introduction

The Anza-Borrego Desert (ABD) constitutes the northwestern portion of the Colorado Desert, which extends from the eastern slope of the Peninsular Ranges to the Colorado River, and from San Gorgonio Pass in the north to Baja California Norte. Anza-Borrego Desert State Park (ABDSP) is one of five parks within the Colorado Desert District of the California Department of Parks and Recreation and encloses the Anza-Borrego Desert. Extensive exposures of late Miocene through Pleistocene fossiliferous sediments have yielded important collections of vertebrate fossils, beginning in the 1930s and 1950s (e.g., Frick 1937; Downs 1957, 1967; Brattstrom 1961; White and Downs 1961, 1965; Downs and Woodard 1962; Howard 1963; White 1965, 1968, 1969; Downs and White 1965, 1966, 1968; Hibbard et al. 1965). These fossils, especially those from the Vallecito Creek – Fish Creek sequence, played a role in the development of biostratigraphic correlations and biochronologic interpretations across western North America, and continue to figure prominently in late Pliocene to early Pleistocene paleontological investigations. The stratigraphic and chronologic significance of the ABD faunas is highlighted by the fact that several ABD specimens were reported previously as either the earliest or latest known records of mammalian taxa in North America. Purported earliest occurrences include Microtus cf. M. californicus, Equus (Equus), Euceratherium, Nothrotheriops, and Sylvilagus; purported latest occurrences include Borophagus diversidens, and cf. Dinohippus sp. (Lundelius et al. 1987; Repenning 1987, 1992; Cassiliano 1999; Bell et al. 2004b). Interpretations of the stratigraphic and chronologic placement of these and other taxa helped to shape discussions of the absolute age of the local boundary between the Blancan and Irvingtonian mammal ages (e.g., Opdyke et al. 1977; Cassiliano 1999), and were important in secondary syntheses of local to continental and global-scale concepts of taxonomic relationships, biochronology, and environmental change (e.g., Opdyke et al. 1977; Lundelius et al. 1987; Lindsay et al. 1990; Martin et al. 2003; Bell et al. 2004b).

Some previous authors noted difficulties in their attempts to reconcile taxonomic identifications and ages of ABD specimens with the known biochronologic and biogeographic distribution of Blancan and Irvingtonian mammals elsewhere (Zakrzewski 1972; Repenning 1992; Repenning et al. 1995; Cassiliano 1999; Bell et al. 2004b). Our examinations of ABD leporid and arvicoline rodent specimens and the documents associated with their collection and curation revealed important inconsistencies among published information, database records, catalogued specimens, and data recorded in field notes. Most of these inconsistencies originated in the complex curatorial history of the ABD collections, but they have important ramifications for biochronologic interpretations within the Anza-Borrego Desert and throughout western North America.