The four arvicoline rodent taxa identified from SCC include extinct (Microtus meadensis, Microtus paroperarius) and extant voles [Microtus sp. (not Microtus meadensis or Microtus paroperarius), Lemmiscus curtatus]. Both Microtus sp. and Lemmiscus curtatus are characterized in the fauna by multiple morphotypes. These include the four-triangle form of Lemmiscus curtatus, a morphotype that persisted only into the early Holocene (Barnosky and Bell 2003;
Bell and Jass 2004). Identifying characteristics and abundance of each identified taxon are discussed below.
Two hundred thirty-nine specimens (80.7%) were identified as Microtus sp., making it the dominant taxon in the SCC deposit. These specimens exhibit greater labial-lingual length in T1 than T2, a character that can be used to distinguish Microtus from Lemmiscus (Barnosky and Rasmussen 1998). Specimens identified as Microtus sp. in this paper are characterized by the presence of at least five closed, alternating triangles, distinguishing them from both Microtus paroperarius and Microtus meadensis. Previous reports on the SCC fauna separated Microtus cf. M. montanus (Goodrich 1965;
Miller 1979) and Microtus cf. M. longicaudus (Mead et al. 1982) as distinct from other specimens referred to Microtus sp. Given the current state of knowledge regarding identification of m1s of Microtus at the species level (see
Bell et al. 2004a for discussion), there is no morphological basis for the identification of Microtus cf. M. montanus and Microtus cf. M. longicaudus from SCC. I consider previous reports of these taxa to be synonymous with Microtus sp. as reported here.
Lower first molars of Microtus show high levels of variation (e.g.,
van der Meulen 1978;
Weddle and Choate 1983;
Bell and Repenning 1999;
Gordon 1999), and specimens recovered from SCC are no exception. Ten morphologic variants of Microtus sp. were recovered. These are a five-triangle form with well-developed secondary wings that are confluent with the anterior cap (196 specimens;
Figure 3A), a five-triangle form where triangles 1 and 2 are confluent with one another (one specimen;
Figure 3B), a five-triangle form where T6 is pinched from T7 and anterior cap (16 specimens;
Figure 3C), a five-triangle form where T6 is pinched from T7 but T7 is closed from the anterior cap (one specimen;
Figure 3D), a five-triangle form where the T6 exhibits incipient closure from T7 and anterior cap (three specimens;
Figure 3E), a five-triangle form where T6 and T7 are confluent but are closed from the anterior cap (three specimens;
Figure 3F), a five-triangle form where T6 and T7 are confluent but are pinched from the anterior cap (one specimen;
Figure 3G), a five-triangle form where T6 and T7 are confluent but exhibit incipient closure from the anterior cap (one specimen; Figure 3H), a six-triangle form where T6 is closed from a confluent T7/anterior cap complex (15 specimens;
Figure 3I), and a seven-triangle form where both secondary wings (T6 and T7) are closed (one specimen;
Figure 3J). A single specimen (SCCAR-115) of the common five-triangle form has a unique morphology where T4 is bifurcated (Figure 3K). This morphology was previously noted on specimens of Microtus pinetorum from late Quaternary deposits in Iowa (Jans 1993;
Wallace 2001). One specimen (SCCAR-203) had T5 pinched from the secondary wings/anterior cap in occlusal view, but T5 was closed in ventral view. I conservatively identified this specimen as Microtus sp. rather than M. paroperarius (see description below).
Two of the 296 specimens examined for this study (< 1%) were identified as Microtus meadensis and are characterized by the presence of three closed, alternating triangles (Triangles 1-3), followed by confluent primary wings (Triangles 4 and 5), which are closed from the secondary wings (Triangles 6 and 7) and anterior cap (Figure 4A). The enamel of the m1 is positively differentiated (terminology follows
Martin 1987), where the leading or anterior edges of triangles retain thicker enamel than on the trailing or posterior edges. Cementum is present in the re-entrant angles. This m1 structure is similar to that found in some extant Microtus quasiater and Microtus pinetorum (Repenning 1983;
Zakrzewski 1985). Previous authors described features that characterize the m1 of Microtus meadensis and other extant taxa with similar morphologies (Martin 1987;
Harris 1988). However, the frequency of distinct m1 morphologies in these populations is undocumented.
Three of the 296 specimens (1.0%) were identified as Microtus paroperarius. These m1s have four closed, alternating triangles, followed by a well-developed fifth triangle that is variably confluent with the secondary wings and anterior cap (Figure 4B, C). Two of these specimens exhibit a state where the fifth triangle is pinched from the secondary wings (e.g.,
Figure 4B). In one specimen (SCCAR-212), the fifth triangle is broadly confluent with the secondary wings (Figure 4C). This variation is consistent with that seen in other samples of Microtus paroperarius, but is also known to be similar or identical to the living Microtus oeconomus (van der Meulen 1978). Additionally, this morphology is known to occur in very low percentages in some other extant species of Microtus (Bell and Repenning 1999).
Fifty of the 296 arvicoline specimens (16.9%) were identified as Lemmiscus curtatus. The only previous report of Lemmiscus from SCC was based on specimens collected from a back-dirt pile in the cave (Bell and Mead 1998). I identified both four- and five-triangle forms (Figure 5). The five-triangle form is most common, comprising 45 of the 50 specimens (90%) of Lemmiscus curtatus (Figure 5A). These specimens are characterized by the presence of five closed triangles and a well-developed T6 that is confluent with the anterior cap. They exhibit equal (or roughly equal) labial-lingual lengths in T1 and T2, a character that distinguishes them from Microtus, which has a labial-lingual width of T1 that is larger than T2 (Barnosky and Rasmussen 1998). Five specimens represent a four-triangle form where the fourth triangle is either confluent (not figured), pinched (Figure 5B, C), or has incipient closure (not figured) with respect to T5.
Four-triangle forms of Lemmiscus curtatus are noteworthy because that morphology is not known to occur in extant populations of sagebrush voles (Bell and Barnosky 2000;
Barnosky and Bell 2003), and the loss of this morphotype may represent one of the few small mammal 'extinction' events near the early Holocene (Bell and Jass 2004). Smith Creek Cave represents
one of the few localities known to preserve specimens of Lemmiscus curtatus that have a four-triangle morphology (other localities were reviewed by
Bell and Jass 2004). Given the fact that three of the localities containing a four-triangle form of Lemmiscus curtatus occur in, or near Smith Creek Canyon, it seems a strong possibility that additional Pleistocene localities in the vicinity (e.g., Crystal Ball Cave) may also preserve this morphotype.
In addition to the specimens reported here, two other arvicoline rodent taxa were reported from SCC. A single specimen (NAUQSP 17910) of Phenacomys that was collected from a back-dirt pile in SCC is housed at the Laboratory of Quaternary Paleontology, Northern Arizona University. The specimen was not examined for this study but was described and illustrated by
Mead et al. (1982) as Phenacomys cf. P. intermedius and by
Repenning and Grady (1988) as Phenacomys albipes.
Specimens of Mictomys borealis were reported from both Cathedral Cave and Smith Creek Cave (Mead et al. 1992). The record from Cathedral Cave consists of multiple specimens, but only a single M2 was reported from SCC. A subsequent re-examination of arvicoline rodents from Cathedral Cave (Bell 1995) revised the identification of Mictomys borealis but did not address the specimen from SCC. During the course of this study, no additional specimens of Mictomys were found in the SCC material housed at the Nevada State Museum. In the absence of additional specimens of Mictomys from SCC (e.g., m1s) that would allow for a more confident taxonomic placement, I consider the single record reported by
Mead et al. (1992) to be Mictomys sp.