Díaz-Sibaja et al.—Mexican Bison antiquus diet 92(6):1130–1139 Finally, the discriminant analysis (Fig. 4.4) classified the
samples of La Cinta-Portalitos and La Piedad-Santa Ana as grazers with posterior probabilities of 88% and 81%, respec- tively, whereas the Viko Vijin sample was classified as a mixed feeder, with a probability of 75%. Also, the multivariate cen- troids of the dietary categories showed statistically significant differences between groups (prob>F ≤ 0.0001).
Discussion
Cluster analysis.—In the cluster analysis (Fig. 4.1), the samples from Michoacán-Guanajuato comprised one cluster, and this was close to the one formed by Connochaetes taurinus and Alcelaphus buselaphus, which are identified as nonstrict grazers (Fortelius and Solounias, 2000). The Viko Vijin sample formed a cluster with the waterbuck (Kobus ellipsiprymnus). The blue wildebeest (C. taurinus) diet consisted of 87.5% monocots, 12% dicots, and 0.5% fruits (Gagnon and Chew, 2000). The blue wildebeest preferred habitat consisted of open bushland and short grassy plains, always with a close (< 20 km) water source (Kingdon, 2013). On the other hand, the hartebeest (A. busela- phus) diet comprised 75% monocots, 20% dicots, and 5% fruits (Gagnon and Chew, 2000). Hartebeests are considered an eco- tone species (Kingdon, 2013); their preferred habitats are boundaries between open grassy plains (or glades) and park- land, woodland, or scrub; their movements are often within the drainage lines in the dry season and in more open plains during the rainy season (Booth, 1985; Kingdon, 2013). Similarly, Kobus ellipsiprymnus is regarded as a nonstrict grazer (Fortelius and Solounias, 2000); its diet consists of 84% monocots, 15% dicots, and1%fruits (Gagnon and Chew, 2000) and its preferred habitat consists of mosaics of savanna woodlands and forest- savanna, with permanent water sources (Kingdon, 2013). The cluster analysis also showed that the modern plains
bison (Bison bison) is classified within the strict grazer category (Fortelius and Solounias, 2000), and it is not near the mesowear pattern of our samples of B. antiquus (Fig. 4.1, bb=B. bison). These data suggest a distinct and broader dietary niche for the analyzed ancient Mexican bison populations, when compared to their alleged modern plains analog, and a wider range of habitat use by the Pleistocene B. antiquus individuals analyzed in this study.
Age of studied bison samples.—The age of the bison samples used in this study is important for comparisons. The radiometric age of la Piedad-Santa Ana site is unknown, but the age of the other two sites has been inferred. Bison material of Viko Vijin has been dated by the uranium series based on the 230Th-234U-238U method, providing an estimated age of 36 ka for the fossil localities (Ordoñez-Regil et al., 2016). We inferred the age of La Cinta- Portalitos by the relationship of dated radiocarbon sections of a core made in the Cuitzeo Lake (Israde-Alcántara et al., 2010) and the depth in which the fossil bed was found, thus providing an estimated age of 24.05 kyr (with a maximum of ca. 35 kyr and a minimum of ca. 18 kyr). This shows that both northern and southern Mexican samples are of comparable age, both occurring during the middle Wisconsin glacial stage. Under this scenario, caution must be taken when comparing different univariate bison scores from the literature. For example, B. antiquus from New
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Mexico and New Mexico-Texas samples date back to 10.5 ka (Rivals et al., 2007b) and 10−15 ka (Barrón-Ortíz, 2016), both close to the YoungerDryas event ca. 10−11.6 ka, and theBølling- Allerødwarm period, respectively (Björck, 2007), and far from the estimated ages of our study sites. Other bison samples also have this age pattern; B. priscus from Alaska dates from 11.99 ka (Rivals et al., 2007b). Only two dated bison samples are known from both glacial (21−60 ka) and late glacial (10−13 ka) stages, unfortunately, these samples were not identified to species level (Barrón-Ortíz, 2016), thus obscuring the paleoecological inference of those mesowear scores because they could represent the signal of several species.
Modern and extinct bison wear patterns.—Our results suggest that the Mexican samples analyzed display a mixed feeder pat- tern, rather than a strict grazer pattern. This is like the diet of Bison antiquus from Rancho La Brea, inferred from dental boluses in which the consumption of grasses was only 13.4% of the diet, compared to 86.6% gymnosperms and dicotyledons (Akersten et al., 1988). On the other hand, our extended mesowear univariate scores
show two patterns: more abrasive scores for La Cinta-Portalitos (1.66) and La Piedad-SantaAna (1.54), and a less abrasive score for Viko Vijin (0.93). The scores from the northern Mexican samples (Fig. 4.2) are similar to one previously known Bison antiquus score (1.56) from Plainview Quarry (BaPV),Hale County, Texas (Rivals and Semprebon, 2012), and are lower than the 1.83 score from Ingleside (BaIP), San Patricio County, Texas (Rivals and Semprebon, 2012), the 1.93 score from Blackwater Draw (BaBC), Clovis Pit, Curry County, New Mexico (Rivals and Semprebon, 2012), and the 1.88 score from Blackwater Draw (BaBD), Dry Cave, and Dark Canyon Cave, New Mexico and Lubbock Lake, and Scharbauer Ranch, Texas (Barrón-Ortíz, 2016). Also, our scores from northern Mexico are higher than the 1.31 score from Folsom Quarry,Union County,NewMexico (Rivals et al., 2007b). In contrast, the Viko Vijin sample score is the lowest for the species (0.93). The low latitude provenance of the sample and the association of the diet and the different environment and vegetation structure can explain this low score, as was noted above. Mesowear univariate scores from other bison fossil species
are known. Two samples of Bison sp. from Dalhart Sideroad Pit, Channing, Hartley County, Texas, and Seminole Field Station B, Pinellas County, Florida display a 1.05 mesowear score (Rivals et al., 2007b). There are two known Bison sp. fossil samples from Alberta, Canada, with mesowear data at two different times, one preglacial sample with a 1.67 score, and one postglacial sample with a 1.86 score (Barrón-Ortíz, 2016). A sample of the steppe bison (B. priscus) from the Fairbanks area, Alaska, displays a score of 1.1 (Rivals et al., 2007b). Despite this mesowear spectrum, the genus Bison and its fossil species still all fall within the grazer univariate score range (Rivals et al., 2007b). The northern Mexican samples display the highest values, and the Viko Vijin sample has the lowest values. The differences between the Mexican samples are probably due to dissimilarities in the diet and habitat use between these more middle- to high-latitude species and our sample individuals. The univariate wear pattern of modern Bison bison ecotypes is also different from our sample patterns. Modern
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