The site of La Milloque (Aquitaine Basin, south-western France), dated to the end of the Late Oligocene (MP29 reference level), has yielded an important fauna of mammals since its discovery in 1868. Notably, three different size species of Moschidae (Ruminantia) were identified at this locality: Dremotherium quercyi, Dremotherium guthi and Bedenomeryx milloquensis. Dental microwear analysis was used to determine their respective feeding preferences in comparison to various extant ungulates. The smallest one, D.quercyi, was close to leaf browsing extant species and to the mixed-feeding species Tragelaphus scriptus, and so fed mainly on leaves. The largest species D.quercyi and B.milloquensis showed affinities with grazing extant species, and so consumed grasses. Thus they constitute the oldest brachydont grazing ruminants analysed up until now. The results, in agreement with previous studies, imply the existence of a mixed environment at La Milloque, ranging from light forest to more open areas constituted of grasses.
Le gisement de La Milloque (Bassin aquitain, Sud-Ouest de la France), daté de la fin de l’Oligocène supérieur (niveau repère MP29), est connu pour avoir livré une faune importante de mammifères depuis sa découverte en 1868. Notamment, trois espèces de Moschidae (Ruminantia) de tailles différentes y ont été identifiées: Dremotherium quercyi, Dremotherium guthi et Bedenomeryx milloquensis. L’analyse des micro-usures dentaires a été utilisée pour déterminer leurs préférences alimentaires respectives, par comparaison avec différentes espèces d’ongulés actuels. La plus petite espèce, D.quercyi, était proche des actuels brouteurs et du mangeur mixte Tragelaphus scriptus, et donc se nourrissait de feuilles. D.guthi et B.milloquensis, espèces de plus grande taille, montrent des affinités avec les paisseurs actuels, et donc consommaient des graminées. Par conséquent, ce sont les plus anciennes espèces de ruminants brachyodontes consommatrices de graminées, analysées à ce jour. Ces résultats, en association avec de précédentes études, impliquent l’existence d’un environnement mixte à La Milloque, s’étendant de la forêt claire à des zones plus ouvertes constituées de graminées.
La Milloque, Late Oligocene, Ruminants, Dental microwear, Feeding preference, Paleoenvironment, FranceLa Milloque, Oligocène supérieur, Ruminants, Micro-usures dentaires, Préférence alimentaire, Paléoenvironnement, FrancepresentedPresented by Philippe TaquetIntroduction
The site of La Milloque (Aquitaine Basin, south-western France) is characterized by a set of molasses deposits called ‘Molasses de l’Agenais’, which underlie the Lower Miocene white limestone called ‘Calcaire blanc de l’Agenais’. These deposits, since their discovery by Landesque in 1868, have yielded important fossil vertebrate fauna. The major part of this material was collected in the second half of the nineteenth century (Brunet, 1979 and Richard, 1948). The site corresponds to the MP29 reference level of the mammal scale of western Europe and is dated close to 24 Ma (Aguilar et al., 1997). The largest part of the collected material has already been described in several publications (Brunet, 1979, Cirot and Wolsan, 1995 and Jehenne, 1985). In particular, three different species of ruminants belonging to the family of Moschidae were discovered: Dremotherium guthi, Dremotherium quercyi and Bedenomeryx milloquensis (Gentry et al., 1999, Jehenne, 1985 and Jehenne, 1988).
The Moschidae, small hornless pecorans, consist of only one living genus today, Moschus, with five species (Novak, 1999). The preferred habitat of these of Asian musk deer is forests and brushlands in high altitudes. They are browsers with a diet consisting of a variety of vegetation, such as mosses, buds, young shoots, leaves, lichens and occasionally grasses. The moschids have an extensive fossil record with an Oligocene-Miocene radiation in Eurasia and a Miocene radiation in North America. The oldest documented representative of this family is Dremotherium. This genus appears at the beginning of the Late Oligocene (Blondel, 1997). D.guthi and D.quercyi appear at the MP28 reference level and survived until the MP30 level at the end of the Late Oligocene (Jehenne and Brunet, 1992). The genus Bedenomeryx appeared at La Milloque (MP 29) with the species B. milloquensis known to the MP 30 level. Dremotherium and Bedenomeryx, as for most of the European genera of moschids, did not reach the Middle Miocene.
