The specimen was found among the undescribed material of the palaeoichthyological collection of the ‘Laboratoire de paléontologie’ (‘Département Histoire de la Terre’) of the ‘Muséum national d’histoire naturelle’ (MNHN), Paris. It consists of a largely incomplete skeleton preserved on laminated diatomitic marls. The fossil was collected during the early years of the 20th century from the diatomaceous deposits of the Chelif Basin, northwestern Algeria. Unfortunately, there is no indication about the exact locality of provenance of the specimen. Diatomaceous sedimentation extensively occurred in the Chelif Basin during the Messinian (Late Miocene) in response to enduring upwelling phenomena [32] and [38], and the resulting deposits represent the basinal relatives of carbonate platforms [34] and [39]. Large-scale correlations proposed by Cornée et al. [21] suggested that in the western Mediterranean, such a diatomaceous sedimentation occurred between 6.7 and 5.95 Ma.

Subdivision Teleostei sensu Patterson & Rosen, 1977.

Order Tetraodontiformes sensu Santini & Tyler, 2003.

Family Molidae sensu Santini & Tyler, 2002.

Genus Ranzania Nardo, 1840.

Ranzania sp. (Fig. 1).

Due to their highly unusual and bizarre morphology, fishes of the family Molidae have long fascinated naturalists and ichthyologists (see [43]). Since the first half of the 19th century, several authors (e.g., [7], [20], [22], [24], [25], [26], [28], [29], [36], [37], [45] and [53]) have investigated various aspects of their peculiar anatomy and life histories, providing a large amount of data about their structure and biology. Santini and Tyler [40] were the first to provide a cladistic study of interrelationships among living taxa belonging to this family. According to their conclusions, the monophyly of the Molidae is supported by 31 synapomorphies, which refer to skeletal features clearly not observable on MNHN ORA1777 because of its evident incompleteness. However, despite its largely fragmentary nature, the material documented herein can be referred to family Molidae, based on the relative dimensions and morphology of the bones (Fig. 1 and Fig. 2C). Within the family Molidae, the well-ossified nature of the bones, as well as their elongate slender morphology and their arrangement in clusters are clearly indicative of the genus Ranzania. The degree of ossification of the skeleton of the ocean sunfishes is considered as a salient distinctive feature at generic level [5], [23], [36], [40], [44] and [45]. Ranzania is characterized by dense, moderately spongy (sub-osseous [23]) texture of the bones, whereas Masturus and Mola possess soft, weakly ossified bones with a highly spongy structure. As far as the morphology of the bones is concerned, the general appearance of the anal pterygiophores described herein is consistent with that characteristic of the genus Ranzania. As reported by Tyler [45] and Santini and Tyler [40], the dorsal- and anal-fin pterygiophores of Masturus and Mola are widely expanded anteroposteriorly, plate-like (Fig. 2A and B), while in Ranzania they are long, slender rods (see Fig. 2C). Moreover, because of their anteroposterior enlargement, the haemal pterygiophores of Masturus and Mola are broadly in contact among each other. On the contrary, the clusters of long and slender elements that characterize Ranzania are separated by a wide gap, as in the Algerian fossil documented herein (Fig. 2C).

The genus Ranzania consists of one living species, R. laevis, and at least four extinct species, R. grahami, R. ogaii, R. tenneyorum, and R. zappai. Ranzania grahami was established based on the premaxillary and dentary beaks, nasal plate, jugular plate, branchial fragments, and part of the armour shield from the lower Middle Miocene Calvert Formation, Virginia [52]. Ranzania ogaii was described by Uyeno and Sakamoto [48] based on an incomplete articulated skeleton from the Middle Miocene Hiranita Formation, Japan. Ranzania tenneyorum is based solely on a premaxillary beak from the lower Middle Miocene basal phosphate horizon of the Calvert Formation, Virginia [52]. Finally, R. zappai was described by Carnevale [12] based on articulated dermal scale plates from the Middle Miocene of Torricella Peligna, central Italy. Therefore, the material described herein can be compared only with R. laevis and R. ogaii in order to interpret its possible systematic affinities. Unfortunately, the inadequate preservation of MNHN ORA1777 and the apparent absence of morphological characters in the anal-fin pterygiophores that are useful for discrimination at the specific level (see [40]) do not allow a more detailed taxonomic analysis. The fossil is therefore left unnamed, awaiting more complete material. In any case, much new comparative information is necessary before the fossil described herein can be definitely assigned at the specific level.

