Pliocene (Blancan) Voles at the Always Welcome Inn Fossil Site,

Baker City, Oregon: A Study in Progress

Eric Bergey, Misty Davis, Jayson Kisselburg, April Leithner, Story Miller, Carli Morris, Takeshi Sugimoto, and Jay Van Tassell

Science Department, Badgley Hall, Eastern Oregon University, One University Boulevard, La Grande, OR 97850

Abstract

              The species of fossil vole found at the Always Welcome Inn appears to be more closely related to Cosomys primus, the species found at Hagerman and in the Coso Mountains of southern California, than to the Ophiomys mcknighti found in the Columbia River drainage area.  It may be an ancestor of Cosomys primus and, perhaps, Ogmodontomys sawrockensis.  Additional lower first molars need to be found and the age of the fossils more precisely defined in order to test this hypothesis.

Introduction:  Debate over the Classification of Microtine Rodents

              One of the exciting fossil finds at the Always Welcome Inn has been the discovery of the teeth of a fossil vole.  Voles are part of a group called microtine rodents.  Microtine rodents are primarily grass eaters and are specialized lineages of cricetic rodents (deer mice, wood rats, and many more).  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.  The large number of species in this group and their rapid evolution has led to a lot of confusion about the group’s evolutionary history and their classification.  As a result, many of the species in this group have been named, and then recombined, synonymized, and reassigned to several different genera (Repenning, 1987, 2003).

              In the late 1930’s, Claude Hibbard of the University of Michigan in the United States and Miklos Kretzoi in Hungary, along with their students, started trying to sort out the history of microtine rodents.  By 1965, Kretzoi recognized that new and more advanced species that appeared abruptly in the European fossil record evolved in the steppe environments of Asia and migrated from there to central Europe.  Kretzoi saw a strong similarity between European and North American fossil microtines, but Hibbard, who had earlier agreed, was convinced that the North American microtine fauna was not the result of invasions from Asia despite considerable evidence of the immigration of other mammals from Asia to North America (Repenning, 1987).

              The development of radiometric dating techniques, especially K-Ar dating, has made it possible to develop a more detailed biochronology of the microtine rodents, but the debate about the origins and evolution of this group continue unabated.  According to Repenning (1987), the primitive microtine Promimomys (also known as Prosomys), immigrated to North America at ~6.7±0.5 Ma.  Prosomys mimus is one of the rodents found in the late Miocene (Hemphillian) fossil locality at McKay Reservoir, near Pendleton, Oregon.  Two microtine subgenera, Mimomys (Ophiomys) and Mimomys (Cosomys) emigrated from Asia to North America at ~ 4.8±0.2 Ma and evolved into a distinct fauna in the United States by ~4.25±0.3 Ma.  Ophiomys fossils are common in the Pliocene fossil localities at Hagerman and other nearby localities in Idaho, and the Pliocene White Bluffs, Blufftop, and Taunton localities in Washington.  Cosomys is very common at Hagerman, but has not been found at the Washington sites.

              Underlying the debate about whether or not these microtines migrated from Asia to North America or evolved on the North American continent is considerable disagreement about the classification of these groups of microtine rodents.  For example, Cosomys primus was first described in the ~3.0 million years-old sequences in the Coso Mountains of southern California by R.W. Wilson (1932), who reasoned that the few differences and the great geographic separation from Europe suggested that assigning this genus to Mimomys would not be appropriate, a conclusion that was disputed by Hinton (1932), who felt that Wilson’s Cosomys mimus was a species of Mimomys.  When Wilson (1934) discovered Cosomys mimus in the fossil beds at Hagerman, Idaho, he described it as Mimomys (Cosomys) primus, agreeing with Hinton’s opinion, but retaining the name Cosomys as a subgenus.  In contrast, Hibbard (1948, 1952) described a microtine from the Saw Rock Canyon fauna of Kansas as Cosomys primus, stating that the Saw Rock microtine was not Mimomys.  Later, Hibbard (1957) reassigned the Saw Rock microtine to a different genus and renamed it Ogmodontomys sawrockensis.  Based on lesser hypsodonty of the teeth of Ogmodontomys sawrockensis, he suggested it was the ancestor of Cosomys primus.  Repenning (1987) returned the species Ogmodontomys sawrockensis to the subgenus Cosomys, where Hibbard (1941, 1952) had first put it (as a genus) and to the genus Mimomys following Hinton (1932) and Wilson (1934), but this reassignment has been challenged by other workers, who have sided with Hibbard rather than Repenning. 

