Wednesday, September 19, 2012

Fossil mammoth teeth tell us about past climates and plant communities

Written August 7, 2012, published September 2012

There is a fossil Columbian mammoth tooth in the museum at the Pacific County Historical Society, South Bend, WA. The tooth was found in the 1930s, in floodplain sediments along the North River. Mammoth fossils from the Pleistocene Epoch are common throughout North America, and in Washington. During the Pleistocene, mammoths lived from Alaska and Canada to Nicaragua and Honduras. Two mammoth species wandered over from Asia into Alaska and Canada, and two species were indigenous to and widespread in North America, including the Columbian mammoth. 

Figure 3. Mounted composite skeleton of a Columbian-type mammoth made from skeletal elements recovered in the 1870s from the
‘swamps’ at the Copelin Ranch along Latah Creek in Spokane County (site 06). When assembled in 1886 in the Field Museum of Natural History in Chicago, Illinois, this ‘mammoth’ was considered to be the first fully mounted specimen, albeit a composite from several individuals, of a mammoth in North America. (Photo from Higley, 1886.)
Washington Geology, vol. 27, no. 2/3/4, December 1999, page 25 “Some Notable Finds of Columbian Mammoths
from Washington State” Bax R. Barton

The Columbian mammoth, Mammuthus columbi, is the state fossil. Bax Barton wrote in Washington Geology (Vol. 27 (2/3/4), 1999, page 23) “Of the 39 counties in Washington, only heavily forested counties on the west side of the Cascade mountains (for example, Skamania and Wahkiakum) and less populated counties on the east side (for example, Ferry and Pend Oreille) have thus far failed to produce mammoth fossils.” The Columbian mammoth was up to 13 feet tall and just under 10 tons, eating around 500 pounds of vegetation per day. This mammoth had long tusks, and was not very hairy, unlike other mammoth species, and also unlike mastodons. 

Mammoth tooth on display at Pacific County Historical Society, South Bend, WA. It was collected from the floodplain of the North River in north Pacific County. Photo by Kathleen Sayce.
This single fossil tooth from the North River tells us what plant communities and climate were like during glacial maxima (when continental ice was most widespread) in the Pleistocene. Mammoths did not live in forests. They grazed on open grasslands, eating grasses and sedges, with sages, mosses, ferns and aquatic plants as minor foods. Grasses and sedges were extensive during glacial maxima because that climate was cooler and drier than today’s. These shifts promote a long-term seesaw between forests and grasslands. Forests shrink during glacial maxima, and expand during warm wet periods. Periods of cold dry weather promote grasslands and sedges. Warm wet periods promote forests, such as today’s climate. Today, the present climate promotes trees. 

Sea level also seesaws between ice ages and temperate periods. During glacial maxima, sea level was as much as 350 feet lower than today. The continental shelf was a wide rolling plain, dissected by rivers flowing from modern day estuaries in deep valleys, including Columbia, Willapa and Grays Harbor. These river valleys can still be seen on bathymetric charts as deep canyons, along with larger side channels; finer stream details are buried under marine sediments. 

Mammoths weren’t the only animals to flourish during the Pleistocene; freshwater river habitats for fish in those streams on the plains were extensive compared to present day. 

Mammoth teeth are common fossils for two reasons. One, teeth are hard, and generally persist with relative ease compared to other body parts. Two, over their lifetimes, each mammoth had six or more full sets of teeth. As grazers, they wore teeth down quickly, and replaced them in sets. Shark teeth are common marine fossils for the same reasons: teeth are hard, and sharks constantly grow new teeth to replace worn and damaged ones.  

As the climate warmed, forests expanded, grasslands shrank, and mammoths found their preferred food plants disappearing. Their teeth were designed for grasses and sedges, not conifer trees. The last mammoths died out around 10-11,000 years ago, based on dating the youngest known fossils from the Midwest. There are gold deposits buried on the continental shelf, and mammoth fossils are out there as well, along with a lot of shark teeth. 

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