Obaichthys africanus

Meunier, François J., Eustache, René-Paul, Dutheil, Didier & Cavin, Lionel, 2016, Histology of ganoid scales from the early Late Cretaceous of the Kem Kem beds, SE Morocco: systematic and evolutionary implications, Cybium 40 (2), pp. 121-132 : 128

Obaichthys africanus

Dentine. – The organisation of the scales with three layers (ganoine, dentine, bony basal plate) is typical of the palaeoniscoid type as defined by Goodrich (1907). This author also defined the lepidosteoid type on the basis of absence of dentine, which is the situation present in all extant Lepisosteidae . Although Obaichthys africanus is clearly a lepisosteiform on the basis of osteological characters, the thick dentine layer in its scales does not fit with the definition of the lepidosteoid type, which is normally present in this group. Dentine has also been observed in the scales of South American extinct obaichthyids (Brito et al., 2000). Nevertheless, in O. africanus , the dentine units are more developed than in the scales of O. decoratus and Dentilepisosteus laevis (Brito et al., 2000) . On the basis of this feature, O. africanus appears more primitive than O. decoratus and of Dentilepisosteus laevis , which are, however, less derived than the extant Lepisosteidae Lepisosteus and Atractosteus , which both possess typical lepidosteoid scales.

Odontodes in the ganoine layer. – The first descriptions of lepidosteoid scales already mentioned the presence of odontodes fixed on the external ganoine layer ( Williamson, 1849; Nickerson, 1893). This observation was confirmed on the scales of Lepisosteus oculatus (Sire, 1994) . In aspidorhynchid scales, odontodes are also present on the ganoine surface (Brito and Meunier, 2000) as well as in polypterid scales ( Hertwig, 1879; Géraudie, 1988), although in the latter they are often temporary. Our study shows that the scales of O. africanus had odontodes at their ganoine surface. These superficial odontodes were partly resorbed before the deposit of a new stratum of ganoine that covers their base. Contrary to O. africanus , however, the scales of O. decoratus and Dentilepisosteus laevis do not show odontodes imbedded within the ganoine (Brito et al., 2000).