Fossil Beaver (Dipoides) Tooth, Always Welcome Inn,

Baker City, Oregon

Elizabeth Burton, Division of Distance Education, Eastern Oregon University, Baker City, OR 97814 and Jay Van Tassell, Science Department, Badgley Science Center, Eastern Oregon University, La Grande, OR 97850

Abstract

An upper first molar of a fossil beaver, Dipoides vallicula, was found at the Always Welcome Inn.  This suggests that there was a water connection between the type locality for this species of Dipoides in the Little Valley area of Oregon between Vale and Harper and the Baker City area sometime around 4-5 Ma..

Introduction

A tooth of an extinct beaver was found by Elizabeth Burton, Director of the Baker County Center of Eastern Oregon University's Division of Distance Education, on October 7, 2005 at the Always Welcome Inn fossil site in Baker City (Fig. 1).  The tooth came from the silt units in the upper part of the outcrop at approximately the same stratigraphic level where teeth from the rodents Pliopotamys and Cosomys have previously been found.  The tooth belongs to the genus Dipoides, a separate lineage from modern beavers and an ancestor to the giant Pleistocene beaver, Castoroides.

Figure 1.  Dipoides tooth found at Always Welcome Inn.  Scale bar is in centimeters and millimeters.

Description of the Tooth

The occlusal surface of the tooth found by Elizabeth Burton has a maximum dimension of 2.7 mm in an anterior to posterior direction and a maximum dimension of 2.8 mm in a transverse direction perpendicular to the posterior-anterior direction (Fig. 2).  This almost square occlusal outline is typical of Dipoides (Shotwell, 1955).  The anterior margin of the occlusal surface has been worn off, probably by abrasion prior to deposition.  The tooth is 11.3 mm long, the greatest measurement of the tooth column measured in a straight line. 

Figure 2.  Occlusal surface of Dipoides tooth.

The occlusal pattern on the tooth consists of a first (anterior) loph with the "s-pattern" typical of Dipoides, plus a second (posterior) loph with a "u-shaped pattern."  The protocone on the first loph is wider and less flattened than the paracone.  The second loph includes a straight anterior loph and a posterior loph with a small plication.  The anterior, medial, and posterior portions of each loph are not inflated.  The mesoflexus is nearly transverse, is pendulous in occlusal outline, and has a narrow termination.  The hypoflexus is also pendulousand has a narrow termination.  Both the mesoflexus and hypoflexus extend the entire length of the tooth.  The distal ends of both the mesoflexus and hypoflexus abut with the outer enamel.  The tooth lacks a paraflexus, a metaflexus, a parastriid, and a metastriid (Fig. 3, terminology based on Stirton, 1935, and Shotwell, 1955).

Comparison with Dipoides Species Found in Oregon, Idaho, and Washington

              Dipoides fossils are found in a number of Tertiary fossil localities in Oregon, Idaho, and Washington (Fig. 4, Table 2).  Pliocene Dipoides fossils have been found in the Blancan Glenns Ferry Formation at Hagerman, Idaho (Zakrzewski, 1969), the Blancan Ringold Formation White Bluffs fossil locality, Washington (Gustavson, 1978), the Hemphillian (Miocene-Pliocene) Drewsey Formation of Eastern Oregon (Shotwell, 1955, 1963, and 1970), the Hemphillian Shotwell Formation (?) at the McKay Reservoir locality in Eastern Oregon, and the Hemphilian to Pleistocene(?) Chalk Butte Formation of Eastern Oregon, which appears to be slightly younger than the Drewsey Formation (Shotwell, 1970).

Figure 3.  Morphology of the Always Welcome Inn Dipoides tooth.  Terminology based on Stirton (1935) and Shotwell (1955).

Figure 4.  Comparison of Dipoides molar teeth from localities in Oregon, Idaho, and Washington.

Table 1.  Sizes of the teeth (mm) of different species of Dipoides from the Oregon, Idaho and Washington areas.

PA:  Posterior-Anterior   TR:  Transverse

Table 2.  Characteristics of Dipoides teeth from Oregon, Idaho, and Washington.

The Always Welcome Inn Dipoides tooth has an overall loph pattern on its occlusal surface that is similar to the occlusal pattern illustrated by Shotwell (1970, p. 31, Fig. 13D) on an upper left first molar in a palate of D. vallicula Shotwell, 1970 (sample UO 26698) from the type section of the species in Little Valley, OR (UO Loc 2516).  The Always Welcome Inn tooth is slightly smaller than the molar teeth of D. vallicula, but within the same range given measurement error.  Both the Always Welcome Inn Dipoides tooth and the upper left first molar of D. vallicula have a transverse and

pendulous mesoflexus, no parastriids or metastriids (which are not present in adult D. vallicula teeth), and a mesoflexus and hypoflexus that abut the opposite side of the teeth.