The three moschids from La Milloque possess brachydont molars and so would be interpreted as browsers. According to this hypothesis, these species would have competed for the same general resources. The dental, cranial and postcranial skeletal anatomy indicates that D.quercyi was smaller than D. guthi, and B.milloquensis was the largest of the three species. This difference in body size suggests the existence of different feeding behaviours, which would then suggest that these species were able to coexist by occupying different ecological niches (Eisenberg, 1981 and Pérez-Barberia and Gordon, 2001). Furthermore, the presence of these three ruminants at La Milloque at the end of the Late Oligocene in association with other herbivorous mammals such as cainotheriids, anthracotheriids and rhinocerotids (Brunet, 1979 and Richard, 1948) could mean that rather favourable climatic conditions existed for developing new feeding strategies and ecological niches. Indeed, it is known that after a climatic cooling in the Lower Oligocene, a trend of warming appears to have begun in the Upper Oligocene and then continued until the climatic optimum of the mid-Miocene (Zachos et al., 2001). In particular, in southern France, several studies give evidence of warmer and wetter closed environments at the end of the Oligocene, in spite of the persistent, dry open areas (Blondel, 1998, Brunet, 1979, Comte, 2000, Costeur, 2005, Costeur and Legendre, 2008, Legendre, 1987 and Vianey-Liaud, 1991).
The present work aimed to analyse the dental microwear patterns of the three ruminants of La Milloque in order to: (1) determine their dietary habits; and (2) estimate their ecological niches. Indeed, dental microwear analysis constitutes valuable method for the reconstruction of past environments. Moreover, this method is already well known through numerous studies on ungulates (Merceron et al., 2004a and Solounias et al., 2000). Microwears are directly linked to the ingested food, which, thanks to its physical properties, marks the enamel surface to differing degrees during the chewing cycle. Microwears result from the abrasion of teeth on items consumed during the last few meals. Their description and quantification allow very different dietary profiles to be determined: the browsers (exclusively consume leaves or fruits/leaves), the grazers (consumers of grasses), the mixed-feeders (graze or browse according to the environmental conditions) and the generalists.
Material
Dental specimens selected for this study had been inventoried by Jehenne (1985) and housed in the CVCU of Poitiers (Centre de Valorisation des Collections Universitaires). Eighteen specimens belonging to D.guthi and sixteen to D. quercyi were used for analyses. Only four specimens of B.milloquensis were included in this study due to the lack of available material. The material chosen included both lower molars (M2), directly selected on mandibles, and isolated upper molars (M1, M2 and M3) (Appendix A). The microwear signature is not significantly different between lower and upper molars, even if the positions of these last could not be determined (Teaford and Walker, 1984). The microwears were analysed on the lingual blade of the paracone and on the buccal blade of the protoconid (Merceron et al., 2005). In cases where these facets were not preserved, their analogous facets were chosen on the metacone or hypoconid.
Methods
The protocol established by Merceron et al. (2004b) was applied for the casting procedures and quantification of the microwear patterns on the enamel surfaces. The dental facets were digitized using a Leica DC 300 digital microscope camera and the image acquisition software Leica IM50 connected to a light stereomicroscope (Leica MZ125) at low magnification (×30). The dental microwear was quantified in a 0.09 mm2 area using Optimas v. 6.2. image analysis software (Media Cybernetics®).
Microwear scars are defined as pits or scratches as follows: pits are circular to sub-circular scars in enamel whereas scratches correspond to lengthened straight scars with parallel edges. A scratch has a ratio between its minor and major axes lower than ¼, whereas a pit has a higher ratio (Grine, 1986).
Microwear quantification was made by a single observer (AN). The number of scratches (Ns) and the number of pits (Np) were counted. The total number of microwear scars (Ntot = Ns + Np) and the percentage of pits (%p = Np/Ntot x100) were calculated for each species.
