Early Pliocene (Blancan) Fossil Beaver (Castor) Jaw Found At the Always Welcome Inn, Baker City, Oregon

Adora Brockman and Troy Tubbs, Pine Eagle High School, 400 N. Main Street, Halfway, Oregon 97834;

Story Miller and Jay Van Tassell, Science Department, Badgley Hall, Eastern Oregon University. One University Boulevard, La Grande, Oregon 97850

Abstract

              The lower left jaw of the beaver Castor californicus was discovered near the base of the Always Welcome Inn sequence in Baker City, Oregon, in April 2007.  It includes the incisor, fourth premolar and first, second and third molars.  It is large compared to other Castor fossils found in nearby areas.  The occlusal pattern of the lower second molar is more similar to teeth found in the 4.1 million year-old Etchegoin Formation of the Kettleman Hills of California than it is to lower second molars of Castor found in the 2.3-2.0 million year-old Jackass Butte faunas of the Snake River Plain.  The overall occlusal patterns of the teeth are less convoluted than those of Castor teeth found in late Pliocene faunas of the Snake River Plain of Idaho.

Figure 1.  The lower left jaw of the beaver Castor californicus found at the base of the Always Welcome Inn sequence (EO-919).  Top:  Lateral view; Middle:  Medial view; Bottom:  Occlusal view.

Introduction

              A lower left jaw of the fossil beaver Castor (Figure 1) was found in April 2007 at the base of the early Pliocene Always Welcome Inn sequence in Baker City, Oregon, by Adora Brockman, who was then a seventh grade student at Pine-Eagle High School in Halfway, Oregon.  The diatom-rich sediments which yielded the beaver jaw also contain turtle shell, gastropods, bivalves, and thin organic-rich layers that suggest shallow water deposition near the shore of a lake.

A Tale of Two Beaver Teeth:  Pliocene Castor Fossils in the Western U.S.

              The tale of the Pliocene relatives of the modern beaver, Castor, has some interesting twists and turns.  The first species discovered in the western United States was Castor californicus, which was named by Kellogg (1911) based on one tooth found in the ~4.1 m.y.-old Etchegoin Formation in the Kettleman Hills area of California.  She assigned the tooth as a new species because the width of the tooth in a transverse direction was greater than its length in an anteroposterior direction, which is not the case in the modern species Castor canadensis.  Wilson (1933) pointed out that Kellogg’s identification of the tooth as an upper second molar might not be  correct, which turned out to be true.  Stirton (1935) identified the tooth as an upper third molar, not an upper second molar as Kellogg (1911) had thought.  He also examined other teeth of the same species from the Etchegoin Formation and described the species (Stirton, 1935, p. 445) as “large beavers, in size equaling or larger than the largest individuals of C. canadensis; external striae on upper cheek teeth and internal striids on lower cheek teeth unequal in length and reaching the base of the tooth less frequently than those of modern beavers; incisors like those of C. canadensis but wider (8.8 mm)” (see Figure 2 for an explanation of the nomenclature of beaver teeth).  He also described material from the Snake River Plain of Idaho that he referred to Castor cf. californicus and mentioned a collection from the Hagerman area of Idaho that he concluded was referrable to the same species.

Figure 2.  Nomenclature for parts of beaver teeth, modified from Korth (2001).

              Another Castor tooth, an upper fourth premolar, was collected by N.F. Drake in 1897 from the ~1.7-1.5 m.y.-old deposits of Idaho’s Snake River Plain near the site of Fromans Ferry.  F.A. Lucas of the U.S. National Museum identified it as Castor and suggested that it was possibly a new species, a conclusion that was affirmed by Merriam (1918).  Hay (1927) assigned the tooth to a new species, Castor accessor.

