Pliocene Vole Fossils, Always Welcome Inn, Oregon
April Leithner and Jay Van Tassell
Science Department, Badgley Hall, Eastern Oregon University, La Grande, OR 97850; jvantass@eou.edu
Abstract
A broken left lower arvicoline rodent molar tooth found at the Always Welcome Inn fossil site in Baker City, Oregon, is similar to a lower left third molar of Ophiomys taylori found in the 3.6-3.0 Ma Hagerman fossil locality of Idaho, but may be a new species. Until more teeth, especially lower first molars, are found, it is difficult to make a precise identification. If the tooth does prove to be related to O. taylori (also known as Mimomys taylori), then it may provide important new information about the migration of microtine rodents and other animals between Lake Idaho, the great lake that occupied the western Snake River Plain in the Pliocene, and the Columbia River basin.
Introduction
Part of a microtine rodent tooth along with two broken microtine rodent incisor teeth were found by April Leithner in May 2005 in the silts in the upper part of the Always Welcome Inn sequence in Baker City, Oregon (Fig. 1, 2).
The tooth is a molar that measures 3.0 mm from tip to tip and the occlusal surface is ~1.4 mm x ~1.5 mm wide (Fig 3-5). Unfortunately, the posterior part of the tooth was broken during sampling, making it difficult to identify the tooth precisely. Dr. Chris Bell, a vertebrate paleontologist at the University of Texas, Austin, tentatively identified the tooth as belonging to an arvicoline rodent, but didn't want to speculate farther than that since identification of this group is based mainly on the shapes of lower first molars (Chris Bell, written communication, Feb. 2006). The tooth is probably from a juvenile (Greg McDonald, written communication, Feb. 2006).
The second lingual reentrant of the Always Welcome Inn tooth points in a posterior direction and the second labial reentrant points in an anterior direction. The first and second triangles are roughly equal in size. A reentrant on the anterior loop makes the loop narrower on the lingual end than the labial end (See Fig. 6 for terminology).
Figure 1. Location of the Always Welcome Inn fossil locality and other fossil localities mentioned in the text.
Figure 2. Rodent incisor fragments (two upper teeth) and microtine tooth found by April Leithner in May 2005.
Figure 3. Occlusal surface of microtine tooth found by April Leithner in May 2005.
Figure 4. Posterior and labial views of microtine tooth.
Figure 5. Ventral view of microtine tooth.
Figure 6. Parts of a lower left second microtine molar. Numbers of triangles are shown in italics. In some species the anterior loop (AL) is broken up into third and fourth triangles.
Comparison with Other Microtine Rodent Teeth from Adjacent Areas
Microtine rodents are primarily grass eaters and are specialized lineages of cricetid rodents (deer mice, wood rats, voles, and many more), which are basically seed eaters. Their major skeletal modification has been an increase in tooth complexity and height, resulting in prismatic cusps that provide greater resistance to wear and more tooth to be worn by abrasive food." (Repenning, 1987, p. 238). Microtine rodents are among the most rapidly evolving mammals known, which has caused many of the problems with their classification (Repenning, 2003, p. 470). As a result, many of the species in this group have been named, and then recombined, synonymized, and reassigned to several different genera (for more information on individual species, see The Paleontology Database, www.paleodb.org).
The very simple dentition of Promimomys represents a possible starting point for the derivation of the other microtines. The microtine genera Ogmodotomys, Propliopenacomys (also known as Cseria), and Pliophenacomys may have been derived from Promimomys by successively increasing the complexity of the anterior loop (Martin, 1975, p. 109). Promimomys may also have given rise to the succession Ophiomys taylori toOphiomys magilli to O. meadensis, and to O. parvus (Chaline and others, 1999). The gradation from Mimomys sawrockensis (Ophiomys magilli/Ophiomys mcknighti) to M. taylori (O. taylori) to M. meadensis (O. meadensis) and to Ophiomys parvus is strikingly displayed in the Glenns Ferry and Ringold Formations of Idaho and Washington in the Columbia and Snake River basins (Repenning, 2003, p. 477).
Figure 7 shows how the Always Welcome Inn microtine molar compares with the teeth of microtine species found in areas adjacent to the Always Welcome Inn locality that have similar shapes. The shape of the Always Welcome Inn anterior loop and shapes of its lingual and labial re-entrants appear most similar to those of Ophiomys taylori from the ~3.6-3.3 million year old Hagerman fossil beds, but the anterior loop is much narrower and the re-entrant angles are greater on the Always Welcome Inn microtine molar. This may be due in part to the juvenile character of the Always Welcome Inn molar. It may also indicate that the Always Welcome tooth is from a new, previously undescribed species.