              As stated by Robert Martin (written communication, 2006): “The problem is not easy, as I'm sure you can tell from the literature.  Most of the archaic voles with rooted molars from the western US that I have seen are referable to Ophiomys. The late Charles Repenning and a few others lump all these ancient voles with a three triangle m1 into Mimomys, but I think the New World experienced a parallel, endemic radiation to what went on in Eurasia, and our Ophiomys and Ogmodontomys represent a New World clade with its own evolutionary tendencies.  Cosomys is another vole in this group. At some point, of course, all these forms "regress" to a common ancestor that would look very much like the more primitive Ogmodontomys and Ophiomys. We do not know if this ancestor gave rise to both Ophiomys and Ogmodontomys in eastern Asia before they dispersed here, or if the common ancestor, perhaps something like Promimomys antiquus, came over here and then diversified.  I have not yet seen anything from the Pacific coast that was clearly Ogmodontomys; almost all of the specimens, such as at Taunton, White Bluffs, Maxum, etc. appear to be close to Gustafson's Ophiomys mcknighti. Yun Mou's "Mimomys" panacaensis, from the earliest Pliocene of Nevada, also seems to be an Ophiomys, and may also be close to O. mcknighti.  But Great Plains Ogmodontomys must have had a western ancestor, so it is certainly there to be found, or perhaps even "recognized" among some fossils in various collections. A careful cladistic analysis of all the western archaic arvicolids would be a fine contribution.”

              This is not just an argument about which genus a species belongs to.  It is about where the microtine rodents in North America came from and how and where they evolved.  Do the Mimomys, Ogmodontomys, or Ophiomys (depending on which

Figure 1.  Vole teeth from the Always Welcome Inn.  L:  left;  R:  right; m:  lower;  M:  1,2,3:  First, second, third molars.  All the molars have two roots except the right second and third upper molars.

Table 1.  ALWAYS WELCOME INN MICROTINE TEETH:

SIZE MEASUREMENTS

Table 2.  COMPARISON OF ALWAYS WELCOME INN MICROTINE TEETH WITH MICROTINE TEETH FROM OTHER AREAS

classification you pick) microtine fossils which are found in the Maxum fauna of the San Francisco area, the ~4.8 Ma Upper Alturas faunas of northeastern California, and the ~4.8 Ma Panaca lake beds of Nevada provide evidence for the migration of Mimomys from North America, as suggested by Repenning (2003) or did they originate in North America as Hibbard (1964) believed?  Did Mimomys (Cosomys) primus (or Cosomys primus) originate from the Ogmodontomys found in the Great Plains or from the ~4.8 Ma species found in California and Nevada?  New evidence is desperately needed to resolve this controversy.

The Always Welcome Inn Vole Fossils

              We have found a total of eight microtine rodent molars at the Always Welcome Inn to date, plus three incisors that may be from the same species (Fig. 1, Tables 1 and 2).  Ever since a molar and two incisors of a juvenile vole were found by April Leithner in May 2005, we have puzzled over the identity of the Always Welcome vole.  The broken lower left third molar found by April Leithner appeared similar to that of Ophiomys taylori from the Snake River Plain of Idaho, but we were unable to tell for sure if that identification was correct.  The shape of a right upper third molar by Jayson Kisselburg in June 2006 suggested that the Always Welcome Inn vole might be more similar to Ogmodontomys poaphagus, a vole found in the Pliocene sediments of the Midwestern United States.  This prompted us to send photos of the Always Welcome Inn vole to Dr. Robert Martin of Murray State University in Kentucky, a paleontologist who is a world leader in the study of microtine rodents.  Dr. Martin responded (Martin, written communication, 2006) that he did not think that the Always Welcome Inn vole was Ogmodontomys, but rather was more closely related to Ophiomys or Cosomys.  Based on his comments, we searched the literature and concluded that the upper third molar of the Always Welcome Inn was most similar to that of Ophiomys cf. mcknighti, a fossil vole described in the Blufftop local fauna of Washington by Gustafson (1978).  We could not say whether or not this identification was correct because the classification of fossil voles is based mainly on the shapes of lower first molars.

              We finally found a lower left first molar of a vole at the Always Welcome Inn in October 2006.  The posterior loop of the tooth is broken off, but the anterior loop was complete (Fig. 1).  We sent Dr. Martin pictures of the tooth and he replied (Martin, written communication, 2006):

“Nice find! Maybe better than we both anticipated. The morphology of that one juvenile specimen does not appear to be the same as in O. mcknighti.  I wish you were here and I could show you the differences among a variety of specimens, but anyway, take out Gustafson's illustrations of O. mcknighti and Hibbard and Zakrzewski's (1967) figures of Ophiomys (Contribs Mus. Paleontology, U. Michigan 21:255-271).  Also, Yun Mou's paper (JVP 17: 376-383) on O. panacaensis.  Look at the anteroconid (the broad area with the enamel atoll).  Notice how narrow your specimen appears. This is in part due to deep penetration by LRA4 (lingual reentrant 4) which, at least when there is an atoll, is almost never seen in Ophiomys, especially not the early ones like O. mcknighti and O. panacaensis.  Instead, this is a feature, along with the bulbous form of the anterior part of

