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

Columbian ground squirrel (Spermophilus columbianus)

Abstract

 Columbian ground squirrel (Spermophilus columbianus) fossils found at the Airport Lane site suggest cooler and wetter conditions at the site during the late Pleistocene.  These are the first late Pleistocene Spermophilus columbianus fossils found in Oregon and the second in the United States.

Introduction

The genus Spermophilus evolved in North America from the more primitive Miospermophilus as climates became dryer and grassland habitats spread in the Great Plains during the Miocene (Black, 1963, 1972).  There are five species of Spermophilus that are found today in northeast Oregon:

1) Belding's ground squirrels (Spermophilus beldingi) have a geographic range from northeast Oregon south to California, southwest Idaho, north-central Nevada, and northwest Utah and has denser populations in agricultural areas at lower elevations than at high elevation sites (Jenkins and Eshelman, 1984).  It is common on the floor of the Grande Ronde Valley (Verts and Carraway, 1998);

2) Columbian ground squirrels (Spermophilus columbianus) are found in the Rocky Mountain region of western Montana, Idaho, northeastern Washington, southeastern British Columbia, and western Alberta.  They are also found on the plains of eastern Washington and in the Blue and Wallowa Mountains of eastern Oregon (Elliott and Flinders, 1991).  They are typically found on the edges of alpine or subalpine meadows and grasslands and on mounds within meadows where flooding occurs in the upper part of the Ponderosa Pine zone to the lower part of the Subalpine Fir zone.  Colonies of Columbian ground squirrels are found along the margins of the Grande Ronde Valley, including the Ladd Canyon area 8 km southwest of the Airport Lane fossil site (Betts, 1976; Verts and Carraway, 1998);

3) Golden ground squirrels (Spermophilus lateralis) are found in the mountains of the western United States and California and range in elevation from 1,220 m to 3,965 m).  Its range includes most of Oregon except for the Coast Range (Bartels and Thompson, 1998);

4) Townsend's ground squirrel s(Spermophilus townsendii) are distributed throughout the Great Basin and the Columbia Plateau, including Nevada, western Utah, eastern California, southern Idaho, eastern Oregon, and south-central Washington (Rickart, 1987); and

5) Washington ground squirrels (Spermophilus washingtonii) have a geographic range restricted to the Columbia Plateau lying south and east of the Columbia River in Washington, and between the John Day River and the Blue Mountains in adjacent northeastern Oregon.  They are found at elevations ranging from 90-450 m, within the upper Sonoran life zone (Rickart and Yensen, 1991).  The range has been reduced in recent years due to the loss of habitat to agriculture (Betts, 1990).

The Airport Lane Spermophilus

One of the workers at the Airport Lane site digging near the mammoth tusks dug up a clot of soil that contained the skull and partial skeleton of a ground squirrel, plus the femur of a second ground squirrel (Figures 1, 2).  A mandible of a third ground squirrel was found on the bulldozed surface approximately 7 m to the southeast.

                  The ground squirrel fossils include a cranium, two mandibles, a left humerus and a right humerus, four articulated vertebrae, a left pelvis, 3 ribs, a right radius and a right ulna, a right femur and 2 left femurs, and a right tibia and a left tibia (Figure 3).

Figure 1. Location of ground squirrel skeleton relative to the mammoth tusks at the Airport Lane site.

Figure 2. Left lateral view of some of the ground squirrel bones found near the mammoth bones.  The bones that were found are colored black on the lower diagram.

Figure 3. Photographs of the Spermophilus columbianus fossils found at the Airport Lane Site:

A. Dorsal view of cranium; B. Ventral view of cranium; C. Lateral view of cranium and mandible; D. Dorsal view of mandible.  The left mandible (with I, p4) was found with the cranium; the right mandible (with I, p4, m1, m2, m3) was found ~7 m to the southwest; E. Dorsal view of palate showing left and right I, P4, P3, M1, M2, and M3.

Figure 3, continued.  F. Vertebrae (4) and other bone fragments in matrix; G. Anterior view of ribs; H. Anterior view of right humerus; I. Anterior view of left humerus; J. Anterior view of right radius; K. Anterior view of right ulna; L. Left lateral view of pelvis; M. Anterior view of right femur; N. Anterior view of left femur; O. Anterior view of left femur found 7 m southeast of the location where the ground squirrel skeleton was found.

Figure 3, continued.  P. Anterior view of right tibia (broken); Q. Anterior view of left tibia.

                  These ground squirrel fossils came from a horizon 5 m below the late Pleistocene terrace surface, well below the depth (~1.5 m) that these rodents normally burrow.  This suggests that they may be the same age or slightly younger than the other late Pleistocene fossils at the Airport Lane site, but are not modern ground squirrels.

                  The skull was complete when it was removed from the soil matrix, but the posterior portion of the skull fell apart shortly afterward.  Because of this, measurement of the greatest length of the skull was impossible.  The length of the maxillary tooth row, the nasal length, and the palatilar length were measured with a digital caliper.  An estimate of the zygomatic breadth of the skull was made from the fragments of the posterior portion of the skull (Figure 4).