The results obtained were compared with those acquired by Merceron et al., 2004a, Merceron et al., 2004b and Merceron et al., 2005 for 25 extant ungulate species with known varied diets extracted from the ‘UNGULATES’ database. These species are classified under five different feeding preferences, each reflected by a particular microwear pattern: leaf browsers, fruit/leaf browsers, grazers, mixed-feeders and generalists (Table 1).
Statistical analyses were undertaken to test whether the observed intergroup differences of microwear patterns were significant or not. All variables were rank transformed to ensure normality and homeoscedasticity. Univariate analyses of variance (ANOVA) were applied to test the significance of the observed differences between samples (Table 2). The combination of two pairwise tests (Fisher's least significant difference and Tukey's honestly significant difference) determined the sources of significant variation for each dental facet (Conover and Iman, 1981 and Sokal and Rohlf, 1998) (Table 3).
Results
The majority of scratches observed on the enamel surface of fossil ruminants are very fine (Fig. 1). However, the microwear pattern observed on D. quercyi teeth tended to be qualitatively coarser than that observed for D. guthi, with the presence of wider scratches and more marked pits.
D.quercyi had a pit percentage (%p) of 56.1% whereas D. guthi possessed one of 42.3% (Table 1; Fig. 2). Furthermore, D. quercyi had a lower number of scratches (Ns) than that obtained for D. guthi. These observed differences were statistically significant (Table 2 and Table 3) and decisive for the assignation of each Dremotherium species to a particular diet.
D.guthi possessed mean Ns, Np and %p values which were not significantly different from those observed for the grazers. This result is clearly illustrated in Fig. 3, where D. guthi can be seen to fall within the range of grazing species (Table 1). In contrast, D. quercyi was statistically closer to both browsers and mixed-feeders (Table 3). This result is also well illustrated in Fig. 3, where D. quercyi is clearly comparable with the mixed-feeders in addition to possessing a mean Ns value which falls within the range of browsers.
Despite the small sample size, B. milloquensis presents a Np (11.0) much lower than those obtained for the two other fossil species and a %p sharply lower than 50% (37.4%) (Table 1). Even if this species was graphically outside of the distribution of dietary categories (Fig. 3), it was close to the grazers in having very low Np and %p values which were not significantly different from those of the grazing extant species (Table 3).
Discussion
All of the analyses indicated that both Dremotherium species had different feeding preferences. From the variables tested, D. quercyi presented important similarities with the extant browsers and mixed-feeders. In particular, it is very close to the extant mixed-feeder Tragelaphus scriptus (Fig. 3). This species adopts different feeding strategies during the year. In the wet season, it preferentially feeds on young items of grasses and non-graminoid herbaceous plants, and throughout rest of the year it principally consumes dicotyledon leaves (about 80%). Thus, it is not surprising that its microwear profile is very close to the extant browsers and some studies classify T. scriptus in this category (Gagnon and Chew, 2000). Nevertheless, D. quercyi also possessed a high number of large pits probably reflecting its occasional consumption of fruits such as Acacia pods (Jacobsen, 1974). Considering these facts, we assumed that D. quercyi principally consumed leaves and also occasionally consumed some fruits such as pods, thus explaining its more marked pits and wide scratches.
The results indicated that D. guthi was very close to extant grazers such as the topi, ourebi and zebra, indicating that it had to have consumed grasses (Gagnon and Chew, 2000). Nevertheless, D. guthi possessed very fine scratches compared to these extant species which would indicate a consumption of C3 grasses (Merceron et al., 2004a, Merceron et al., 2005 and Solounias and Semprebon, 2002). Indeed, such fine scratches are clearly observable in an extant species of Equidae, Equus przewalskii, which exclusively consumes C3 grasses. On the contrary, the microwear pattern was different from those observed in extant species which only feed on C4 grasses, such as the zebra Equus burchelli.