Figure 2.  Comparison of the occlusal pattern of the lower left first molar of the fossil vole found at the Always Welcome Inn with those of fossil voles from Alturas, California (Mimomys sawrockensis);  Panaca, Nevada (Mimomys panacaensis); White Bluffs, Washington (Ophiomys mcknighti); and Hagerman, Idaho (Cosomy) primus).

the acd above LRA4, that is seen in Cosomys from the Hagerman l.f.  At least based on this single specimen, I would suggest you might have an ancestor for Cosomys; maybe even for Ogmodontomys as well, as Great Plains Ogmodontomys had to evolve from a west coast ancestor in the early Blancan.  This is really cool!  You need to get out there and get a larger sample of m1s.  If this turns out to be the case, the next step would be to borrow a sample of Cosomys primus from Idaho State and maybe some Ogmodontomys and do a multivariate comparison.”

              We recently found a lower right first molar, but the top of the occlusal surface is broken off on parts of the anterior and posterior loops.  It is not clear what this tooth will tell us about the Always Welcome Inn vole.

              We still do not know exactly where the species of fossil vole found at the Always Welcome Inn fits into the evolutionary tree of microtine rodents in North America, but now we know that it may be a very important piece of the puzzle (Fig. 2).  We recently discovered a pocket of volcanic ash ~0.15 meters below the layer that has produced the vole teeth.  We sent a sample of the ash has been sent to the Geochronology Laboratory of the University of Alaska at Fairbanks for Ar/Ar radiometric dating.  Hopefully this will tell us how old the Always Welcome Inn fossils are.  If we can find some more lower first molars, perhaps someday we will solve the puzzle of the voles present in the Always Welcome Inn sequence.

Conclusions

              The species of fossil vole present at the Always Welcome Inn appears to be more closely related to Cosomys primus, the species found at Hagerman and in the Coso Mountains of southern California, than to the Ophiomys mcknighti found in the Columbia River drainage area.  It may be an ancestor of Cosomys primus and, perhaps, Ogmodontomys sawrockensis, but additional lower first molars will need to be found and the age of the fossils more precisely defined before we can test this hypothesis.

Acknowledgements

              We are very grateful to the Langrells, the owners of the Always Welcome Inn, for making this research possible, and to Greg McDonald and Bob Martin, for sharing their expertise and helping us try to identify the vole fossils from the Always Welcome Inn.

References Cited

Gustafson, E.P., 1978, The vertebrate faunas of the Pliocene Ringold Formation, south-central Washington:  University of Oregon Museum of Natural History Bulletin 23, p. 1-62.

Hibbard, C.W., 1941, Mammals from the Rexroad fauna of the upper Pliocene of southwestern Kansas:  Kansas Academy of Science Transactions 44, p. 265-313.

Hibbard, C.W., 1949, Pliocene Saw Rock Canyon fauna in Kansas:  University of Michigan Museum Contributions 7, p. 91-105.

Hibbard, C.W., 1952, Vertebrate fossils from late Cenozoic deposits of central Kansas:  University of Kansas Paleontologic Contributions, Vertebrata 2, p. 1-14.

Hibbard, C.W., 1957, Two new Cenozoic microtine rodents:  Journal of Mammalogy, v. 38, p. 39-44.

Hibbard, C.W., 1964, A contribution of the Saw Rock Canyon local fauna of Kansas:  Papers of the Michigan Academy of Science, Arts, and Letters 49, p. 115-127.

Hibbard, C.W., and Zakrzewski, R.J., 1967, Phyletic trends in the late Cenozoic microtine Ophiomys gen. nov., from Idaho:  University of Michigan Museum of Paleontology Contributions 21, p. 255-271.

Hinton, M.A.C., 1932, Note on “Cosomys” Wilson, from the Pliocene of California:  Journal of Mammalogy, v. 13, p. 280-281.

Mou, Y., 1997, A new arvicoline species (Rodentia: Cricetidae) from the Pliocene Panaca Formation, southeast Nevada:  Journal of Vertebrate Paleontology, 17, no. 2, p. 376-383.

Repenning, C.A., 1987, Biochronology of the microtine rodents of the United States, in M.O. Woodburne, ed., Cenozoic mammals of North America: Geochronology and biostratigraphy:  Berkeley, University of California Press, p. 236-268.

Repenning, C.A., 2003, Mimomys in North America:  Bulletin American Museum of Natural History, no. 279, p. 469-512.

Wilson, R.W., 1932, Cosomys, a new genus of vole from the Pliocene of California:  Journal of Mammalogy, v. 13, p. 150-154.

Wilson, R.W., 1934, A rodent from late Cenozoic beds of southwestern Idaho:  Carnegie Institute of Washington Contributions to Paleontology 490, p. 119-135.

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