Microtus species with five or six triangles are quite common in Irvingtonian and especially Rancholabrean faunas; at least 12 species are still extant in North America (listed in the previous section). In addition to Porcupine Cave, other early North American specimens known to show this morphology are from the Anza-Borrego Desert (California; Zakrzewski, 1972; Repenning, 1992), the Elsinore Fault Zone (California; Pajak et al., 1996), the Irvington fauna (California; Savage, 1951), Hansen Bluff (Colorado; Bell and Repenning, 1999), SAM Cave (New Mexico; Rogers et al., 2000), and Wellsch Valley (Saskatchewan, Canada; Stalker and Churcher, 1982; Churcher, 1984; illustrated by Repenning, 1992:37).
The oldest reported record is from the Anza-Borrego Desert, where it may be as old as 1.4 Ma (Repenning, 1992), but locality data for that material may be problematic (L. K. Murray, pers. comm.). The Wellsch Valley record also seems to have a great antiquity, but age control for those materials is lacking, and reliable temporal placement is not possible at this time. The SAM Cave record is probably the next oldest and is estimated to be as old as approximately 840 Ka (Rogers et al., 2000).
LEMMISCUS CURTATUS THOMAS, 1912: FIVE- AND SIX-TRIANGLE MORPHOTYPES
IDENTIFICATION The molars are rootless with cementum in the reentrant angles. The m1 consists of a posterior loop and five or six closed, alternating triangles; in the majority of specimens (fossil and modern) the sixth triangle is confluent with the anterior cap. Incipient development of a seventh triangle is very rare. Triangle 2 of the m1 is as long as or longer than triangle 1.
DISTRIBUTION WITHIN PORCUPINE CAVE CM 1925, Pit; CM 1926, Gypsum Room; CM 1927, CM Velvet Room; DMNH 644, DMNH Velvet Room; DMNH 1349, Mark's Sink; UCMP V93173, Pit; UCMP V93174, Gypsum Room; UCMP V93175, CM Velvet Room; UCMP V97002, undifferentiated cave deposits.
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