              At this point, the tale starts to get complicated.  Shotwell (1970) noted that the tooth from Fromans Ferry displays an occlusal pattern common in both fossil and living beavers of the genus Castor.  He concluded that it is not possible to determine the relative length of the striids compared to those of other beavers because the tooth from Fromans Ferry is from a young individual.  Because of this, Shotwell (1970) defined a sample from Jackass Butte, the youngest of the Grandview Faunas (2.32-2.0 Ma), as a supplementary type (plesiotype) in order that the beaver could be characterized.  Shotwell (1970, p. 39) defined Castor accessor as:  “a large beaver, approximately 10% larger than the living Castor canadensis but similar in gross characters of dentition and skeleton.  In the lower fourth premolar the hypostriid and mesostriid are directly opposite.  Stria and striids of cheek teeth longer than in C. californicus and shorter than in C. canadensis.  Occlusal pattern of lower P4 variable in occurrence of isolated lophs and development of fossettes to about the same degree as C. canadensis.  Hypostria and hypostriid to base of teeth.”

              Shotwell (1970) was careful to point out that future work might prove his definition of Castor accessor to be unsound.  This caveat turned out to be correct.  Gustafson (1978) pointed out that no adequate quantitative data was available for the lengths of the stria and striids in Castor californicus and Castor accessor.  This lack of data and the fact that Castor californicus was the better founded and earlier name led Gustafson (1978) to choose Castor californicus for the name of the Castor species found in the ~4.3 million year-old White Bluffs fauna of the Ringold Formation in Washington.  Later detailed measurements of Castor teeth by Conrad (1980) showed that the ~3.7-3.3 million year-old Hagerman Castor was no different from the beaver from Jackass Butte assigned to Castor accessor by Shotwell (1970).  Conrad (1980) concluded that both the Hagerman and Jackass Butte species belong to Castor californicus and he suggested the possibility that it was likely that the Fromans Ferry Castor specimen was also Castor. californicus.  This was confirmed when Repenning and others (1995) pointed out that:  1) Shotwell’s (1970) observations that the opposition of the hypostriid and mesostriid in Castor accessor was dissimilar to living Castor canadensis and that the striae and striids in cheek teeth of Castor accessor were longer than in Castor californicus were not always true, and 2) Shotwell’s (1970) assumption that the Fromans Ferry and Jackass Butte faunas were the same age was not valid.  As a result, Repenning and others (1995) reassigned the Fromans Ferry Castor to Castor californicus.  The two original Castor species in the western United States are now one, at least for the moment.  This means that, unless the Castor species found at the Always Welcome Inn is a new species (which does not appear likely) the Always Welcome Inn jaw can be assigned to Castor californicus.

How Does The Always Welcome Inn Castor Compare with Other Castor Fossils from the Western United States of Approximately the Same Age?

              A comparison of the sizes of the teeth of the Always Welcome Inn Castor with others described in the literature suggests that the Always Welcome Inn Castor jaw ranks with larger fossil Castor fossils found at Hagerman, Idaho, and other localities in the western United States (Table 1).

I1:  Lower incisor

P4: Lower 4th premolar          M1:  Lower 1st molar              M2:  Lower 2nd molar              M3:  Lower 3rd molar

AP: Anterioposterior                                TR:  Transverse

Figure 3.  Comparison of the occlusal patterns of the Always Welcome Inn Castor jaw with Castor teeth from Idaho and California (not to scale).

The anteroposterior dimensions of the premolar and molars, the transverse dimensions of the second and third molars, and the overall length from the fourth premolar to the third molar of the Always Welcome Inn Castor jaw are large compared to Castor fossils from the other areas.  Only the transverse dimensions of the Always Welcome Inn Castor lower incisor and first molar fall within the range of Castor tooth dimensions found in the Hagerman area.

              The occlusal patterns of the teeth in the Always Welcome Inn jaw are less convoluted than those of the teeth of a lower left Castor jaw from Jackass Butte illustrated by Shotwell (1970).  The overall shape of the occlusal pattern of the lower left second molar from the Always Welcome Inn is very similar to a lower second molar (or possibly a lower first molar, according to Stirton, 1935) from the 4.1 million year-old Etchegoin Formation (Figure 3).  This tooth is one of the paratypes of Castor californicus.

Castor and Dipoides

              Modern beaver belong to the genus Castor, which apparently migrated to North America over seven million years ago and has undergone little evolutionary change ever since.  Dipoides, the other type of beaver found at the Always Welcome Inn, evolved into the giant Pleistocene beavers Procastoroides and Castoroides, which reached the size of a small bear (Korth, 2001).

