Evaluación de la dieta de una población de caballos hipparionin del Mioceno medio de Oaxaca, sur de México

Victor M. Bravo–Cuevas^1,* y Jaime Priego–Vargas²

¹ Museo de Paleontología, Área Académica de Biología, Universidad Autónoma del Estado de Hidalgo, Ciudad Universitaria s/n, Carretera Pachuca–Tulancingo Km. 4.5, 42184 Pachuca, Hidalgo, Mexico. *vbravo@uaeh.edu.mx

² Licenciatura en Biología, Universidad Autónoma del Estado de Hidalgo, Ciudad Universitaria s/n, Carretera Pachuca–Tulancingo Km. 4.5, 42184 Pachuca, Hidalgo, Mexico.

Manuscript received: January 21, 2008 ]]> Corrected manuscript received: January 21, 2009
Manuscript accepted: February 3, 2009

ABSTRACT

The first paleodietary interpretation of a hipparionin horse population from the middle Miocene El Camarón Formation (late early Barstovian, K–Ar dated 15.0 ± 0.8 to 16.7 ± 0.71 Ma) of Oaxaca, southeastern Mexico is presented. The species is formally referred to Cormohipparion aff. C. quinni and its dietary behavior were evaluated by the extended mesowear analysis method. A cluster analysis compared the Oaxacan species with 23 selected extant species and fossil hipparionins from the Neogene of North America, Europe, and Africa. The mesowear pattern of the Oaxacan hipparionin is comparable to that of the mixed feeder Cormohipparion quinni from the late Barstovian of northern Great Plains. These species display a combination of high relief and round cusps characteristic of a relatively high–abrasion diet. This suggests that the population of C. aff. C. quinni was a mixed feeder that incorporated abrasive food items into the diet (grass and/or extrinsic grit). The results provide evidence for the existence of local grazing habitats in southern tropical North America during the early middle Miocene (ca. 15 Ma), and give additional support that savanna–like habitats were present in high and low latitudes of North America during the mid Miocene (~18–12 Ma).

Key words: hipparionins, mesowear, paleodiet, paleoecology, middle Miocene, Oaxaca, Mexico.

RESUMEN

Se presenta la primera interpretación sobre la dieta de una población de caballos hipparionines cuyo material fósil procede de estratos pertenecientes a la formación El Camarón del Mioceno medio (parte más tardía del Barstoviano temprano, con fechamiento isotópico por K–Ar de 15.0 ± 0.8 a 16.7 ± 0.71 Ma) de Oaxaca, sureste de México. La especie se designa formalmente Cormohipparion aff. C. quinni y sus hábitos dietarios se evaluaron mediante la aplicación del método conocido como mesodesgaste extendido. Para fines de comparación se realizó un análisis de cluster de la población oaxaqueña con 23 especies recientes y poblaciones selectas de hipparionines del Neógeno de Norteamérica, Europa y África. El patrón de mesodesgaste del hipparionine oaxaqueño es semejante al de la especie de hábitos mixtos Cormohipparion quinni del Barstoviano tardío de las Grandes Planicies. Estas especies muestran una combinación de relieve alto con cúspides redondeadas, que es característica de una dieta que incluye material abrasivo. Nuestras observaciones sugieren que la población de C. aff. C. quinni tuvo hábitos alimentarios mixtos con una ingesta de recursos abrasivos (pastos, polvo y/o arena). La información disponible evidencia que a principios del Mioceno medio (ca. 15 Ma) existieron zonas cubiertas por pastos en lo que ahora es parte de Norteamérica meridional; asimismo, apoya la propuesta de la existencia de biomas de tipo sabana en latitudes altas y bajas de Norteamérica durante el Mioceno medio (~18–12 Ma).

Palabras clave: hipparionines, mesodesgaste, paleodieta, paleoecología, Mioceno medio, Oaxaca, México.