To a lesser degree, B. milloquensis also displayed similarities with grazers. However, this extinct species possessed a very low number of scratches (mean Ns = 18.5) compared to D. guthi (mean Ns = 22.8) and all of the extant grazers studied (Ns ranging from 22.6 to 29.8). Consequently, it is possible that B. milloquensis had a diet principally composed of grasses, but maybe different to that of D. guhi. Nevertheless, this result was only supported by four specimens of B. milloquensis and has to be tested with supplementary material.
The existence of grasses at the end of the Oligocene is not surprising since their presence had been already attested in North American deposits dated back to the same period (Strömberg, 2002). Considering that the C3/C4 transition took place in the Late Miocene, Cerling et al. (1997) supposed that only C3 grasses were widespread in Europe at the period studied. This assumption was notably supported by the presence of very fine scratches in the fossil species.
The consumption of grasses by two brachydont ruminants is unexpected. Indeed, the presence of low-crowned cheek teeth in an ungulate is often perceived as a browsing adaptation (Janis, 1990 and Janis et al., 2000). Thus, Janis et al. (2002) linked the expansion of grasses to the appearance of hypsodonty in ungulates. High-crowned cheek teeth are considered to be an adaptation to an abrasive diet such as grasses, which are richer in phytoliths than other types of plants. Moreover, the evolution of hypsodonty seems to be closely connected to the diversification of C4 grasses (richer in phytoliths than C3 grasses) (Bouchenak-Khelladi et al., 2009 and Hodson et al., 2005). To sum up, the hypsodonty/grasses co-evolution credibly became more marked during the Miocene, in spite of the previous establishment of C3-grasslands during the Oligocene. Therefore, it is likely that brachydont D.guthi and B.milloquensis were forerunners in grass consumption at the end of the Oligocene. Additionally, it is necessary to keep in mind that the grass type supposedly consumed in our study was the C3 type, which possessed a less important degree of abrasion than the C4 type.
This feeding behavior probably enabled the avoidance of more intense interspecific competition during this period and may have been necessary for the survival of these moschids. Moreover, the appearance of a grazing adaptation early in the evolution of ruminants has already been demonstrated in the Early Miocene brachydont Procervidae and some Miocene Giraffidae (DeMiguel et al., 2008, Solounias et al., 1988 and Solounias et al., 2000).
The diversity of resources consumed by the ruminants of La Milloque justifies their body size differences and so implicates the presence of a relatively heterogeneous environment at the end of Oligocene. The smallest species, D. quercyi, had to live in rather closed environments by having a diet mainly composed of leaves. Notably, studies concerning the post-cranial skeleton of this species revealed that it must have been adapted to rather light forests (Blondel, 1998). The presence of rodents such as Eomyidae, Gliridae, and anthracotheriids such as Microbunodon minimum at La Milloque also confirms the existence of close forested areas (Brunet, 1979, Comte, 2000, Lihoreau, 2003 and Vianey-Liaud, 1991). D.guthi and B. milloquensis, whose diet consisted of grasses, probably fed in more open areas. Such environments were notably attested by the presence of the cursorial rhinoceros Mesaceratherium aff. paulhiacense and the Dipodidae (Brunet et al., 1981 and Comte, 2000). Moreover, the La Milloque cenogram shows few large and medium mammals and a high number of small mammals, which characterizes an open and slightly arid environment in this locality (Legendre, 1987).
Conclusions
The dental microwear analysis revealed different feeding behavior for the moschids of La Milloque. The smallest species, D.quercyi, possessed a dental microwear pattern close to extant leaf browsers, and to the mixed-feeder T.scriptus. The largest species, D.guthi and B.milloquensis, were close to extant grazers. Thus, they constitute the oldest brachyodont ruminants to have included a significant proportion of grass in their diets. Notably, during this period, only C3 grasses were likely to be widespread. Considering these results, we assert the existence of a mixed-environment at La Milloque, which probably ranged from light forest to more open areas composed of C3 grasses. This conclusion is in agreement with previous studies. At last, DeMiguel et al. (2010) suggested that dietary specialization on leaves or grass occurred independently in several lineages of ruminants and that mixed feeding may have been the primitive dietary state of this group. In order to check this hypothesis, it would be interesting in future works to take into account the other Oligocene ruminant species and to combine dental microwear and mesowear analyses.
