Aznag (Ouarzazate basin, Morocco), a new African middle Eocene (Lutetian) vertebrate-bearing locality with selachians and mammals (7 fig., 1 tabl.)

• Autores: Rodolphe Tabuce, Sylvain Adnet, Henri Cappetta, Abdelmajid Noubhani, Frédéric Quillevere
• Localización: Bulletin de la Société Géologique de France, ISSN 0037-9409, Vol. 176, Nº. 4, 2005, págs. 381-400
• Idioma: inglés
• Títulos paralelos:
  • Aznag (bassin d'Ouarzazate, Maroc), nouvelle localité à sélaciens et mammifères de l'Eocène moyen (Lutétien) d'Afrique
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• Resumen
  • français

    Les dépôts paléogènes d'environnements margino-littoraux du bassin d'Ouarzazate (sud du Haut-Atlas central, Maroc) avaient livré diverses faunes à sélaciens et mammifères du Thanétien et de l'Yprésien. Nous décrivons ici la première découverte d'une faune du Lutétien. Le locus d'Aznag constitue le seul site de l'Eocène africain parfaitement calibré ; ceci grâce à la découverte de foraminifères planctoniques associés aux vertébrés. Le gisement est situé dans l'intervalle biozonal P11, dont l'équivalent biochronal est daté entre 45,8 et 43,6 Ma. La faune de sélaciens est riche avec près de 30 espèces, certaines étant probablement nouvelles. On note la présence, pour la première fois dans l'Eocène du Maroc, de taxons "septentrionaux" tels Protoginglymostoma, Hemiscyllium et Rhinobatos bruxelliensis. Il semble ainsi que des échanges fauniques Nord-Sud soient marqués au Lutétien, contrairement à la tendance Sud-Nord de l'Eocène inférieur. Malgré tout, ce gisement garde un cachet tropical avec la présence d'espèces ou de genres actuellement endémiques au Maroc ou plus généralement à la Téthys équatoriale-Atlantique central. Les dents de mammifères découvertes, bien que peu nombreuses et très fragmentaires, permettent d'identifier au moins 7 espèces. Parmi les "insectivores", un probable soricomorphe est très atypique par sa taille minuscule et par l'entoconide et l'hypoconulide totalement fondus sur la postcristide. Un chiroptère, ne pouvant être attribué à l'une des super-familles de microchiroptères connues, se caractérise par un ensemble de caractères à la fois dérivés (protocône peu cuspidé, hypo-cône présent) et primitifs, telle la présence d'un paraconule qui évoque le grade "éochiroptère" et les vespertilionoïdés primitifs (paléochiroptérygidés). Un petit primate bunodonte, caractérisé par un paraconide puissant, rappelle le tarsiiforme Altiatlasius du Thanétien du site voisin de l'Adrar Mgorn 1. Deux "condylarthres" évoquent des formes européennes ; une M3 est probablement à attribuer au genre Paschatherium et une DP3ou4 rappelle celle de Microhyus. D'un point de vue paléobiogéographique, ces mammifères confirment les échanges Europe-Afrique de l'Yprésien. Des données palynologiques antérieures ont suggéré la présence d'un arrièrepays relativement aride, puis de mangroves et marais salants sur la côte des golfes atlasiques ; de fait, les mammifères découverts à Aznag, lesquels sont inféodés à un environnement fermé, devaient être confinés à une mince ceinture de végétation côtière du type forêt galerie.

  • English

    Early Paleogene mammals are rare in Africa. They are mainly found in the northwestern part of the continent. Nearshore marine deposits from the Ouarzazate basin, in the southern rim of the Central High Atlas (Morocco), have yielded diverse Thanetian and Ypresian selachian and mammalian faunas [Cappetta et al., 1978, 1987; Gheerbrant et al., 1993]. These faunas from the Paleocene/Eocene transition document the oldest known placental mammals of Africa [Gheerbrant, 1992, 1994, 1995; Gheerbrant et al., 1998; Sigé et al., 1990; Sudre et al., 1993]. New field work in the central part of the basin, near El-Kelaâ M'Gouna, has led to the discovery of a new locality with continental and marine remains, including some non-determinable ostracods and charophytes plus planktonic and benthic foraminifera.