INTRODUCTION

The interpretation of ancient dietary regimes in fossil ungulates is an important source of paleobiological information. It provides evidence on the availability of food resources in a particular area and insight into the resource partitioning of mammalian herbivore communities. The indirect evidence provided is a useful ecological indicator and is of key importance in the interpretation of ancient terrestrial environments. The methods to assess dietary traits include direct comparison with living animals, the application of general functional principles (e.g., Rensberger et al., 1984; Fortelius, 1985), carbon isotopic studies of the dental tissue (e.g., MacFadden and Cerling, 1996; Cerling et al., 1997; Cerling and Harris, 1999), and the analysis of dental wear patterns at microscopic and/or macroscopic inspection (e.g., Hayek et al., 1992; Fortelius and Solounias, 2000; Solounias and Semprebon, 2002; Kaiser and Solounias, 2003; Kaiser et al., 2003; Semprebon et al., 2004). In recent years, Fortelius and Solounias (2000) introduced the method of dietary analysis known as mesowear, which is based on relative facet development on the occlusal surface of the teeth. The technique was formerly restricted to investigate the upper second molar, and consisted of a visual evaluation of the molar cusps morphology providing information on both attrition (tooth–on–tooth contact) and abrasion (tooth–on–food contact). Later, Kaiser and Solounias (2003) extended the method for the remaining upper cheek tooth positions, allowing for the investigation of the dietary preferences in fossil populations represented by a small number of individuals. In order to obtain stable dietary classifications and to know ancient dietary regimes, the mesowear methodology has been applied to various extant and/or extinct ungulates (e.g., Fortelius and Solounias, 2000; Croitor and Kaiser, 2002; Franz–Odendaal and Kaiser, 2003; Kaiser and Fortelius, 2003; Kaiser and Franz–Odendaal, 2004). It has been shown that mesowear analysis is a reliable and simple technique used to indicate diet of a particular population.

There are many papers concerning the dietary evaluation of fossil hipparionin horses from the Neogene of Europe and Africa (e.g., Kaiser et al., 2000, 2003; Franz–Odendaal et al., 2003; Kaiser and Solounias, 2003; Bernor et al., 2004). The hipparionins are a group of three toed horses that make up a significant element of North American terrestrial mammalian faunas throughout the Neogene. In the early Miocene (~15–18 Ma), horses experienced an adaptive radiation of taxa that developed high–crowned cheek teeth (MacFadden and Hulbert, 1988; Hulbert and MacFadden, 1991). The acquisition of hypsodonty has classically been considered as an adaptive shift from browsing to grazing in order to exploit the spread of savanna grasses habitats in North America during the Miocene (see Webb, 1977, 1983). Recently, the paleodietary investigation of fossil horses (and several other groups of ungulates) found that the advent of hypsodonty is not necessarily coupled with a grazing behavior and the expansion of grasslands, providing evidence about the paleocology of hypsodont horses as well as the global changes in climate and ecology that affected them (MacFadden and Cerling, 1994; MacFadden et al., 1999; Passey et al., 2002; Solounias and Semprebon, 2002).

The purpose of this study is to present for the first time the paleodietary preference of a hipparionin population from the middle Miocene of southeastern Mexico, including a comparison of dietary strategies previously reported for other selected hipparionins from the Neogene of temperate North America.

Study area and geologic setting

The horse material comes from fossiliferous localities that lie in the Nejapa de Madero area (hereafter Nejapa), northwest of the Isthmus of Tehuantepec, roughly midway between the cities of Oaxaca and Tehuantepec, and geographically located between 16°30'–16°40' N and 95°55'–96°10' W (Figure 1).

The study area is an exposed sedimentary sequence that belongs to the synorogenic, tuffaceous, fluviolacustrine El Camarón Formation (late early Barstovian, K–Ar dated 15.0±0.8 to 16.7±0.71 Ma: Ferrusquía–Villafranca and McDowell, 1991; Ferrusquía–Villafranca, 1992; Tedford et al., 2004). The strata that yield the fossil horse material is the lower member. It is dominantly fine grained (clayey, silty to finely sandy) and laminar to thinly bedded. The upper member is coarse grained (sandy, granular to pebbly conglomeratic) and thin– to thickly bedded (Ferrusquía–Villafranca, 2002).

The horse record from the middle Miocene of Oaxaca, southeastern Mexico

Thirty years of paleontological work carried out in localities of the Nejapa Area on the middle Miocene (Barstovian) El Camarón Formation, have recovered a limited but significant collection of fossil horse material. The sample is chiefly represented by isolated teeth and scarce mandibular and maxillary fragments. The taxonomic analysis of this collection indicates the presence of at least four genera and five species (Jimenéz–Hidalgo et al., 2002; Ferrusquía–Villafranca, 2003). The horse record includes the relict "Merychippus" cf. "M." primus, the merychippine hipparionine grade species "M." cf. "M." sejunctus, Calippus sp., Pliohippus aff. P. pernix, and Cormohipparion aff. C. quinni (Jiménez–Hidalgo et al., 2002; Ferrusquía–Villafranca, 2003; Tedford et al., 2004; Bravo–Cuevas and Ferrusquía–Villafranca, 2006, 2008). The material referred to Cormohipparion aff. C. quinni is more abundant in comparison to that of other horse species known from the Nejapa fauna.

