Une espèce nouvelle du genre Palaeocarassius Obrhelová dans le Miocène supérieur lacustre d'Alissas (Ardèche)

Abstract

The occurrence of fossil fishes in the diatomites outcropping at Le Ranc, in the vicinity of Rochessauve castle (fig. 1) was first reported by Faujas Saint-Fond (1803) in his Essai de Géologie. One Century later, Priem (1912) briefly described, and figured as Cobitis sp., a fish from Rochessauve that is redescribed in the present paper. Finally, Grangeon (1958) noted the occurrence of the genus Cyprinus in the diatomites from Combier. According to the mammal remains found in quarries open near Combier and La Grangette, the age of the fossiliferous diatomite that was worked corresponds to the upper part of the MN 11 mammal zone, or may be slightly younger (Demarcq et al., 1989). Radiometric ages obtained from basalts give a fork between 8.0 ± 0.65 Ma and 6.4 ± 0.21 Ma for this upper diatomite (fig. 2). The reexamination of the fish studied by Priem (1912) has shown that it does not really belong to the Cobitids, as demonstrated by its less elongate body and its higher number of postabdominal vertebrae (17 instead of 12-14 in the Middle Miocene species Cobitis centrochir Agassiz). On the contrary, the composition of its vertebral column and its number of dorsal fin-rays are similar to those characterizing a Cyprinid found in a nearby locality: "Leuciscus" demasi Civet, from the Upper Miocene of Montagne d'Andance, Saint-Bauzile (Ardèche). For this reason, the fish from Rochessauve is provisionally named "Leuciscus" cf. demasi Civet. Three still undescribed fishes from the same area are also housed in Paris at the Palaeontological Institute of the National Museum of Natural History. They are preserved in a grey-reddish diatomite. These fishes are characterized by their rather massive body, the maximum height of which equals 1/3 to 2/5 of standard length, and by their rather large head, the length of which is approximately 1/3 of standard length. The vertebral column is made up of 30 or 31 vertebrae, 13 of which are postabdominal. The caudal fin, rather large, is moderately forked; it is made up of 19 principal fin-rays, 17 of which are forked. The dorsal fin includes three short anterior fin-rays, one ossified ray having a notched posterior edge and 12 to 14 fin-rays both articulated and forked. Like the dorsal fin, the anal fin begins begins with three short anterior fin-rays whereas, behind the ossified ray, similar to that of the dorsal fin, it has only five fin-rays. About 15 rays are present in the pectoral fin and one can distinguish 7 finrays in the pelvic fin. The body is covered with cycloid scales ornamented by concentric circuli. Radii are present both on the anterior and posterior fields. From the preceding description, it appears that these three fishes undoubtedly belong to the genus Palaeocarassius Obrhelová, as shown by the shape of their operculum, their rather short vertebral column, the occurrence of notched ossified rays in both the dorsal and anal fins, and their relatively high number o dorsal fin-rays. The genus Palaeocarassius Obrhelová is mainly based on a disarticulated head and a distorted body part, so that its type-species P. mydlovarensis Obrhelová is a nominal species which does not allow any reliable taxonomical comparison with other congeneric materials. Otherwise, the general body shape of the fishe found in the vicinity of Alissas is similar to that of P. priscus (von Meyer), from the Lower Miocene (Ottnangian) of Unterkirchberg near Ulm (Wurtemberg, Germany), and differs significantly from that of P. obesus Gaudant, from the Middle Miocene of Sansan (Gers, France), which is more stocky. However, the specimens found near Alissas have more fin-rays in the dorsal-fin than P. priscus (12-14 instead of a number not exceeding 10). Consequently, the Palaeocarassius studied in the present paper should be considered as being the representatives of a new species named P. basalticus nov. sp. It may be defined as: "Palaeocarassius having a moderately stocky body, the maximum height of which does not exceed 2/5 of standard length. Rather large dorsal fin with 12 to 14 fin-rays, both articulated and forked, occurring behind the ossified ray". Holotype: Specimen M.N.H.N.P-PTF 2, housed in Paris at the Palaeontological Institute of the National Museum of Natural History. Type locality: Upper diatomites, Combier, near Alissas (Ardèche, France). Age: Lower Turolian (upper part of the MN 11 mammal zone) or Middle Turolian (lower part of the MN 12 mammal zone).