SUPERFICIAL ORNAMENTATION AND STRUCTURE O F THE OSSEOUS DERMAL PLATESIN SOME ARMORED SILURIFORMS (LORICARIIDAE, CALLICHTHYIDAE, DORADIDAE)

The present study is devoted to the superficial ornamentation, the str ucture and the mineralization of the postcranial osseous dermal plates (scutes) in various species representative of three families of armou red Siluriformes (Callichthyidae, Loricariidae and Doradidae). In Call ichthyidae and Loricariidae, the scute surface shows numerous odontode s but they lack in Doradidae the scutes of which are characterized by a large central osseous spine. The odontodes are conical true teeth ; they have various species specific shapes but they show the same organ ization : dentin, covered by an hypermineralized substance (enamel or enameloid), surrounding a central pulpar cavity which communicates wit h the vascular network of the scute. The odontodes are always attached to the scute surface by a ligament. The base of the latter remains ea sily visible after the fall of the odontodes and looks like a crown (p edestal) the center of which is occupied by the opening of a vascular canal. The scute surface, examined with the scanning electron microsco pe, is ornamentated with small anchoring bundles. In toto micrographs show that the scutes always have a well-developed vacular network whic h is the largest in the loricariid Liposarcus disjunctivus. Althouth t he scutes have various shapes their structure is relatively homogeneou s when examined on microradiographs of ground sections and on semi-thi n sections. A scute is mainly constituted by an osseous cellular plate composed of woven bone in the central region and surrounded by parall el-fibered bone. This plate is covered by a layer of hyaloine, more or less developed following the species. Hyaloine is a well-mineralized tissue which is not bone. Its thin organic matrix is striated by lines of arrested growth. The structure of the scutes is compared to that o f the ganoid and elasmoid scales and the evolutionary problems raised by the existence of the scutes are discussed.