MATERIAL AND METHODS

The material includes 15 upper cheek teeth recovered from the El Camarón Formation, late early Barstovian of Oaxaca, southeastern Mexico. All specimens are housed in the Colección Nacional de Paleontología at the Instituto de Geología, Universidad Nacional Autónoma de México (IGM).

A minimum of 10 dental specimens are needed to give a reasonable scoring of paleodietary preference (cf. Fortelius and Solounias, 2000; Kaiser et al., 2000); given that, the available dental sample is adequate to accomplish an analysis of tooth mesowear. The extended mesowear analysis was used to characterize the dietary behavior of the Oaxacan hipparionin Cormohipparion aff. C. quinni, including a combination of upper tooth positions P4, M1, M2, and M3 (after Kaiser and Solounias, 2003). The conventional mesowear variables occlusal relief (OR) and cusp shape (CS) were scored to each upper cheek tooth (Table 1). The OR is described as low (l) or high (h), whereas CS is described as sharp (s), rounded (r) or blunt (b) (after Fortelius and Solounias, 2000).

The mesowear analysis is sensitive to wear on the dental enamel of the occlusal surface caused by the effect of tooth–on–tooth and food–on–tooth contact, thus unworn teeth, teeth in very early wear, and teeth in very late and late wear are omitted from the analysis (after Fortelius and Solounias, 2000; Kaiser et al. 2003; Rivals et al., 2007). The wear stage for cheekteeth was performed following Kaiser et al. (2003, p.106), including teeth with entire occlusal surface beginning to wear (pre– and postfossette are still fused with anterior or posterior enamel band) and/or tooth wear less than 70% of maximum crown height (pre– or postfossette are isolated from anterior or posterior enamel band).

We used Statistica '98© Edition software to compute chi–squared statistics to test for significance of differences observed between individual datasets using the absolute frequencies of mesowear variables (high, sharp, and blunt). Hierarchical cluster analysis with complete linkage (furthest neighbors) was applied. In order to observe frequency differences in the mesowear signature of selected extant and fossil species, histograms of the mesowear variables %low, %high, %sharp, and %blunt were plotted.

In order to compare the population of Cormohipparion aff. C. quinni with other extant ungulates, the mesowear variables were converted into a mesowear score following methods described by Rivals and Semprebon (2006) and Rivals et al. (2007): a combination of high relief and sharp cusps was assigned a score of 0, a combination of high relief and round cusps was assigned a score of 1, a combination of low relief and round cusps was assigned a score of 2, and a combination of low relief and blunt cusps was assigned a score of 3. A mesowear score for C. aff. C quinni was then calculated and compared with data published in Rivals and Semprebon (2006).

RESULTS

In the Oaxacan hipparionin population, occlusal relief is 53.3% high. Cusp shape scorings are 86.2% round, 10.3% sharp and 3.4% blunt (Figure 2, Table 2). There are three main clusters in the hierarchical tree plot based on the selected recent and fossil species and the population under study, one containing browsers and mixed feeders (ED [Euclidean distance] = 38), one containing the abrasion–dominated grazers (ED = 34), and one that includes the abrasion–dominated mixed feeders or grazers (ED = 40) (Figure 3). The hipparionins are distributed within any of these clusters, as it is described in following paragraph.

The species Hippotherium kammerschmittae is linked with the browser species Odocoileus virginianus (white–tailed deer) and Okapia johnstoni (okapi) (ED = 20). The population of H. primigenium from Dorn–Dürkheim and that of "Cormohipparion" sp. from Chorora are grouped with the mixed feeders Cervus canadensis (wapiti), Taurotragus oryx (eland), Tragelaphus scriptus (bushbuck), and Capricornis sumatrensis (serow) (ED = 16). Cormohipparion aff. C. quinni is linked with the extant grazer Alcelaphus buselaphus (hartebeest) and the population of 'Eurygnathohippus' cf. baardi from Pelletal Phosphate Member (ED = 19). The species C. goorisi is linked with the extant grazer species Hippotragus equinus (roan antelope) and Redunca redunca (reedbuck) (ED = 10). The species Merychippus insignis is linked with the mixed feeder Aepyceros melampus (impala), whereas the populations of H. primigenium from Höwenegg and Eppelsheim, and Cormohipparion quinni are grouped with the former two species (ED = 16). The hipparionin population of 'Eurygnathohippus' cf. baardi from Quartzose Sand Member is linked with the abrasion–dominated grazer species Equus burchellii (plains zebra) and E. grevyi (imperial zebra) (ED = 18) (Figure 3).