    The tests of the planktonic foraminifera were globally poorly preserved and highly re-crystallized. However, we found Globigerapsis index (dominant in the sample), Globigerapsis aff. kugleri, Subbotina frontosa, Subbotina inaequispira, Acarinina bullbrooki, Acarinina aff. pentacamerata, Morozovella aragonensis, Morozovella caucasica, Morozovella crater and Pseudohastigerina sp. Following Berggren et al. [1995], the lowest occurrence of the genus Globigerapsis denotes the P10/P11 zonal boundary and the highest occurrence of M. aragonensis denotes the P11/P12 zonal boundary. Both these taxa were represented by several relatively well-preserved specimens. Then, we consider that the sample is correlative to the planktonic foraminiferal Zone P11, for which a biochronal equivalent has been dated between 45.8 and 43.6 Ma. Consequently, the middle Lutetian Aznag locality represents the only known Eocene mammalian site to be adequately dated in Africa.

    The selachian fauna is relatively rich with nearly 30 species, many of them being probably new. The genera Squatiscyllium, Protoginglymostoma, Ouledia and Garabatis are confidently identified; these taxa were unknown in the Lutetian. We mention the first occurrence in the Eocene of Morocco of "septentrional" taxa Protoginglymostoma, Hemiscyllium and Rhinobatos bruxelliensis and we confirm the presence of genera Eomobula and Rhynchobatus. These data suggest significant North to South faunal exchanges, that contrast with the South to North tendency observed during the early Eocene [Noubhani and Cappetta, 1997]. Nevertheless, the selachian fauna from Aznag is clearly tropical with the occurrence of endemic taxa only known from Morocco (Garabatis, Orectolobiformes nov. gen.) and from the equatorial Tethys-Central Atlantic (Chiloscyllium aff. meraense, Ginglymostoma aff. angolense, Squatiscyllium, "Dasyatis" aff. sudrei, Ouledia, Arechia, Odontorhytis).

    Only 15 mammalian teeth have been recovered, most of them are very fragmentary and of small size; they document at least 7 species. A probable soricomorph "insectivore" documented by a complete lower molar is very atypical by its tiny size and the entoconid and hypoconulid poorly differentiated from the postcristid. Another "insectivore" is documented by a trigonid of p4, it is reminiscent to the zalambdodont groups. An incisor of rodent shows an enamel microstructure with uniserial Hunter-Schreger bands ; this structure is observed in several Eocene rodents from Eurasia and could be related to the pauciserial to uniserial transition observed in Zegdoumys sebtlai from Chambi (early Eocene, Tunisia) [Martin, 1999]. An upper molar of chiropteran cannot be assigned to one of the known microchiropteran superfamilies; the species from Aznag is characterized by derived traits (e.g. crestiform protocone, hypocone present) and primitive ones such as the presence of a paraconule which evokes the "eochiropteran" grade and the primitive vespertilionoids (paleochiropterygids). A small primate is documented by two fragmentary lower molars and characterized by a well-marked bunodonty and a large paraconid ; this form differs from contemporaneous anthropoids but is very similar to Altiatlasius from the Thanetian of the Adrar Mgorn 1. Two "condylarths" evoke European taxa; a M3 is tentatively assigned to the genus Paschatherium and a DP3or4 is reminiscent to that of Microhyus. This specimen shows also affinities with the ?DP4 of Chambius, a primitive macroscelidid from Chambi. The presence of these mammals strengthen the hypothesis of faunal exchanges between Africa and Europe during the early Eocene.

    During the middle Eocene, in the south of the Central High Atlas, palynologic data suggested a fairly dry, scarcely vegetated hinterland, and a tropical coastal vegetation (mangrove swamp and salt-marsh) [Fechner, 1988]. The mammals of Aznag, which are characteristic of a closed environment (chiropterans, primate, small ungulates), were certainly confined to a thin belt of coastal vegetation (forest gallery). The fossiliferous level is characterized by a cyclicity of the deposits which could suggest a seasonal variation. Among the selacians, the abundance of the batoids, the absence of both macrofauna and deep-water selacians, and the fragmentary elements indicate hydrodynamical transport in a shallow channel or delta.