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Reliable characterization of a species is an essential step toward eventual reconstruction of phylogenetic alliances among related taxa (Musser et al. 1998). Although characterization of species within the genus Oryzomys has met with some confusion in the past, significant work has taken place to help better define specific limits within this group (Musser et al. 1998; Bonvicino and Moreira 2001; Langguth and Bonvicino 2002).
In spite of several recent surveys performed in the eastern Bolivian Panhandle (Emmons 1993; Taber et al. 1997; Brooks et al. 2002), our knowledge of the mammalian fauna in this region is still incomplete, and further studies are warranted. For example, of 1,259 collecting localities in Bolivia analyzed by Anderson (1997), less than two percent are from the eastern panhandle of Santa Cruz Department. Thus, this region constitutes a priority for mammalian exploration and conservation.
In mid-April 1999, during an expedition to the eastern Bolivian panhandle (Brooks et al. 2002), we collected a single specimen of the genus Oryzomys that could not be assigned to any known species previously reported for the region in former studies (e.g., Anderson 1993, 1997). Extensive morphological comparisons with deposited voucher specimens revealed that this specimen may represent an undescribed species most closely related to the O. subflavus group (Guy Musser, pers. comm.). To confirm this taxonomic hypothesis, molecular analyses using a portion of the mitochondrial cytochrome-b gene were perfonned to establish phylogenetic relationships. Molecular data supported our conclusion that this specimen represents a new taxon within the genus. In this study, we describe a new form of Oryzomys from the Department of Santa Cruz, Bolivia.