Chronologic Significance

The chronologic significance of arvicoline rodents is well established. The known temporal ranges of arvicolines recovered from Porcupine Cave are summarized in table 19.1 and are based on localities outside Porcupine Cave. The biochronology and age interpretations of the stratified sequences from Porcupine Cave are discussed in chapter 7, but a few comments are in order here.

One of the most interesting (and frustrating) arvicoline assemblages from the Cave is from the Mark's Sink locality (DMNH 1349). Although the fossils from Mark's Sink are mixed, it is clear from the species recovered that this is the oldest deposit yet located within the cave. Pliolemmus antiquus is known elsewhere only from Pliocene deposits in the Great Plains. Its presence, combined with that of Ophiomys parvus and Mictomys vetus or M. landesi, argues strongly that at least part of the Mark's Sink sediments contain a Pliocene (mid- to late Blancan) fauna. No other localities in Porcupine Cave include these three taxa together.

The Badger Room and Generator Dome appear to represent the next oldest assemblages of arvicolines within the cave. They include Mimomys cf. M. virginianus, Phenacomys gryci, and Allophaiomys pliocaenicus. Fissure Fill A is probably somewhat younger because, in addition to those three taxa, it contains Microtus paroperarius. Sample sizes are adequate in the Badger Room and Fissure Fill A, and low in Generator Dome, but it is intriguing that none of these localities contains any specimens of Lemmiscus, which is quite abundant in other localities. It is possible that these localities predate the arrival of Lemmiscus in the vicinity of Porcupine Cave. However, Lemmiscus is present in the Ferret Room in association with Phenacomys gryci and Mictomys vetus or M. landesi. It may also be that taphonomic and sample size considerations result in the apparent absence of Lemmiscus in some localities.

When Lemmiscus occurs, it is generally represented by both four- and five-triangle morphotypes. The exception is the Ferret Room, where only a single four-triangle specimen is known; but only three arvicoline teeth were recovered from this locality, so sample size is prohibitively small. This is true of the other Irvingtonian localities of Lemmiscus as well (Cathedral Cave and SAM Cave). The clear indication from studies of the change in relative abundance of the two morphotypes from the Pit (Barnosky et al., 1996; Bell and Barnosky, 2000; see figure 23.8), as well as from the Kennewick locality in Washington (Rensberger et al., 1984; Rensberger table 19.1

Known Temporal Range of Arvicoline Rodent Taxa Recovered from Porcupine Cave

Taxon

Oldest Reported Occurrence

Youngest Reported Occurrence

Pliolemmus antiquus

Ophiomys parvus

Mictomys vetus / M. landesi

Ondatra sp.

Phenacomys gryci

Mictomys kansasensis

Allophaiomys pliocaenicus

Mimomys cf. M. virginianus

Microtus sp. five-triangle morphotype

Phenacomys sp. (not P. gryci) Microtus paroperarius

Lemmiscus curtatus five-triangle morphotype Lemmiscus curtatus four-triangle morphotype Microtus/Terricola meadensis Mictomys meltoni

Slightly younger than 2.1 Ma Slightly younger than 2.1 Ma "Early Irvingtonian" 1.4 Ma?

Alternatively ~840 Ka

~840 Ka

~840 Ka

840 Ka

840 Ka

820 Ka

670 Ka

~2.58 Ma 1.5 Ma 1.5 Ma Extant 1.5 Ma ~1.2 Ma ~840 Ka ~840 Ka Extant

Extant 252 ± 30 Ka Extant ~9.5 Ka 252 ± 30 Ka 252 ± 30 Ka notes: Oldest and youngest known reliable dates are provided for each taxon and are based on radiometric dating, paleomagnetics, or biochronologic correlation of localities outside Porcupine Cave. See text for references and details on localities. Mictomys kansasensis and M. meltoni are both shown, but the species identification from Porcupine Cave is not established.

and Barnosky, 1993), is that the early populations of Lemmiscus have more four-triangle than five-triangle morphotypes, but this relative abundance shifts through time in favor of the five-triangle morphology. In modern populations, the five-triangle morphology predominates, but six-triangle forms are not uncommon (particularly from northern Nevada; Barnosky and Bell, unpubl. data). No four-triangle morphotypes are known from extant populations, and that morphology appears to have become extinct in the latest Pleistocene or early Holocene (Bell and Mead, 1998; Bell and Jass, in press). Presumably populations of Lemmiscus existed that were older than those now known, and when they are found we predict that they will contain only four-triangle morphotypes. Early reports of the fauna from SAM Cave in New Mexico (Repenning, 1992; Bell and Barnosky, 2000) indicated that only four-triangle morphotypes were known from that site. Those reports were based on an incomplete sample; a more thorough report on the fauna confirms the presence of five-triangle morphotypes there (Rogers et al., 2000). None of the wellsampled localities within Porcupine Cave records a Lemmiscus population with only four-triangle morphotypes.

Clearly the work on Porcupine Cave arvicolines is still in its early stages. The wealth of material from this unique site guarantees that the faunas from the cave will play a key role in helping us reshape our thoughts on arvicoline biochronology in the western United States.

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