DISCUSSION

Lepus is one of several taxa suggested as stratigraphic markers signaling the beginning of the Irvingtonian land mammal age (Lundelius et al. 1987), especially in faunas where other, more preferable marker taxa (e.g., Mammuthus, Microtus) are unavailable. In his review of the Blancan-Irvingtonian boundary at ABD, Cassiliano (1999, p. 183) concluded that "the use of Lepus to define the boundary may be unreliable (Lindsay 1995), but, at present, is the best choice in the FCVC [VCFC] section."

Based on our reanalysis of the ABD leporines there are currently no specimens in the fossil collection that can be diagnosed unequivocally as Lepus. This is not to say Lepus is not present in the ABD fossiliferous sediments, but with the fossil material currently available its presence is not verifiable. The best course of action at present is to remove Lepus from the ABD faunal list and eliminate the purported ABD records from discussions of biochronology. Further work on leporines in the VCFC section is needed.

A similar re-evaluation and removal of 'Lepus' from the Curtis Ranch and 111 Ranch faunas (Galusha et al. 1984; Lindsay et al. 1990; White 1991a) resulted in the emendation of southern Arizona Pliocene-Pleistocene biochronology and the stratigraphic repositioning of the local Blancan-Irvingtonian boundary. Our study further reduces the number of localities recording the early presence of Lepus and increases the importance of the remaining early Lepus localities, Big Springs and Borchers, as well as the recently published Blancan records of Lepus from Bear Springs, Anita, and San Simon faunas in Arizona (Morgan and White 2005). It also places greater emphasis (Lundelius et al. 1987; Martin et al. 2003) on the caveats regarding the difficulty in diagnosing leporine fossils based on the enamel pattern and size of p3 alone (White 1984, 1991a).

The isolated arvicoline tooth ABDSP(LACM) V24540 was one of the most important specimens reported from ABD. For years it was accepted as the oldest known Microtus with five closed triangles in North America, and because of its advanced morphology (five well-developed and fully closed triangles) it became central to hypotheses about arvicoline evolution, phylogeny, dispersal, and biochronology.

The three localities published or proposed for V24540 are of different ages spanning about 1.75 million years. Locality ABDSP(LACM) 6814 lies within the lower normally-magnetized portion of the Gauss (chron C2An3n) between 3.33 and 3.58 Ma. Locality ABDSP(LACM) 6683 is in part of the section currently uncorrelated to the paleomagnetic stratigraphy and may be anywhere from just above the top of the Olduvai (chron C2n; slightly younger than 1.77 Ma) to below the base of the Olduvai, about 2.0 Ma (future magnetic polarity and stratigraphic research in the section may resolve this issue). Locality ABDSP(LACM) 1357 lies stratigraphically below locality 6683 in the same uncorrelated area. Other potential localities from which the specimen might have been collected span almost the entire range of ages in ABD, so any speculation about actual provenience is unwarranted, and will not remove the taint of bad data associated with the specimen.

Microtus (sensu Repenning 1992) was used by Repenning to mark the beginning of his Irvingtonian I division in the southern portion of the United States west of the Rocky Mountains. The earliest appearance of Microtus with five closed triangles was placed variously at 1.4 Ma (Repenning 1992, pp. 38, 80), 1.6 Ma (Repenning et al. 1995, pp. 29–31), and 1.7 Ma (Repenning 1998, p. 52). All of those age assessments were based on the single isolated tooth ABDSP(LACM) V24540 and now must be disregarded. Other early occurrences were summarized by Bell et al. (2004b), and greater attention now needs to be given to those records (Bell and Bever 2006).

The stratigraphic position of the 'Lasiopodomys' specimen, in reversed polarity sediments dating to between 1.07 and 1.77 Ma, makes it the oldest record of that morphotype in North America (Repenning 1992). It is one of only a few specimens showing a 'Lasiopodomys' morphotype from west of the Rocky Mountains (Repenning 1992; Bell et al. 2004a). Elsewhere in North America, Lasiopodomys morphotypes are known from faunas younger than 850,000 years old (Repenning 1992).

The reported specimen of Microtus (=Terricola) meadensis from ABD is from the top of the Borrego Badlands section, not VCFC. The specimen locality is stratigraphically above the Bishop Ash (average age 758.9 ± 1.8 ka; Sarna-Wojcicki et al. 2000); a stratigraphic change equivalent to an age reduction of at least 1 million years. Repenning's attribution of this specimen to the VCFC formed the basis for his reinterpretation of the geomagnetic polarity stratigraphy in the upper part of that sequence (Repenning 1992; Bell et al. 2004b), an assessment that now must be disregarded.