              How did two types of beaver live together at the Always Welcome Inn site?  Studies by Natalia Rybyczynski (2004) of the Canadian Museum of Nature in Ottawa help give us the answer.  She modeled the incisors of both genera in steel and experimented with different wood types to see how well each incisor worked.  She discovered that Dipoides teeth were specialized for chewing trees similar to red cedar, while Castor incisors were better suited for exploiting a number of woody vegetation types compared to those of Dipoides.  The two types of beaver were able to live together in the same areas because they ate different types of vegetation.  The Castor jaw at the Always Welcome Inn was found in lake deposits, while the Dipoides tooth was found in stream sediments.  This suggests that the two types of beavers may have occupied different habitats along the margin of the lake system that occupied the Powder Valley during the early Pliocene.

Conclusions

              The lower left jaw of a beaver discovered near the base of the Always Welcome Inn sequence belongs to the species Castor californicus Kellogg 1911.  It is large compared to other Castor fossils found in nearby areas.  The occlusal pattern of the lower second molar is more similar to a tooth found in the 4.1 million year-old Etchegoin Formation of the Kettleman Hills of California than it is to lower second molars of Castor found in the 2.3-2.0 million year-old Jackass Butte faunas of the Snake River Plain.  Castor and Dipoides fossils have both been found at the Always Welcome Inn site and were able to live together because they ate different types of vegetation and, perhaps, because they occupied different habitats in the area.

Acknowledgments

              We are very grateful to the Langrells, the owners of the Always Welcome Inn, for allowing us to dig at the site and bring classes there so that students can share in the excitement of finding real “picture puzzle pieces of the past.”  We would also like to thank the students of Phi Beta Rock, the Eastern Oregon University Geology Club, who helped organize and run the dig on which the Castor jaw was found.  Special thanks go to Matt Smith, the preparator at the John Day Fossil Beds National Monument for the outstanding job he did separating the beaver jaw from its matrix and uncovering the teeth and the rest of the jaw.  Ted Fremd and Greg McDonald of the National Park Service were the first to identify the jaw as belonging to Castor.

References Cited

Conrad, G.S., 1980, The biostratigraphy and mammalian paleontology of the Glenns Ferry Formation from Hammett to Oreana, Idaho:  Pocatello, Idaho, Idaho State University Ph.D. dissertation, 334 p.

Gustafson, E.P., 1975, The vertebrate faunas of the Pliocene Ringold Formation, south-central Washington: Eugene, Oregon, University of Oregon Bulletin 23, 62 p.

Hay, O.P., 1927, The Pleistocene of the western region of North America and its vertebrate animals:  Carnegie Institute of Washington Publication 322, 346 p.

Kellogg, L., 1911, A fossil beaver from the Kettleman Hills, California:  University of California Publications, Bulletin of the Department of Geology, v. 6, no. 17, p. 401-402.

Korth, W.W., 2001, Comments on the systematics and classification of the beavers (Rodentia, Castoridae):  Journal of Mammalian Evolution, v. 8, no. 4, p. 279-296.

Merriam, J.C., 1918, New mammals from the Idaho Formation:  University of California Publications, Bulletin of the Department of Geology, v.10, p. 523-530.

Repenning, C.A., Weasma, T.R., and Scott, G.R., 1995, The early Pleistocene (latest Blancan-earliest Irvingtonian) Froman Ferry fauna and history of the Glenns Ferry Formation, southwestern Idaho:  U.S. Geological Survey Bulletin  2105, 86 p.

Rybyczynski, N., 2004, Optimized for chewing:  Science, v. 306, p. 2045.

Shotwell, J.A, 1970, Pliocene mammals of southeastern Oregon and adjacent Idaho:  University of Oregon Museum of Natural History Bulletin 10, 103 p.

Stirton, R.A., 1935, A review of the Tertiary beavers:  University of California Publications, Bulletin of the Department of Geological Sciences, v. 23, p. 391-398.

Wilson, R.A., 1933, A rodent fauna from later Cenozoic beds of southwestern Idaho: Contributions to Paleontology, Carnegie Institute of Washington, v. 473, p. 15-34.

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