Figure 4. Measurements made on the squirrel skull.  MTR: Maxillary tooth row; NL: Nasal length; PL: Palatilar length; and ZB: Zygomatic breadth.  Modified from Helgen and others (2009).

                  The measurements of the skull (Table 1) show that the Airport Lane Spermophilus has a palatilar length in the range of Spermophilus columbianus.  The estimated zygomatic breadth falls within the ranges of Spermophilus beldingi and Spermophilus lateralis and is slightly smaller than the lower limit of the range of Spermophilus columbianus.  The nasal length falls within the range of Spermophilus beldingi, Spermophilus columbianus, and Spermophilus lateralis.  The length of the maxillary tooth row of the Airport Lane Spermophilus is slightly different on the left and right sides.  Both measurements fall within the range of Spermophilus columbianus and are larger than the values for the other species.

Table 1.  Spermophilus Skull Measurements

                  Based on this analysis, the Airport Lane Spermophilus is most similar to Spermophilus columbianus, the species presently found on the margins of the Grande Ronde Valley and in the nearby mountains.  This identification is confirmed by the nearly parallel upper tooth rows of the palate (Figure 3E), which is a diagnostic feature of Spermophilus columbianus according to Hall (1981) and Elliott and Flinders (1991).

                  This suggests that the Airport Lane ground squirrel can be classified as:

                                                      Class                          Mammalia

                                                      Order                        Rodentia

                                                      Suborder               Sciuromorpha

                                                      Family                      Sciuridae

                                                      Subfamily             Sciurinae

                                                      Tribe                         Marmotini

                                                      Subtribe                Spermophilina

                                                      Genus                       Spermophilus F. Cuvier, 1825

                                                      Species                                    Spermophilus columbianus (Ord, 1915)

Discussion

 The presence of Spermophilus columbianus at the Airport Lane site indicates wetter and colder conditions on the floor of the Grande Ronde Valley during the late Pleistocene than at present.  The ground squirrels may have lived at the site at the same time as the other animals or they may have burrowed down into the fossil horizon shortly after the other fossils were deposited.  The depth of burial (5 m) is well below the burrowing depth of modern ground squirrels.

                  Spermophilus columbianus fossils have been previously described only at the late Pleistocene (Rancholabrean) Wasden Site (Owl Cave) in Idaho (Guilday, 1969; Kurtén and Andersen, 1980).  This is the first time they have been found in Oregon.

Conclusions

 The Airport Lane ground squirrel is Spermophilus columbianus, not Spermophilus beldingi, the species currently found in the area.  This suggests colder and wetter conditions at the site during the late Pleistocene.  The Airport Lane ground squirrels may be the same age or slightly younger than the other fossils discovered at the site.  The Airport Lane site is the first locality in Oregon and the second in the United States where Late Pleistocene Spermophilus columbianus fossils have been found.

References Cited

Bartels, M.A., and Thompson, D.P., 1993, Spermophilus lateralis: Mammalian Species, no. 440, p. 1-8.

Betts, B.J., 1976, Behavior in a population of Columbian ground squirrels, Spermophilus columbianus columbianus: Animal Behavior, v. 24, p. 652-680.

Betts, B.J., 1990, Geographic distribution and habitat preferences of Washington ground squirrels (Spermophilus washingtonii): Northwest Naturalist, v. 71, p. 27-37.

Black, C.C., 1963, A review of the North American Tertiary Sciuridae:  Bulletin of the Museum of Comparative Zoology, v. 130, p. 109-248.

Black, C.C., 1972, Holarctic evolution and dispersal of squirrels (Rodentia: Sciuridae), in Dobzhansky, T.; Hecht, M.K; and Steere, W.C., eds.; Evolutionary biology: New York, Meredith Corporation, v. 6, p. 1-445.

Elliott, C.L., and Flinders, J.T., 1991, Spermophilus columbianus: Mammalian Species, no. 372, p. 1-9.

Guilday, J.E., J.E., 1969, Small mammal remains from the Wasden site (Owl Cave), Bonneville, Idaho: Tebiwa, v. 12, p. 47-57.

Hall, E.R., 1981, The mammals of North America, Second edition: New York, John Wiley and Sons, 600 p.

Helgen, K.M.; Cole, F.R.; Helgen, L.E.; and Wilson, D.E., 2009, Generic revision in the holarctic ground squirrel genus Spermophilus: Journal of Mammalogy, v. 90, no. 2, p. 270-305.

Jenkins, S.H., and Eshelma, B.D., 1984, Spermophilus beldingi: Mammalian Species, no. 221, p. 1-8.

Kurtén, B., and Anderson, E., 1980, Pleistocene mammals of North America: New York, Columbia University Press, 442 p.

Ord, G., 1815, North American zoology, in Guthrie, W., ed., A new geographical, historical, and commercial grammar, 2nd edition: Philadelphia, Lippincott, p. 303.

Rickart, E.A., 1987, Spermophilus townsendii: Mammalian Species, no. 268, p. 1-6.

Rickart, E.A., and Yensen, E., 1991, Spermophilus washingtonii: Mammalian Species, no. 371, p. 1-5.

Verts, B.J., and Carraway, L.N., 1998, Land mammals of Oregon: Berkeley, University of California Press, 668 p.