1136
Journal of Paleontology
plains bison (B. bison bison), grouped with the strict grazer guild in the cluster analysis (Fig. 4.1), display a very high mesowear score (2.73), the highest value among the extant model species (Fortelius and Solounias, 2000; Rivals et al., 2007b). On the other hand, the wood bison (B. bison ‘athabascae’ Rhoads, 1897) grouped within the mixed feeder guild in the cluster analysis (Fig. 4.1), and displays a mesowear score of 1.0, which is outside the range of typical grazers (Rivals et al., 2007b). We consider these modern scores as two different ecological behaviors of the same species because the modern status of wood bison as a separate subspecies is dubious (Geist, 1991; Groves and Grubb, 2011). The modern bison and its ecotypes display a wide range of variation among its wear signals. Our samples display a similar wear spectrum, and this might also reflect notable differences between the diets of our northern and southern Mexican samples and niche plasticity as wide as that described for modern bison. The Viko Vijin sample has the lowest wear values of all known bison (extant and extinct), although it is very similar to the pattern of modern wood bison (Fig. 4.2). This could reflect a similar composition in the abrasiveness in the diet of modern wood bison and extinct B. antiquus from Viko Vijin, as well as important differences between the feeding behavior of northern Mexican samples of B. antiquus and modern plains bison, making the latter an unsuitable ecological analog for B. antiquus, because its diet is apparently more abrasive and restricted to grasses. This is supported by the analysis of dental boluses of B. antiquus from California (Akersten et al., 1988).
Dietary geographical variation and grit consumption.—A cline of wear (Fig. 4.2, 4.3) of the studied samples (both for extended mesowear and mesowear III) is apparent and seems to be asso- ciated with latitude (i.e., the northern samples show more wear in their teeth than the southern ones). This can be explained by differences in the vegetation structure and the diet of the indi- viduals (Rivals et al., 2007b) or by the presence of exogenous materials ingested by the northern bison individuals (Damuth and Janis, 2011; Hoffman et al., 2015). There is some evidence against dust, quartz sand, and other exogenous materials as the main source of the wear patterns observed. For example, Antil- ocapra americana (Ord, 1815) inhabits highly dusty and open environments and its mesowear signature contains a very high proportion (88.6%) of sharp cusps (Fortelius and Solounias, 2000). Also, there is an experimental approach that shows the importance of vegetal endogenous SiO2 particles in the abrasion of enamel, pointing out that wear is mainly caused by the mechanical forces involving chewing of the vegetal material rather than just ingesting exogenous grit (Xia et al., 2015). However, we cannot completely discard that volcanic ash, dust, and other quartz-related exogenous materials were involved to some degree in shaping of the observed wear patterns. Also, these exogenous particles are more abundant in open areas (Janis and Fortelius, 1988; Fortelius and Solounias, 2000), such as the ones previously inferred for the fossil localities of the northern samples (Gutiérrez Bedolla et al., 2016; Marín-Leyva et al., 2016b), which supports the ingestion of grasses. Given that the studied sites have similar elevations—La Cinta- Portalitos ranges from 1750 − 2350 masl (meters above sea level), La Piedad-Santa Ana ranges from 1700−2500 masl, and
Viko Vijin ranges from 1800 −2400 masl (Jiménez-Hidalgo et al., 2011; Díaz-Sibaja, 2013)—altitude seems not to be rela- ted to the observed dietary trends.
Mesowear and previous paleoenvironmental reconstructions of localities.—Our data support and expand on previous paleoen- vironmental scenarios for La Cinta-Portalitos and La Piedad- Santa Ana. The evidence suggests the main presence of open areas with a dominance of grasses. All in agreement with iso- topic (δ13C and δ18O) and microwear data from Equus spp. and Mammuthus columbi (Falconer, 1858) (Gutiérrez Bedolla et al., 2016; Marín-Leyva et al., 2016a), mesowear data from horses (Marín-Leyva et al., 2016a), palynological data (Israde-Alcán- tara et al., 2010), and the scarcity of typical closed-habitat mammals in these paleocommunities such as tapirs. Our meso- wear bison data expand this scenario and suggest the presence of a more heterogeneous vegetation structure, mainly for La Cinta- Portalitos, where previous palynological data point to the pre- sence of other botanical elements such as Cheno-Am, Ambrosia spp., Asteraceae indet., Cirsium spp., and Thalictrum spp. during the time interval in which bison inhabited the area (Israde-Alcántara et al., 2010), and several typical mixed feeder mammals such as pronghorns and camelids (Díaz-Sibaja et al., 2014a, b; Plata-Ramírez et al., 2015). Our mesowear data also support previous paleoenviron-
mental scenarios for Viko Vijin local fauna. The bison samples of this fauna come from localities Oax-3 La Pedrera and Oax-7
Río Tejupam. The inferred paleoenvironment for Oax-3 is a floodplain (Jiménez-Hidalgo et al., 2011) with the presence of short-lived vegetation in a wetland setting (Guerrero-Arenas et al., 2013). Locality Oax-7 was fluvial, possibly a meandering system in an open setting (Jiménez-Hidalgo et al., 2011). Our mesowear data support these environmental settings and suggest a more heterogeneous vegetation structure, with mosaic vegetation, and little dominance of grasses. This is supported by the presence of typical open habitat mammals such as Equus spp. and Mammuthus spp., as well as typically closed habitat mammals such as Odocoileus spp. and Cuvieronius spp., coexisting with some genera identified as mixed feeders such as Hemiauchenia spp. and Camelops spp. (Jiménez-Hidalgo et al., 2011).
Conclusions
The combined mesowear analysis showed that our two northern Mexican samples of Bison antiquus fall within the grazer guild and the southern one falls within the mixed feeder guild. These mesowear signatures are different from those of most of the previously reported samples of B. antiquus, and from modern B. bison. The Viko Vijin mesowear score is close to that of the wood bison (B. bison ‘athabascae’) suggesting similar dietary behavior. These mesowear patterns show broader dietary habits for B. antiquus and a low abrasive ingestion in the southernmost populations. These interpretations fit within a more generalist diet than previously assumed for this species. The difference among mesowear scores is related to lati-
tude, reflecting different environmental conditions. Based on previous evidence (i.e., isotopic, microwear, mesowear, paly- nological, and sedimentological data, as well as the composition
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