Wednesday, January 29, 2014

Little island mammoths of California

Doing my bit to encourage warm thoughts for those of us in the frozen north...

I present to you an island off the coast of what is now California, approximately 15,000 years ago. This island is called Santarosae (or Santa Rosae); as sea level rises following the melting of the great continental ice sheets, it will be dissected into the four northern Channel Islands (San Miguel, Santa Rosa, Santa Cruz, and Anacapa), which are eventually reunited as Channel Islands National Park (with Santa Barbara Island). These events, though, are in the future. For "now", we have one large island a handful of miles from the coast:

A. The modern northern Channel Islands. B. The geography of the islands at various times in the recent past. From Collins (2009).

The geologic history of the northern Channel Islands is unusual; you're looking at a slice of land that until about 19 million years ago was quite comfortably attached to the San Diego area. At this point, the North American plate experienced some indigestion from overriding the East Pacific Rise, one of the effects being the crustal block containing the islands was wrenched out of place and rotated about 90 degrees clockwise (it is actually linked to the Santa Monica Mountains, which also run east-west). A summary of this can be found in Weigand et al. (2002). At least part of the islands have probably been above sea level since the Pliocene or early Pleistocene, but they were under water before that, and surveys of the sea bottom have pretty much nixed old ideas of land bridges, so everything on the islands got there by air or water (swimming under their own power, or passively rafting on debris from the mainland).

Just start swimming west; you can't miss it.
The most iconic fossil organism of the northern Channel Islands is the pygmy mammoth (Mammuthus exilis). This species of mammoth acquired its nickname honestly, averaging perhaps 1,750 lb (750 kg) and 5.6 ft (1.7 m) at the shoulder (Agenbroad 2009). The mammoths were not alone, but the fauna of Santarosae was rather limited, as you might suspect. The vertebrates included lizards, rattlesnakes, an abundance of birds, vampire bats, shrews, voles, deer mice, and the famous Island Fox (but see Rick et al. 2009) (Guthrie 1993, 1998). There was a species of giant deer mice (Peromyscus nesodytes, which has its own Wikipedia page, a true mark of distinction for a fossil rodent; do you have any idea how many species of fossil rodents are out there?); unfortunately, "giant" in this case is relative, so instead of being impressively giant like our friend Castoroides, we're talking more like rat-sized. The flightless sea duck Chendytes also found Santarosae to its liking, and provides an example of the common island phenomenon of birds becoming flightless. There was also the ubiquitous land snail Helminthoglypta ayresiana (Johnson 1971). Plant fossils from what is now Santa Cruz Island indicate that a tree and shrub woodland comparable to the surroundings of Fort Bragg was present on at least part of Santarosae (Chaney and Mason 1930). Other plants are represented by the "caliche forests", carbonate cement casts of plant roots (Johnson 1967).

The human, historical side of the Channel Islands mammoth is covered in Orr (1968) and Roth (1996). Mammoth bones have been reported from the three largest northern Channel Islands since the 1870s, and interest has followed some definite cycles of waxing and waning. The first burst of interest occurred in the 1920s and 1930s, during which the species received its scientific name (Stock and Furlong 1928). More pressing issues took peoples' minds away from miniature mammoths until the late 1940s, when Phil Orr began a long association with the Channel Islands (more on his work in a moment). After he stopped work in the late 1960s, the mammoth scene on the islands quieted down again for a couple of decades, with the exception of the old "land-bridge" hypothesis receiving its mortal wounds: improved seafloor mapping showed no sign of an appropriate submerged bridge, and it turns out that elephants are capable swimmers (Johnson 1978). The discovery of a nearly complete specimen in 1994 turned the spotlight back on M. exilis, and many additional fossils have been found in the following two decades. A tusk was found on Santa Rosa Island just last year that pushes back the record of this species to about 80,000 years ago.
A cast of the 1994 specimen at the Channel Islands NP Visitor Center (from here; NPS)

...And as it was in the field, to give you a sense of scale (same source; NPS)
Pygmy mammoths are thought to have descended from Columbian mammoths who swam to Santarosae, and a few specimens of this normal-sized mammoth species have been found on the islands (Agenbroad 2012). The mammoths have long been presented as textbook examples of insular (or island) dwarfism, the phenomenon where species of big animals, confined on an island or other small area, become smaller. Aside from size, pygmy mammoths differed from their larger ancestors by having a lower center of gravity and more flexed limbs, which made them more agile and permitted them to negotiate steeper grades (Roth 1996; Agenbroad 2012).