Significant differences between the mesowear variables (high, sharp, and blunt) obtained for Cormohipparion aff. C. quinni, the hipparionin C. goorisi, and the extant grazer Alcelaphus buselaphus were found (x², P<0.05). Low levels of significance, however, were found when a comparison was made between the Oaxacan hipparionin population with C. quinni (P = 0.1294) and the population of 'Eurygnathohippus' cf. baardi from Pelletal Phosphate Member. Histograms of the mesowear variables for Cormohipparion aff. C. quinni and C. quinni (Figure 4) show the similarities in their mesowear pattern, by having a combination of relatively high relief and round cusps.

Figure 5 shows a plot of the hypsodonty index versus the mesowear score (hypsodonty indices as reported in Fortelius and Solounias, 2000; mesowear scores from data reported in Rivals and Semprebon, 2006) for selected extant ungulates and the fossil population considered in present study. The mesowear score for the Oaxacan hipparionin population of Cormohipparion aff. C. quinni (1.3) falls within the range observed in extant abrasion–dominated mixed feeders and fresh grass grazers, whereas the calculated hypsodonty (1.9) places this species with extant mixed feeders with high abrasion mesowear pattern and diets and away from modern extant browser which are rarely hypsodont.

Dietary behavior of Cormohipparion aff. C. quinni from the middle Miocene of southeastern Mexico

Results of cluster analysis link the population of Cormohipparion aff. C. quinni with the extant African antelope Alcelaphus buselaphus (hartebeest), however mesowear pattern of these species is significantly different (x², P < 0.05). The mesowear pattern of Alcelaphus buselaphus (hartebeest), dominated by rounded cusps with also a significant proportion of blunt cusps (Figure 4), is typical of other extant ungulates with diets that comprised a predominance of abrasive food items (particularly grass), such as Connochaetes taurinis (wildebeest) and Damaliscus lunatus (topi) (Estes, 1991; Nowak, 1999; Schuette et al., 1998; Gagnon and Chew, 2000). Therefore, it is likely that the population of Cormohipparion aff. C. quinni may have been engaged in feeding on abrasive foods (grass and/or extrinsic grit [dust and sand]). A few dental conditions observed in the Oaxacan cheek teeth are related to feeding on abrasive items, including hypsodont cheek teeth (relative molar crown height of 45 mm and hypsodonty index of 1.9), moderate curvature (mean radius of curvature of 55 mm), thick layer of cement (~1.5 mm thick), and moderately plicated enamel fossette borders that implies an increase of resistant dental tissue at the occlusal surface of the teeth (Janis and Fortelius, 1988; Bennett, 1992; MacFadden, 1992). This evidence also supported that the Oaxacan hipparionin population incorporated abrasive food materials in its diet.

The mesowear pattern of Cormohipparion aff. C. quinni is defined by predominant high relief and rounded cusp apices (Table 2, Figure 4), and its mesowear score overlaps with the upper extreme for extant mixed feeders and lower extreme for grazers (mesowear score interval of 1–1.5 in Figure 5). Overall differences, however, are observed among extant grazers with extremely blunted mesowear signatures and highest mesowear scores (>2) related to a hyper abrasive grazing diet (upper extreme for grazers in Figure 5). Examples include zebras (Equus burchelli and E. grevyi) and the modern plains bison (Bison bison). The similarities between the mesowear pattern and mesowear score of the Oaxacan hipparionin population and those observed for some highly abrasion–dominated mixed feeders and fresh grass grazers indicate a mixed feeding diet including abrasive items, which in this present study is categorized as graze–dominated mixed feeder.

Additionally, evidence shows that hypsodonty (high crowned cheek teeth) in horses and other extinct ungulates is not necessarily correlated with grazing, a contention previously stated by other authors (see MacFadden and Cerling, 1994; MacFadden et al., 1999; Strömberg, 2006; Rivals and Semprebom, 2006). Thus, dental wear analyses are informative of the diets of extinct and extant species, but they should be cautiously used as sole indicators of ancient terrestrial environments.

