocene elephants of Santa Rosa Island, California. Science 68:140_141.
Tikhonov, A., (brief report) 1997. Zoological Institute Russian Academy of Sciences, St. Petersburg, Russia. Department of History of fauna. EroMam Newsletter:14_15.
Valentine, J. W., and J. H. Lipps, 1967. Late Cenozoic history of the southern California Islands. Proceedings of the Symposium of the Biology of the California Islands (Santa Barbara: Santa Barbara Botanic Garden, R. N. Philbrick, ed.), pp. 13_20.
Vartanyan, S. L., V.E. Garutt, and A. V. Sher, 1993. Holcene dwarf mammoths from Wrangel Island in Siberian Artic. Nature 362:337-340.
Von Bloeker, J.R., Jr., 1967. The land mammals of the southern California Islands. In Proceedings of the aymposium on the Biology of the California Islands (ed., R. N. Philbrick), Santa Barbara Botanic Garden, Santa Barbara, California, p. 245-263.
Weaver, D. W., and D. P. Doerner, 1967. Western Anacapia- a summary of the Cenozoic history of the Northern Channel Islands. In Proceedings of the Symposium on the Biology of the California Islands (ed., R. N. Philbrick), Santa Barbara Botanic Garden, Santa Barbara, California, p. 13-20.
Wenner, A. M., J. Cushing, E. Noble, and M. Daly, 1991. Mammoth radiocarbon dates from the northern Channel Islands, California. Proceedings of the Society for California Archaeology 4:1-6.
______, and D. L. Johnson, 1980. Land vertebrates on the California Channel islands: sweepstakes or bridges? In The California Islands: proceedings of a multidisciplinary symposium (ed., D. M. Power), Santa Barbara Museum of Natural History, p. 497-530.