              The Always Welcome Inn Dipoides tooth is much smaller than the upper third molar teeth of D. intermedius Zakrzewski, 1969, the species found at Hagerman (Table 1) and its mesoflexus is pendulous, not slightly crescentic like that of the upper third molar of D. intermedius.  The Always Welcome Inn tooth is also much smaller than the lower molars of D. rexroadensis Hibbard and Riggs, 1949, the species found at the White Bluffs locality.  The loph pattern of D. rexroadensis lower molar teeth is somewhat similar to that of the Always Welcome Inn tooth because due to the bifurcation of the posterior lophs by the metaflexus, but the D. rexroadensis tooth has a concave rather than pendulous mesoflexus, a nearly straight hypoflexus, and a metaflexus, which is not present on the Always Welcome Inn tooth.  The Always Welcome Inn tooth is also much smaller than the upper first and second molar teeth of D. stirtoni Wilson, 1934, the species found in the Drewsey Formation at Rome, Little Valley, Juniper Creek, and the Juntura Basin in Eastern Oregon.  In contrast to the Always Welcome Inn tooth, D. stirtoni upper first and second molars have a hypoflexus that is crescentic to chevron and a hypoflexus that does not abut the opposite side of the tooth.  The Always Welcome Inn tooth is smaller than the first and second molars of D. smithi Shotwell (1955) found at McKay Reservoir and its hypoflexus does not open posteriorly and have a narrow end like the hypoflexus of D. smithi.

This evidence suggests that the Always Welcome Inn Dipoides tooth is a left upper first molar (M1) of the species D. vallicula and is classified as:

Class Mammalia

              Order Rodentia

                            Suborder Castorimorpha

                                          Family Castoride

                                                        Subfamily Castorinidae

                                                                      Genus Dipoides schlosser, 1902

                                                                                    Species Dipoides vallicula Shotwell, 1970

Paleoenvironment and Paleoecology

              Shotwell (1963) notes that Dipoides (beaver) is peculiar to pond-bank environments and slow-moving streams.  In the Hagerman area, the beaver, Castor, is common and Dipoides intermedius is rare.  Both are found with marmot-like rodents, ground squirrels, gophers, pocket mice, kangaroo rats and other rodents along with a fish, frogs, water snakes, and water and shore birds, suggesting the presence of numerous bodies of fresh water, both ponds and streams.  The  beaver lived in a marsh and meadow habitat in close proximity to trees.  The paleoclimate was more humid and slightly wetter than at present.  Dipoides and Castor may have competed for similar resources (Zakrzewski, 1969).  Similar paleoenvironments and paleoecological relationships likely occurred in the Baker City area at the time the upper part of the Always Welcome Inn sequence was deposited.

              The Little Valley fossil site where D. vallicula was first described by Shotwell (1970) also contains other rodents (Spermophilus, Peromyscus, Diprionomys, and Pliosaccomys  rabbit (Hypolagus), insectivores (shrews and Scapanus), and carnivores (Canis and Martes).  Pliopotamys and Cosomys, the two microtine rodents that have been found at the Always Welcome Inn site, have not been described in the Little Valley sequence but are common in the Hagerman, Idaho, area.  Further collecting at the Always Welcome Inn site may reveal additional fossils that may make the relationships between these areas more clear.

Acknowledgments

              Greg McDonald, Senior Curator of Natural History for the National Park Service's Museum Management Program, and Ted Fremd, Paleontologist at the John Day Fossil Beds National Monument, helped confirm that the Always Welcome Inn beaver tooth is Dipoides.  Greg McDonald pointed the way to Shotwell's classic (1955) paper on Dipoides.  Any errors in description and classification of the tooth to the genus and species level are the responsibility of the author.

References Cited

Gustafson, E.P., 1978, The vertebrate faunas of the Pliocene Ringold Formation, south-central Washington:  Bulletin No. 23, Museum of  Natural History, University of Oregon, Eugene, Oregon, 62 p.

Shotwell, J.A., 1955, Review of the Pliocene beaver Dipoides:  Journal of Paleontology, v. 29, no. 1, p. 129-144.

Shotwell, J.A., 1956, Hemphillian mammalian assemblage from northeastern Oregon: Bulletin of the Geological Society of America, v. 67, p. 717-738.

Shotwell, J.A., 1963, The Juntura Basin: Studies in earth history and paleoecology:  Transactions of the American Philosophical Society, v. 53, part I, 77 p.

Shotwell, J.A., 1970, Pliocene mammals of southeast Oregon and Idaho:  Bulletin No. 17, Museum of Natural History, University of Oregon, 103 p.

Stirton, R.A., 1935, A review of the Tertiary beavers:  University California Publications, Bulletin of the Department of Geology 23, v. 13, p. 391-458.

Wilson, R.W., 1934, A new species of Dipoides from the Pliocene of Oregon: Carnegie Institute of Washington, Publication 440, p. 19-28.

Zakrzewski, R.J., 1969, The rodents from the Hagerman local fauna, upper Pliocene of Idaho:  Contributions from the Museum of Paleontology, The University of Michigan, v. 23, no. 1, p. 1-36.

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