LATE PLEISTOCENE AIRPORT LANE FOSSIL SITE, LA GRANDE, NE OREGON.

Columbian Mammoth (Mammuthus columbi)

Abstract

 The Airport Lane fossils include a right tibia and two tusks from a juvenile male Columbian mammoth.  The Airport Lane mammoth was approximately 18 years old when it died.  Fragments of the mammoth left tusk were radiocarbon dated at 10,630 ± 490 (collagen) and 10,640 ± 340 (bioapatite) radiocarbon years before present.

Introduction

The taxonomy of North American mammoths has been condensed since Osborn (1942) proposed 15 species of mammoths, but this has led to confusion because several different classifications (Maglio, 1973; Kurtén and Anderson, 1980; Madden, 1981; Agenbroad, 1984, 1994; and Graham, 1986) have been proposed.  Most of these include 5 species, including the early Pleistocene (Irvingtonian) primitive form Mammuthus meridionalis; the middle Irvingtonian to middle Rancholabrean Mammuthus imperator; the Wisconsin glacial stage Mammuthus columbi (including Mammuthus jeffersonii and Elephas washingtonii)); the Rancholabrean Mammuthus primigenius (wooly mammoths); and the late Pleistocene to Holocene pygmy mammoths, Mammuthus exilis.  The earliest (Mammuthus meridionalis) and latest (Mammuthus primigenius) mammoths immigrated into North America from Eurasia over the Bering Land Bridge during interglacial periods.  Mammuthus imperator and the Columbian mammoth Mammuthus columbi, which was adapted to temperate grasslands, were derived from the ancestral mammoths.  The pygmy mammoth Mammuthus exilis was an island form derived from Columbian mammoths that swam to the Channel Islands off California during a low sea level stage ~20,000 years ago (Agenbroad, 2005).

                  The greatest geographic dispersion and highest frequency of dated mammoth localities falls in the 10,000-15,000 year period.  This is due to the association of mammoths with Clovis and other archaeological sites.  Mammoth sites with ages less than 10,000 radiocarbon years before present have been found in the Michigan Peninsula, Wisconsin, and in scattered locations in the Canadian Prairie Provinces, Montana, and some of the southwestern United States, but these dates are viewed with skepticism.  If valid, they indicate an intense shrinkage of the former distribution of mammoths to small refugial populations (Agenbroad, 2005).  The youngest North American wooly mammoth (Mammuthus primigenius) fossils are the ones dated at 5,700 radiocarbon years found on St. Paul Island, one of the isolated Pribilof Islands in the Bering Sea off Alaska (Crossen and others, 2005).  Remains of dwarf wooly mammoths that are 7,000-4,000 radiocarbon years old have been found on Wrangel Island in the Siberian Arctic (Vartanyan and others, 1993).

                  Mammoth fossils are relatively common in Oregon.  Some of the first Oregon mammoth fossils were collected by Thomas Condon at Fossil Lake.  Many teeth and bones were found during placer mining operations in Baker County.  Other sites along the Columbia River were discovered during excavations for hydroelectric dams.  The highest number of mammoth sites in Oregon is concentrated in the Willamette Valley.  Mammuthus columbi is by far the most common mammoth found in Oregon (Orr and Orr, 2009).

                  Fossils of Columbian mammoths from the late Pleistocene La Grande alluvial fan terrace include two molars found on the Eastern Oregon University campus in 1939; a radius found on the east side of the alluvial fan in 1940; the proximal end of a humerus found at the old hospital site on the northwest side of the alluvial fan terrace; and two molars found under home plate of the old Eastern Oregon University baseball field in 1979.  One of the molars found in 1979 was radiocarbon-dated at 15,280 ± 280 radiocarbon years before present by Geochron Laboratories.  The right tibia and two tusks discovered in January 2010 at the late Pleistocene Airport Lane fossil site are the latest mammoth fossils discovered in this area.

The Airport Lane Mammuthus

Tibia 

The mammoth right tibia found at the Airport Lane site was broken by the bulldozer excavating the site.  The fragments were reassembled (Figure 1) and the dimensions of the tibia (Figure 2) were measured according to the specifications of von den Driesch (1976).  The measurements (Table 1) show that the greatest length and other dimensions of the Airport Lane tibia are less than those of adult Mammuthus primigenius and Mammuthus columbi, except for the smallest breadth of the diaphysis, which falls between values reported for juvenile and adult M. primigenius reported by Lister (2009).  These results suggest that the Airport Lane tibia came from a juvenile mammoth.  This was confirmed by an X-ray radiograph of the tibia which shows that the epiphyseal plate is present (Figure 3).

Figure 1.  Views of the right tibia of a mammoth found at the Airport Lane site:  A.  Anterior view; B. Posterior view; C. Lateral view; D. Proximal view, and E. Distal view.

Figure 2.  Measurements of the tibia (after von den Driesch, 1976).  GL: Greatest length, Bp; Greatest breadth of proximal end; Dp: Greatest depth of proximal end; Bd: Greatest depth of distal end; Dd: Greatest depth of distal end.

Figure 3. X-ray radiograph of the proximal end of the tibia.  Note the prominent epiphyseal plate (arrow).

