CONCLUSIONS

The lagomorph specimens from the ABD section that are nearest to Lepus in size and morphology can not be identified definitively to the genus Lepus. Of the four specimens variously referred to 'Lepus' ABDSP(IVCM) 113/V413 is the nearest in overall appearance to Lepus but only meets the minimum size criterion for p3 of White (1991a). It displays characters also found in specimens of Nekrolagus and Sylvilagus, and may be a modern specimen. ABDSP(IVCM) 113/V1275 has possible problems with locality provenience and may represent Nekrolagus progressus, although more comprehensive work on the collection is required; it is best referred to Leporinae, genus and species indeterminate. ABDSP(LACM) 1906/V24889 is similar to both medium-size Lepus and large Sylvilagus but is undiagnostic beyond Leporinae, genus and species indeterminate. ABDSP(IVCM) 110/V407 was determined to be a specimen from a modern leporine, was discarded prior to this review, and is unavailable for further study. Based on fossil material currently in the collection Lepus does not demonstrably occur in the ABD faunas.

Of the ABD specimens originally published as Microtus californicus? (Zakrzewski 1972) ABDSP(LACM) 1942/V8252 appears consistent with Repenning's (1992) taxonomic reassignment of the specimen to 'Lasiopodomys,' and represents the oldest known Lasiopodomys morphotype in North America. ABDSP(LACM)?/V24540 is most accurately identified as Microtus sp. with five closed triangles, but lacks locality data. ABDSP(LACM) 6683/V24649 is diagnostic only to Arvicolinae. The specimen of Microtus meadensis, ABDSP(LACM) 68123/V24828, is not from VCFC (contra Repenning 1992, 1998) but from the Borrego Badlands, stratigraphically above the Bishop Ash. This is within the known temporal extent of the species elsewhere in North America (Repenning 1987, 1992; Bell et al. 2004a).

The Anza-Borrego Desert specimens from the VCFC sequence no longer contribute to discussions of the early appearance of Lepus or the earliest five-triangle Microtus in North America. The Blancan-Irvingtonian boundary in the ABD is currently undefined, and its recognition is complicated by uncertain taxonomic affinities and inconsistent primary data associated with many relevant specimens. The VCFC section still represents one of the most important fossil sequences for Blancan and Irvingtonian land mammal ages and the transition between them. Further assessment will improve the understanding of its relevance to local, regional, and continental biochronology, as well as to a general understanding of the evolution of the vertebrate biota in western North America during a dynamic interval of time.