Comments on the dietary behavior of selected late Neogene North American hipparionins

By the Barstovian (12.7–16.0 Ma; Tedford et al., 2004), North American species of Cormohipparion were distributed in the Great Plains, Gulf Coastal Plain, and southeastern Mexico. The record includes C. goorisi from the late early to early late Barstovian (15.0–15.6 Ma) of Texas, Florida, and probably Lousiana (MacFadden and Skinner, 1981; Hulbert and MacFadden, 1991; Schiebout and Ting, 2000); C. aff. C. quinni from the late early Barstovian (15.0–16 Ma) of Oaxaca (Bravo–Cuevas and Ferrusquía–Villafranca, 2008); and C. quinni from the late Barstovian (13.0–14.0 Ma) of Nebraska and Colorado (Woodburne, 1996).

The species Cormohipparion goorisi is identified as a grazer (see Fortelius and Solounias, 2000), whereas the population of C. aff. C. quinni is characterized as a graze–dominated mixed feeder. Nevertheless, abrasive food sources were incorporated in their dietary regimes and it would be expected that C. goorisi ingested a greater amount of abrasive items than C. aff. C. quinni. The diet probably consisted of grasses with silica–rich phytoliths, given they make up a major source of abrasives eaten by extant herbivorous mammals (MacNaughton et al. 1985). It should be noted that both species are approximately coetaneous, but come from localities in different biogeographic regions that at are least 1800 km apart. This suggests the existence of local grazing habitats in areas of temperate North America (Gulf Coastal Plain) and tropical North America (southeastern Mexico) during the early middle Miocene (Figure 6).

The species Cormohipparion quinni has been considered an abrasion–dominated mixed feeder (Fortelius and Solounias, 2000). Our observations supported this contention and also provide evidence that C. aff. C. quinni have a comparable mesowear signature and dietary category (Figure 4, Table 2). These species are known from localities in different biogeographic regions and the Oaxacan hipparionin population antedates the record of C. quinni by at least one million years, its probable closest North American sister species.

A mixed feeding strategy is related to ecological scenarios with a broad spectrum of possible food resources. It is hypothesized that by the middle middle Miocene (late early Barstovian to late Barstovian) were areas of temperate North America (Great Plains) and tropical North America (southeastern Mexico) with variable vegetation in the habitat mosaic and the concomitant availability of diverse food resources. Such requirements could best be met in ecosystems where variable amounts of grasses, trees and/or bush cover are resources for different kind of herbivores, as occurs in present–day savannas (Potts and Behrensmeyer, 1992). This explanation agrees with the available information about the ungulate communities, palaeobotanical evidence, and geochemical studies from several fossil localities in temperate North America. It indicates that savanna–like habitats with an important grass component were common in North America during the middle Miocene (~18–12 Ma) (see Janis et al., 2004; Strömberg, 2002, 2004; Retallack, 2007). The known Barstovian Oaxacan large herbivore communities (cf. Jiménez–Hidalgo et al. 2002; Ferrusquía–Villafranca, 2003; Tedford et al., 2004), the complex geologic makeup and physiographic features of southeastern Mexico during the middle Miocene (Morán–Zenteno, 1984; Sedlock et al., 1993; Martínez–Serrano et al., 2008), fit confidently into a scenario for the developing of savanna–like habitats also in tropical North America (Ferrusquía–Villafranca, 2003).

CONCLUSIONS

The mesowear analysis performed identifies as a graze–dominated mixed feeder the population of Cormohipparion aff. C. quinni, from the middle Miocene (Barstovian) El Camarón Formation of Oaxaca. The dietary behavior that characterizes the Oaxacan hipparionin population suggests the presence of local graze habitats in southern tropical regions of North America (specifically southeastern Mexico) during the early middle Miocene. Our observations also provide additional support that during the mid Miocene (~18–12 Ma) savanna–like habitats were present in high– and low–latitudes of North America.

ACKNOWLEDGMENTS

We are indebted to Florent–Rivals for critically reviewing the manuscript and give us access to their published mesowear scores data. Also we thank the anonymous reviewers whose comments and suggestions greatly improved the manuscript. We thank to Mr. Jose Ruíz who made the map of Figure 1, Biologist Berenice Alemán García in the elaboration and edition of the final version of Figure 3, and to Mrs. Elizabeth O. de Bravo for the preparation of previous draft. At last but no to least we are also grateful to Christopher Fielitz for linguistically editing this article.

The senior author express special thanks to Bruce J. MacFadden and Richard C. Hulbert Jr. for their valuable comments about hipparionin horses during my visit to the Florida Museum of Natural History. The visit was partially supported by the 2003 International Student Travel Grant to Study the Vertebrate Paleontology Collection at the Florida Museum of Natural History.

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