To get back to Phil Orr: he was interested in a possible connection of the extinction of the mammoths to the appearance of humans. He came to interpret certain features as roasting hearths, and envisioned humans on Santarosae as far back as 40,000 years ago (Orr 1968). His interpretations in this matter have not been widely supported. However, it is interesting to note that the Channel Islands have one of the longest records of humans in North America, with Arlington Springs Man (or Woman, depending on the publication date of whatever you're reading) living there around 10,960 ± 80 radiocarbon years ago (Agenbroad et al. 2005) (between about 13,000 and 12,700 calibrated years ago). Meanwhile, the most recent mammoth remains are 11,030 ± 50 radiocarbon years old (Agenbroad et al. 2005), which provides essentially the same calibrated age. You can see where this is going...

Regardless of whether or nor people got to Santarosae in time to meet the Channel Islands mammoths and found them tasty, I suspect that the mammoths were in for a rough time; take another look at those maps at the top of the post. The switch from the late Pleistocene and the great continental ice sheets to the warmer early Holocene was going to do a number on the available real estate, and the vegetation was also changing.


Agenbroad, L. D. 2009. Mammuthus exilis from the California Channel Islands: height, mass, and geologic age. Pages 15-19 in Damiani, C. C. and D. K. Garcelon, editors. Proceedings of the 7th California Islands Symposium. Institute for Wildlife Studies, Arcata, California.

Agenbroad, L. D. 2012. Giants and pygmies: mammoths of Santa Rosa Island, California (USA). Quaternary International 255:2-8.

Agenbroad, L. D., J. R. Johnson, D. Morris, and T. W. Stafford, Jr. 2005. Mammoths and humans as Late Pleistocene contemporaries on Santa Rosa Island. Pages 3-7 in Garcelon, D. K. and C. A. Schwemm, editors. Proceedings of the Sixth California Islands Symposium. Santa Barbara Museum of Natural History, Santa Barbara, California.

Chaney, R. W. and H. L. Mason. 1930. A Pleistocene flora from Santa Cruz Island, California. Carnegie Institution of Washington Publication 415:1-24.

Collins, P. W. 2009. Historic and prehistoric record for the occurrence of Island Scrub-Jays (Aphelocoma insularis) on the northern Channel Islands, Santa Barbara County, California. Santa Barbara Museum of Natural History Technical Report 5.

Guthrie, D. A. 1993. New information on the prehistoric fauna of San Miguel Island, California. Pages 405-416 in Hochberg, F. G., editor. Third California Islands symposium: recent advances in research on the California Islands. Santa Barbara Museum of Natural History, Santa Barbara, California.

Guthrie, D. A. 1998. Fossil vertebrates from Pleistocene terrestrial deposits on the northern Channel Islands, southern California. Pages 187-192 in Weigand, P. W., editor. Contributions to the geology of the northern Channel Islands, southern California. American Association of Petroleum Geologists, Pacific Section, Bakersfield, California. Miscellaneous Publication 45.

Johnson, D. L. 1967. Caliche on the Channel Islands. California Division of Mines and Geology Mineral Information Service 20:151-158.

Johnson, D. L. 1971. Pleistocene land snails on the Channel Islands, California: a call for research. The Nautilus 85(1):32-35.

Johnson, D. L. 1978. The origin of island mammoths and the Quaternary land bridge history of the northern Channel Islands, California. Quaternary Research 10(2):204-225.

Orr, P. C. 1968. Prehistory of Santa Rosa Island. Santa Barbara Museum of Natural History, Santa Barbara, CA.

Roth, V. L. 1996. Pleistocene dwarf elephants from the California Islands. Pages 249-253 in Shoshani, J. H. and P. Tassy, editors. The Proboscidea. University of Oxford Press, Oxford, United Kingdom.

Rick, T. C., J. M. Erlandson, R. L. Vellanoweth, T. J. Braje, P. W. Collins, D. A. Guthrie, and T. W. Stafford, Jr. 2009. Origins and antiquity of the island fox (Urocyon littoralis) on California’s Channel Islands. Quaternary Research 71(2):93-98.

Stock, C. and E. L. Furlong. 1928. The Pleistocene elephants of Santa Rosa Island, California. Science 68(1754):140-141.

Weigand, P. W., K. L. Savage, and C. Nicholson. 2002. The Conejo Volcanics and other Miocene volcanic suites in southwestern California. Pages 187-204 in Barth, A., editor. Contributions to crustal evolution of the southwestern United States. Geological Society of America, Boulder, Colorado. Special Paper 365.

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