Hibbard and Zakrzewski (1967, p. 258) noted that Ophiomys is a microtine rodent with alternating triangles are not as closed as in Pliophenacomys and with apices of the lingual re-entrant angles of the lower molars that are constricted and are generally directed forward. Ophiomys taylori is a small vole that has a highly variable m1 occlusal pattern (Hibbard and Zakrzewski, 1967, p. 262).
Until more teeth become available for further study, the Always Welcome Inn tooth is tentatively assigned to:
Class MAMMALIA Linnaeus, 1758
Order RODENTIA Bowditch, 1821
Family CRICETIDAE Rochebrune, 1883
GenusOPHIOMYS Hibbard and Zakrzewski 1967
Species OPHIOMYS cf. TAYLORI (Hibbard 1959)
Microtine Rodent Migrations in the Pacific Northwest
Repenning (2003) reassigned several Pacific Northwest Ophiomys species to Mimomys. Early Mimomys evolved from Promimomys in West Siberia and both genera migrated to North America and traveled south along the Pacific Coast ~5 m.y. ago. South of Canada, there were two primary dispersal routes eastward: Along the ancient Columbia River, not then connected to the Snake River that led from Yellowstone Pass, and along the Sacramento River, which then emptied into an inland sea in the position of the Central Valley of California. From the inland sea in California, Mimomys dispersed both up the Sacramento Basin northward and then eastward up the old Snake River and eastward south of the young Sierra Nevada and old Laramide Rocky Mountains to the Great Plains. The known localities of Promimomys, including the McKay Reservoir locality near Pendleton, Oregon, suggest that it moved eastward only up the Columbia River. The records of early Mimomys suggest that it migrated eastward from both the Columbia and from central California. M. sawrockensis (White Bluffs fauna, WA, 4.25 Ma) and the transitional species, M. sawrockensis-taylori (Blufftop fauna, WA, 3.9 Ma) are known only in the Columbia Basin. At ~3.6 Ma, when the drainage of the Snake River was diverted to the Columbia Basin, M. taylori appeared in the Snake River Plain at the Hagerman, ID, fossil locality and later evolved in the Snake River Plain area into M. meadensis (Sand Point fauna, ID, 3.0 Ma and Ophiomys parvus (Jackass Butte and other faunas, ID, 2.3 Ma, and Froman's Ferry fauna, ID, 1.5 Ma). M. meadensis migrated down the Snake River to Washington (Taunton fauna, WA, 2.9 Ma) and up the Snake River to Yellowstone Pass and the Great Plains between ~3.0-2.8 Ma during the initial stages of climate imbalance leading to the onset of extensive continental glaciation in North America (Repennning, 2003; Barnosky, 1985).
If the Always Welcome Inn microtine species is related to Mimomys (Ophiomys) taylori, it may represent a transitional stage between the M. sawrockensis-taylori (O. mcknighti of Gustafson, 1989) found in the White Bluffs Fauna and M. taylori of the Hagerman fauna. This hypothesis is difficult to test due to the lack of M2 teeth from M. sawrockensis-taylori at the White Bluffs locality for comparison. According to Repenning (2003, p. 496). An alternate possibility is that the Always Welcome Inn species represents a transitional stage between M. taylori of the Hagerman fauna and M. meadensis (Ophiomys taylori of Gustafson, 1995) of the Taunton fauna. The diatom fossil record in the Grande Ronde Valley of Eastern Oregon (Van Tassell and others, 2001) suggests that Lake Idaho, the great lake that occupied the western Snake River Plain during the Pliocene, may have spilled over into the Columbia River drainage at both 3.6 Ma and 3.2 Ma, allowing microtine rodents to migrate between the two drainage systems.
Further collecting of fossils at the Always Welcome Inn locality is needed to more precisely identify the type(s) of microtine rodents present in the sequence and their ages. Hopefully this will help refine our picture of migrations between Washington and Idaho and their relationship to the stages of Lake Idaho during the Pliocene.
Acknowledgments
We are grateful to Greg McDonald and Chris Bell for trying hard to identify the microtine rodent tooth from the Always Welcome Inn locality. Nate Carpenter and Laura Mahrt also provided valuable assistance and suggestions.
References Cited
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