INTRODUCTION

Identifying fossil specimens taxonomically relies almost exclusively on morphology. This identification can be especially difficult at the species level, where there may be only slight differences distinguishing unique forms. Therefore, intraspecific variation should be documented in order to understand the limits within and between fossil species. Additionally, understanding life-history changes can facilitate the formation of possible evolutionary scenarios and yield insights into the population dynamics of extinct taxa. Unfortunately, such documentation of intraspecific variation has not, or cannot, be done with most extinct species. In many fossil mammals, the diagnostic characters are confined to the dentition; among fossil cotton rats (Sigmodon) species-level diagnoses have been further restricted to characters from the lower molars. This paper focuses on the ontogenetic variation in occlusal and root morphology of the lower dentition of two samples of the late Pliocene (Blancan) Sigmodon curtisi. Ontogeny as used here extends beyond embryonic development and includes all changes during the life of an individual (sensu Hale and Margham 1991). Although other definitions of ontogeny do not include the time beyond sexual maturity, it is not known when the extinct S. curtisi became mature.

Sigmodon curtisi is an early species of cotton rat that occurs in late Pliocene and early Pleistocene localities in Arizona (Harrison 1978; Johnson et al. 1975; Lindsay and Tessman 1974); Colorado (Hager 1975); Florida (Morgan and Hulbert 1995; Ruez 2001); Kansas (Izett and Honey 1995; Martin et al. 2003; Peláez-Campomanes and Martin 2005); and Sonora, Mexico (Lindsay 1984). However, S. curtisi is abundant only at two of the Florida localities, Inglis 1A and Inglis 1C. The most similar species morphologically are the modern Sigmodon leucotis, and the extinct S. hudspethensis and S. lindsayi from the late Pliocene of Texas (Strain 1966; Akersten 1970; Martin and Prince 1989).

Inglis 1A and Inglis 1C (29º 00' 43" N, 82º 40' 05" W; Citrus County, Florida; Figure 1) are sand-filled sinkhole deposits in the Eocene Ocala Limestone that were intersected during the now-aborted attempt to cut a canal across peninsular Florida (Ruez 2001). Although each of these deposits originally was assigned to the Irvingtonian land mammal age (Morgan and Hulbert 1995; Ruez 2001), a subsequent revision of land mammal age boundaries in North America (Bell et al. 2004) places Inglis 1A and Inglis 1C within the Blancan. In terms of absolute age, both localities were suggested as being deposited between 2.01 and 1.78 Ma, with Inglis 1C being younger by an unknown amount (Ruez 2001). These localities are important to the fossil history of Florida because they are the only Blancan localities from the state with diverse and abundant microfaunas; the temporally closest locality comparable in diversity and abundance is the late Irvingtonian Coleman 2A from about 400 ka years ago (Martin 1974; Ruez 2001). Detailed locality data for Inglis 1A and Inglis 1C, including the geology (Emslie 1998; Ruez 2002), excavation history (Emslie 1998; Klein 1971; Ruez 2001), and the mammalian fauna (Morgan and Hulbert 1995; Ruez 2001) are discussed elsewhere.

The more taxonomically diverse of the two localities, Inglis 1A, contains at least 51 species of mammals, including at least 12 rodents Ruez 2001). Only a portion of the mammalian fossils from this site have been described (e.g., Frazier 1981; Klein 1971; Morgan 1991; White 1991; Wilkins 1984), along with the birds (Carr 1981; Emslie 1996; Steadman 1980) and squamates (Meylan 1982).

Inglis 1C contains 33 species of mammals, including 11 species of rodents (Ruez 2001). Emslie (1998) and Emslie and Czaplewski (1999) evaluated the avian fauna, but the abundant herpetological specimens remain undescribed. The Inglis 1C deposit was shown to be attritional in nature, with the small-mammalian fossils accumulated as the result of predation and fluvial transport (Ruez 2002).