Like the other members of the family Molidae, Ranzania is a primarily pelagic fish that mostly inhabits the upper layers of the water column. Due to the loss of the caudal fin, the major locomotor thrust is furnished in these fishes by the greatly enlarged and powerful dorsal and anal fins (see [6]). The reduced capacity to active swimming is reflected by their alimentary attitudes, given that these fishes feed primarily on jellyfishes, but also on algae, seaweed, eelgrass, hydroids, ctenophores, molluscs, crustaceans, brittlestars, salps, and fish larvae (e.g., [33] and [52]). The occurrence of a slender mola in the Messinian deposits of the Chelif Basin provides new data about the structure of the Upper Miocene pelagic communities of this semi-enclosed enclave (see [38]) of the western sector of the Mediterranean. The fish communities of the pelagic realm of this basin were characterized by a broad palaeobiodiversity during the Messinian. A large number of large predatory sharks (e.g., Carcharhinus, Carcharias, Carcharodon, Hexanchus, Isurus, Sphyrna) and typically offshore teleost taxa of the families Bregmacerotidae (Bregmaceros), Carangidae (e.g., Caranx, Decapterus, Seriola, Trachurus), Clupeidae (e.g., Alosa, Brevoortia, Etrumeus, Spratelloides), Diodontidae (Diodon), Gempylidae (Chelifichthys, Hemithyrsites), Istiophoridae (Makaira), Myctophidae (e.g., Ceratoscopelus, Lampanyctus), Scomberesocidae (Scomberesox), Scombridae (Euthynnus, Sarda, Scomber), Syngnathidae [Syngnathus albyi; see 12], Tetraodontidae (Archaeotetraodon), Trichiuridae (Lepidopus), and Zeidae (Zeus) have been described from the main fossiliferous localities of this basin (e.g., [1]). Such amazing diversity was probably related to the heterogeneous palaeogeography and physiography that characterized the shelf environments of the Chelif Basin, and to the nutrient enrichment and eutrophication during biosiliceous deposition. In this setting, the large availability of prey in response to the water fertilization related to the enduring upwelling phenomena probably attracted a conspicuous and diverse assemblage of pelagic fishes to the basin, thereby producing a miscellaneous contingent of coastal pelagic, epipelagic, and migratory meso- and bathypelagic fishes.

As pointed out by Tyler and Santini [47], because of their pelagic lifestyle and the weakly ossified and spongy nature of the skeleton (at least in the genera Masturus and Mola), representatives of the family Molidae are rather rare in the fossil record and mostly represented by isolated jaws and dermal scale plates. The earliest molid, Eomola bimaxillaria, has been described from the Bartonian (∼41 Myr) marls of the Kuma Horizon, North Caucaus, Russia [46]. The material consists of premaxillary beaks and other isolated bones of the upper jaw, suspensorium, and opercular series. This Eocene fossil is characterized by separate premaxillae that articulate medially by means of interdigitating processes (versus fused in the midline in extant molids). According to Tyler and Bannikov [46], such a separation represents the retention of the plesiomorphic condition typical of other tetraodontoid families, such as the Triodontidae and Tetraodontidae. The Neogene record of the molids is characterized by the repetitive occurrence of isolated jaws, often associated with shark teeth or marine tetrapod remains. Upper and lower beaks, isolated bones and dermal scale plates assigned to the Molidae (Mola and Ranzania) have been reported from several Neogene localities from Belgium, France, the Netherlands, and the United States [27], [30], [31], [35], [49], [50], [51] and [52]. A single largely incomplete articulated skeleton assigned to the genus Ranzania was described by Uyeno and Sakamoto [48] from the Middle Miocene Hiranita Formation, Chibumachi Group, Saitama Prefecture, Japan. Finally, an incomplete articulated dermal armour shield from the Serravallian (Middle Miocene) laminated calciturbidites outcropping near Torricella Peligna, Abruzzo, central Italy, has been recently presented by Carnevale [12] and referred to the genus Ranzania. Thus, it is evident that the meagre fossil record of the Molidae poorly contributes to the understanding of the evolutionary patterns of this extremely derived group of fishes. For this reason, the fossil remains documented herein, even if strongly incomplete, provide precious information about the palaeodistributional history of this family, furnishing a further support to the idea that the Miocene ichthyofaunas from the Chelif Basin are of crucial importance for the fossil record of the Teleostei (see [11]).