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L’odontologie de Gomphotherium angustidens (Cuvier, 1817) (Proboscidea, Mammalia) : données issues du gisement d’En Péjouan (Miocène moyen du Gers, France)

Pascal TASSY

fr Geodiversitas 36 (1) - Pages 35-115

Published on 28 March 2014

Odontology of Gomphotherium angustidens (Cuvier, 1817) (Proboscidea, Mammalia): data from the Middle Miocene locality of En Péjouan (Gers, France)

The dental anatomy of Gomphotherium angustidens (Cuvier, 1817), the commonest proboscidean in the faunal lists of the Middle Miocene of Europe, is detailed on the basis of the numerous fossils found at En Péjouan in the vicinity of Simorre (Gers, France), a locality of Astaracian age (mammal zone MN7 or MN7/8). The examination of the morphologic and biometric variation of tusks, premolars and molars, based on an unprecedented number of specimens of one species in one locality, yields several results. The first is a better understanding of the evolutionary level of G. angustidens during the Astaracian. A second is a better knowledge of dental characters and their variation. The presence or absence of both upper and lower incisors (tusks), due to sexual dimorphism, is documented. In females, when they exist, upper tusks are much reduced. The relation between shape and growth stages of evergrowing tusks is illustrated in relation to a dental-age scale based on crania and mandibles with different premolars and/or molars and different wear stages. Variation of the orientation of the enamel band of upper tusks and of the piriform transverse sections of lower tusks is directly observed based on numerous specimens. The upper tusks are slightly curved outwards; they are not much curved ventrally although they display a twist even on rather straight tusks: in lateral view they are slightly concave dorsally at the premaxilla, then rather straight, and finally ventrally curved at the tip. The fabrication of enamel is stopped on male upper tusks at the dental ages XVII-XVIII (that is, with worn M2/m2 and first loph [id] of M3/m3 worn). No lower tusk shows circular or flat transverse sections, even if the longitudinal sulcus (on both dorsal and ventral sides) is more or less marked; the dorsal wear facet is strong but short. The bunolophodont pattern of decidual premolars, premolars and molars is extremely variable. The range of this variation is understood in details through several parameters: 1) the subdivision of lophs(ids); 2) the shape of central conules and the resulting trefoiled wear figure; 3) the degree of subdivision of the central conules; 4) the posttrite conules; 5) the labial cristae of upper molars; 6) the development of the cingulum; 7) the degree of development of the fourth loph of M3s and of the fifth lophid of m3; and 8) the amount of cement. It appears that: 1) the binary subdivision of lophs(ids) is the rule, posttrite half lophs with three cusps are rare and variable on right and left sides; on upper molars, the mesoconelet is often fused with the anterior central conule; 2) the pretrite trefoiled wear figure of upper molars shows an enlargement of the posterior central conule compared to the anterior, and this asymetry is seen even on worn teeth – this trait seems to be significant at the Astaracian, especially for M2 and M3; 3) pretrite central conules are subdivided (“serridentine” pattern), or not, and variation can be seen of right and left molars of same individuals; 4) posttrite conules are weak or absent; 5) the labial cristae of upper molars (mainly postparacrista and premetacrista) have the shape of smooth ridges and are never strongly developed; 6) the cingulum is mostly weak on the pretrite side, the development of the postingulum is extremely variable, as that of the last loph(id); 7) all M3s are tetralophodont, even the teeth which bear weak fourth loph and only one postcingulum cusp show an entoflexus between third and fourth lophs; m3s have four or five lophids ; and 8) the cement is usually present in the interlophs (ids) and around the cusps. The biometric variation of molars, especially M3s and m3s, is compatible with the explanation of sexual dimorphism. As a consequence, the variation of the dental traits described at En Péjouan brings a rationale for understanding any interspecific variation among the Elephantida. The study of wear facets of associated upper and lower premolars and molars lead to the description of the mode of occlusion in respect to horizontal tooth displacement and anatomical characters of the skull (cranium and mandible). The occlusion depicted for G. angustidens is probably valid for any species that belongs to the trilophodont grade of Elephantida and even, based on comparisons with the mammutid Zygolophodon turicensis (Schinz, 1824), of Elephantimorpha. Occlusal movements are mainly due to an upward component, as for deinotheres, with a secondary component, transversal (inward). The latter component is illustrated by the angle made by the longitudinal axis of occluding molars (= shear angle of elephantine molars). Yet, the extension of the pendulous movement (both vertical and postero-anterior) of the mandible toward the maxilla is limited in G. angustidens compared to elephants due to the different orientation of temporalis, digastricus and masseter muscles. The odontology of the “population” of G. angustidens at En Péjouan, and comparisons with Miocene European taxa, bring some reliable taxonomic statements. No subtapiroid or annectens-like morphotypes (rather frequent in MN4-MN5 zones) is seen at En Péjouan. Molar and incisor characteristics of the contemporaneous species G. steinheimense are also absent in the variation described at En Péjouan. From the viewpoint of dental morphology, G. angustidens is closer to Archaeobelodon filholi (Frick, 1933). Yet, an association of characters seen only in G. angustidens is made of the double twist of upper tusks and enamel band, the piriform transverse section of lower tusks, the enlargement of the posterior central conules with resulting asymmetrical trefoil and reduced posttrite conules of upper molars, the alternate pattern of the lophs(ids) of dP3/dp3. As a consequence, derived traits seen on the dentition of Gomphotherium angustidens in Astaracian localities lead to the conclusion that most molars of bunolophodont trilophodont grade of Orleanian localities rather belong to Gomphotherium subtapiroideum (Schlesinger, 1917). Identification of isolated teeth, especially lower molars, remains sometimes tentative if not hazardous.

 

Keywords :

Mammalia, Proboscidea, gomphotheres, anatomy, teeth, Simorre, Miocene.

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