Acknowledgements
We thank the ministère français de l’Éducation Nationale (UFR SFA, université de Poitiers), the ministère de la Recherche (projet ANR 09-BLAN-0238; CNRS: INEE and ECLIPSE). We are grateful to Géraldine Garcia for the access to the collections of Poitiers Université (CVCU) and Xavier Valentin (IPHEP, Université Poitiers) for his technical help. We thank the two anonymous reviewers for improving the quality of the manuscript.
List of fossil material including specimen number, type of tooth and facet position
Fossil speciesToothFacet positionNumber of specimensDremotherium guthiM2ProtoconidLM1966-MA14; LM1966-MA18; LM1966-MA82LM1966-MA89; LM1966-MA444; LM1968-MA15LM1968-MA19; LM1968-MA64; LM1968-MA70HypoconidLM1966-MA10; LM1966-MA66; LM1967-MA1M1 or M2 or M3ParaconeLM1966-MA1041; LM1966-MA1044; LM1966-MA1061MetaconeLM1966-MA1215; LM1966-MA1232; LM1969-MA194Dremotherium quercyiM2ProtoconidLM1966-MA1; LM1966-MA36; LM1966-MA460LM1968-MA3; LM1968-MA11; LM1968-MA24LM1968-MA33; LM1969-MA3; LM1969-MA4HypoconidLM1966-MA80; LM1966-MA456; LM1966-MA458M1 or M2 or M3ParaconeLM1966-MA1176; LM1966-MA1260; LM1966-MA1289MetaconeLM1966-MA280Bedenomeryx milloquensisM2ProtoconidLM1966-MA1; LM1966-MA2; LM1966-MA3LM1966-MA6
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Digitized photographs of the molar shearing facets from La Milloque ruminants. A. Dremotherium quercyi, LM1968-MA33. B. Dremotherium guthi, LM1968-MA64. C. Bedenomeryx milloquensis, LM1966-MA3. Scale bar = 100 μm.
Photographies numériques des facettes coupantes des secondes molaires des ruminants de La Milloque. A. Dremotherium quercyi, LM1968-MA33. B. Dremotherium guthi, LM1968-MA64. C. Bedenomeryx milloquensis, LM1966-MA3. Barre d’échelle = 100 μm.
Mean values of pit percentage (%p) for the extant ungulates (black) categorized following their feeding styles and extinct species from La Milloque (grey*).
Valeurs moyennes du pourcentage de ponctuations (%p) chez les Ongulés actuels (noir), suivant leur préférence alimentaire et chez les espèces fossiles de La Milloque (gris*).
Bivariate plot of the average number of pits versus average number of scratches for the extant ungulates categorized following their feeding styles and the ruminants from La Milloque. Standard deviations of mean Ns and Np are shown for each fossil species.
Représentation graphique bivariée du nombre moyen de ponctuations par rapport au nombre moyen de stries chez les Ongulés actuels, suivant leur préférence alimentaire et les ruminants de La Milloque. Les erreurs standard des Ns et Np moyens sont présentés pour chacune des espèces fossiles.
Summary statistics (mean [M] and standard error [sem]) of dental microwear variables for extant species from the “UNGULATES” database and for extinct species.
Résumé statistique (moyenne [M] et erreur standard [sem]) des variables de micro-usures pour les espèces actuelles de la base de données « UNGULATES » et les espèces fossiles.
Results on pairwise comparisons of extinct and extant species according to their diet categories. Significance at α < 0.05 indicated in normal font for Fisher's LSD tests in bold for both Tukey's HSD and Fisher's LSD tests.
Résultats des comparaisons entre espèces fossiles et espèces actuelles, d’après leurs catégories alimentaires. Significativité à α < 0,05 indiquée en caractères normaux pour le test LSD de Fisher, en caractères gras pour le test LSD de Fisher et le test HSD de Tukey.