The Middle Miocene-age Barstow Formation (17.2 – 13.2 Ma) was deposited during the height of the Middle Miocene Climatic Optimum (MMCO) and the initial period of cooling that followed. Significant collections of late Hemingfordian- (He2), early- (Ba1), and late Barstovian-age (Ba2) mammal fossils have been recovered from its strata. I examined the distribution of approximately 2300 small-mammal specimens recovered from 17 stratigraphic levels within the Barstow Fm. Among the small mammals there is little change in species richness or overall taxonomic composition between communities recovered from strata deposited during the height of the MMCO (He2 and Ba1) and the period of cooling that followed (Ba2). At each of the examined levels geomyoid and cricetid rodents together account for 70% or more of specimens. With regard to relative abundance, small-mammal collections made from localities low in the section (He2 and Ba1) are generally dominated by geomyoids, 55-88% of specimens, while cricetids account for 3-34%. Throughout most of the Ba2-age portion of the section geomyoids and cricetids are more evenly distributed accounting for 23-68% and 23-61% of specimens respectively. At the three highest stratigraphic levels cricetids account for the largest percentages of specimens (36-59%) while the abundance of geomyoids declines sharply (22-33%) relative to lower stratigraphic levels.

Among specimens recovered from strata deposited during the MMCO the pattern of geomyoid dominance is reversed at Oreodont Quarry where cricetids account for 66% of specimens. I used existing geochronologic data for the formation to estimate an age of 15.77 Ma for Oreodont Q. and compared this age with the benthic marine oxygen isotope curve. At that time there was a brief episode of pronounced cooling (Mi2) during which δ¹⁸O values were more typical of those observed during the late Barstovian. I then used δ¹⁸O values derived from the oxygen isotope curve as a proxy to test for a correlation between temperature and the relative abundances of geomyoid and cricetid rodents in the Barstow Fm. Based on these data there is a statistically significant negative correlation between δ¹⁸O values and geomyoid relative abundance (r²=0.51, p=0.001); and a positive correlation between δ¹⁸O values and cricetid relative abundance (r²=0.45, p=0.003).