Charles A Repenning

Denver Museum of Nature and Science

The evolution and paleobiogeography of wood rats (packrats), genus Neotoma, is poorly known, not because they are uncommon as fossils, but because their teeth all look alike. Living species are identified by features that are seldom preserved in fossils. Usually all that is mentioned of the teeth is the anterointernal reentrant (or "groove," as it is most frequently called in the description of modern forms) of the M1, the alveolar length of the tooth row, and the nature of the m3. The great majority of fossil records consist only of isolated teeth, which generally have not been considered sufficient to identify species.

Thus, at present, all that can be said of the history of Neotoma is that it belongs to a New World lineage first known in the late Miocene (circa 5.5-5.3 Ma ago) of central Mexico. These early Mexican representatives (Caranza-Castaneda and Walton, 1992) are very small forms that appear to be only slightly more derived than the generalized sigmodontine condition. Similar early Pliocene forms have a wide distribution over the southwestern quarter of the United States. Such late Miocene and early Pliocene forms are universally placed in the extinct genus Paraneotoma and are diagnosed as having an S-shaped m3.

Near the Pliocene-Pleistocene transition (circa 1.8 Ma ago), the S of the m3 is almost universally altered to an 8 that is present in all but one living species of Neotoma, N. (Hodomys) alleni, which retains the S-shaped last lower molar and is found in the western part of central Mexico (southern Sinaloa to northern Oaxaca). It thus seems that wood rats probably evolved in Mexico or Central America and dispersed northward into the southwestern United States near the beginning of the Pliocene. From there they appear to have continued dispersing northward along the eastern and western sides of the Rocky Mountains. Today the recognized species on either side of the Rocky Mountains are distinct, with little hybridization between them except just north of Mexico, south and southwest of the Colorado Rockies.

With the exception of the bushy-tailed wood rat, Neotoma cinerea, which has adapted to cooler habitats, the northernmost range of wood rats reaches coastal northern Oregon west of the Rockies and southwestern South Dakota to the east. Most species seem never to have crossed the Rocky Mountains. In the eastern United States Neotoma extends into southern New York; in the Mississippi Valley it ranges northward only to the southern tip of Illinois. East of the Rocky Mountains the northern limit of the wood rat is strikingly close to the former extreme southern limits of continental glaciation, a situation that has a parallel record with mountain glaciation in the Rocky Mountains. Neotoma cinerea ranges along the Rocky Mountains as far north as southern Yukon Territory and adjacent Northwest Territories.

Neotoma largely avoids full tropical regions but extends southward to northern Nicaragua in the mountains of Central America. Following the nomenclature of Hall (1981), the subgenera N. (Hodomys) and N. (Teanopus), each monospecific, are the primary exceptions and live in a small tropical area on the Pacific coast of Mexico. Thus, except for the bushy-tailed wood rat, Neotoma cinerea, the modern ranges of most species of Neotoma are found in warm-temperate to subtropical areas, with about half the living species preferring dry regions or deserts.

Neotoma cinerea, which currently lives in the vicinity of Porcupine Cave at about 2900 m elevation, and as low as 1800 m near Colorado Springs, is found as fossils in the cave. At the latitude of Porcupine Cave, 1800 m is about the upper eleva-tional limit of the other species present in the fauna of the Pit: N. floridana, N. micropus, N. mexicana, and N. stephensi. Finley (chapter 8) describes the modern habitat of these species.

Of these, only N. stephensi is not currently widely ranging in Colorado. It occurs throughout most of northern Arizona and the western third of New Mexico practically to the Colorado state line southeast of Durango. Throughout its range this wood rat is closely associated with juniper bushes or small trees, on which it depends for food, water, and shelter. It builds its nests near (within 10 m), beneath, or in junipers (Vaughan, 1980, 1982). Its former range was apparently much greater, as evidenced by Porcupine Cave and a few other fossil localities, and the relict nature of its modern distribution was pointed out by Vaughan (1982).

The enamel structure (Schmelzmuster) of species of Neotoma is not well known. Descriptions are available only for Neotoma fuscipes (Koenigswald, 1980:63). From that study it seems that the enamel structure is basically quite primitive in the genus, and it is possible that a wider examination might prove of value. But it is also possible, to judge from arvicolines with similar primitive enamel, that the enamel structure will not prove diagnostic of species.

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