LATE PLEISTOCENE AIRPORT LANE FOSSIL SITE, LA GRANDE, NE OREGON.
Taphonomy
Abstract
The bison, mammoth, and bear found at the Airport Lane fossil site may have drowned in a spring flood or a jÖkulhaup released when an ice dam burst or they may have been killed and butchered by late Pleistocene hunters, or they may have drowned and then been butchered.
Introduction
The late Pleistocene Airport Lane fossils were discovered by a bulldozer operator removing the top of a hill in a farmer's potato field in the southern Grande Ronde Valley in January 2010. The site is located on an erosional remnant of the distal margin of a late Pleistocene outwash fan. Diatoms and sponge spicules in the silty fine sands at the site suggest deposition in a shallow stream and adjacent areas. Vegetation in the area included grasses, juniper, and pondweed. Mountain hemlock grew in the hills surrounding the valley.
The Airport Lane fossils include: 1) the tusks and tibia of a juvenile male Columbian mammoth (Mammuthus columbi); 2) seven articulated vertebrae, an articulated radius/ulna and carpals, and the proximal end of a calcaneus from a large male bison that is similar in size to Bison latifrons; 3) the proximal end of the femur of a giant short-faced bear (Arctodus simus simus); and 4) a partial skeleton and other bones from several Columbian ground squirrels (Spermophilus columbianus), a species which is found today in the colder and wetter areas on the margins of the Grande Ronde Valley and not at the Airport Lane fossil site.
The mammoth tusks were found together in a position that suggests they came from the same animal. The right sides of the bison vertebrae were sheared off by the bulldozer, an indication that the bison skeleton was lying on its left side. The mammoth tusks yielded a radiocarbon date of 10,650 +/- 380 14C years. AMS radiocarbon dating of fragments of the bison vertebrae gave a date of 12,950 +/- 50 14C years. The younger age of the tusks may be due to alteration as the tusks dissolved and filled with sediment.
Taphonomy
The articulation of the bison fossils and finding two mammoth tusks from what appears to be the same individual in late river sediments at the Airport Lane fossil site suggests that the bison, mammoth, and bear may have drowned in a flood and been washed to the distal margins of the late Pleistocene La Grande outwash fan. This may have occurred during the spring as rivers were swollen with melt waters from the glaciers in the nearby mountains. Bilderback (1999) and Geraghty (1999) have mapped a Pleistocene ice-dammed lake in the headwaters of the rivers in the Elkhorn Mountains. It is possible that an ice dam may have burst producing a jÖkulhaup which killed the larger animals at the Airport Lane fossil site. It is likely that the ground squirrels burrowed into these flood deposits some time after the flood.
This hypothesis does not answer the question of why the fossils of the large animals at the Airport Lane fossil site consist only of tusks, articulated vertebrae, and leg bones. Where are the rest of the skeletons? Are they still in the ground or were they washed away? Another possibility is suggested by Agenbroad's (1978) study of the early Holocene Hudson-Meng bison kill site in Nebraska, where butchering of the bison left behind numerous articulated vertebrae and limb elements, including abundant femurs, tibiae, radii, and calcanei. Although no tools or butchering marks have been found, butchering of the animals at the site would explain why these particular skeletal elements are present and others are missing. Perhaps the Airport Lane large animals were killed and butchered by late Pleistocene hunters.
It is also possible that the larger animals at the Airport Lane fossil site were killed in a flood and butchered after they drowned by late Pleistocene hunters. We will not know for sure how the bison, mammoth, and bear died until further evidence is uncovered at the site.
Conclusions
It is not known how the bison, mammoth, and bear found at the Airport Lane fossil site died. They may have drowned in a spring flood or a jÖkulhaup released when an ice dam high in the mountains burst or they may have been killed and butchered by late Pleistocene hunters. Further investigation at the site may help clarify this mystery.
References Cited
Agenbroad, L.D., 1978, The Hudson-Meng Site: An Alberta bison kill in the Nebraska High Plains: Washington, D.C., University Press of America, 230 p.
Bilderback, E., 1999, Late Quaternary glacial geology and paleoclimate interpretations of the Anthony Lakes drainages, Elkhorn Mountains, Oregon, in Twelfth Keck Research Symposium in Geology Proceedings, p. 275-278.
Geraghty, E., 1999, Glaciation of the Elkhorn Mountains, northeastern Oregon: in Twelfth Keck Research Symposium in Geology Proceedings, p. 283-286.