Table 1  Measurements of Mammoth Tibia

                  According to Lister (1994), epiphyseal fusion of the proximal end of male M. primigenius tibiae takes place at a dental age of ~28 years.  The timing of fusion to dental stage is the same in Mammuthus primigenius and Mammuthus columbi according to Haynes (1990), but Lister (1994) advanced the hypothesis that dental ages of Columbian mammoths may underestimate true ages at an unknown rate.  The presence of the epiphyseal plate in the Airport Lane mammoth tibia suggests an age less than 28 years or somewhat greater if Columbian mammoths had a slower rate of epiphyseal fusion as postulated by Lister (1994).

Tusks

The two mammoth tusks uncovered at the Airport Lane site were in a position that suggests that they may came from the same mammoth (Figure 4).  The tip of the right tusk was missing and the end of the left tusk was badly shattered.  Both tusks were hollow and filled with sediment.  When they were moved to the laboratory, only the tip of the left tusk and the anterior half of the right tusk survived intact (Figure 5).

Figure 4.  The position of the Airport Lane tusks in the field and measurements that were made on the tusks in the field and laboratory.

Figure 5. Mammuthus columbi tusks found at the Airport Lane Site.  The tip is from the left tusk and the other piece is the anterior half of the right tusk.  The right tusk (bottom left) is filled with silty fine sand.

                  The straight-line (chord) length and maximum length measured along the curve (curvilinear length) of each tusk were used to calculate the index of curvature of each tusk.  Since the right tusk was missing its tip, the length measurements for this tusk are minimum  values.  The diameter and circumference at the base of each tusk were also measured (Figure 4).

                  The right Airport Lane mammoth tusk is larger than the left (Table 2).  This may indicate that the tusks came from an animal with two different-sized tusks, or it may be an indication that the tusks came from two different mammoths.  The chord lengths and maximum lengths measured along the curve of the Airport Lane Mammuthus tusks fall within the ranges of both M. primigenius and M. columbi.  The index of curvature of the Airport Lane tusks is smaller than M. primigenius values and falls within the range of the index of curvature of M. columbi tusks.  This suggests that the Airport Lane mammoths belong to the species M. columbi like the other mammoths that have been previously found in the late Pleistocene La Grande alluvial fan terrace deposits.  The diameter at the base of the Airport Lane mammoth tusks falls in the lower range of M. columbi from Hot Springs, South Dakota and the relationship between the chord length and the maximum length measured along the curve of each tusk falls close to the trend of these parameters for the Hot Springs Mammuthus columbi (Figure 6).  The Hot Springs assemblage is primarily young adult males (Lister and Agenbroad, 1994).  It is likely that the Airport Lane mammoth was also male.

The girth (circumference) at the proximal ends of both tusks from the Airport lane site is 38 cm.  Haynes (1991) has noted that the age of African elephants correlates with the girth (circumference) at the alveolar insertion (lip line).  Plotting values for Columbian mammoths listed in Haynes (1991) on the same graph suggests that male Columbian mammoths had a growth curve similar to but faster than that of male African elephants.  Assuming that the girth at the ends of the Airport Lane tusks is approximately equal to the girth at the lip line (alveolar insertion), the girth of the Airport Lane tusks indicates that the Airport Lane mammoth was roughly 18 years old (Figure 7).

Table 2.  Measurements of Mammoth Tusks

a Maximum length minus chord length, divided by chord length.

Figure 6.  Plot of the base-tip (chord)  length vs. the maximum curvilinear length of tusks found at Hot Springs, South Dakota, and the tusk from the Airport Lane site.  L: Left Airport Lane tusk,  R: Right Airport Lane tusk,       Squares: Hot Springs, South Dakota, Mammuthus columbi.

Figure 7.  Growth rates of the tusks of male African elephants and Columbian mammoths plotted using the data of Haynes (1991).  Assuming that the girth of the Airport Lane tusks approximately equals the girth at the lip line suggests an age of ~18 years for the Airport Lane mammoths.

Fragments of the left tusk were radiocarbon dated by Geochron Laboratories:

                                    10,630 ± 490 14C years BP (13C corrected) on collagen

                                    10,650 ± 340 14C years BP (13C corrected) on bioapatite

These are the youngest Pleistocene fossils found in the La Grande alluvial fan terrace to date, but the dates fall in the range that Agenbroad (2005) feels should be viewed with skepticism.  If valid, they suggest that the Airport Lane mammoth was part of a small population that survived in the refuge of the Grande Ronde Valley after most mammoths went extinct.

Conclusions

The Airport Lane mammoth tibia and tusks came from a Columbian mammoth that can be classified as:

                                                      Class                          Mammalia

                                                      Order                        Proboscidea

                                                      Family                      Elephantidae

                                                      Genus                       Mammuthus Brookes, 1828

                                                      Species                                    Mammuthus columbi (Falconer, 1857)

                  The Airport Lane mammoth was a juvenile male that was approximately 18 years old.  It may have lived ~10,650 radiocarbon years (~12,700 calendar years) ago, although this date should be viewed with skepticism.  It is possible that the M. columbi tibia and tusks come from more than one individual, but the only evidence to support this hypothesis is the different lengths of the tusks, which could be a characteristic of a single individual.  Further excavation at the site may uncover other bones and teeth that may help us answer whether the tibia and tusks came from one or more juvenile mammoths.

Acknowledgments

 We are grateful to Bill Orr, who first suggested that the curvature of the tusks indicated that they were from a mammoth.  Dr. Mark Omann of Country Animal Clinic in La Grande x-rayed the mammoth tibia and identified the epiphyseal plate.

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