The development of the tooth pattern and dentigerous bones in Polypterus senegalus (Cladistia, Actinopterygii).

The formation sequence of the tooth-bearing bones and the tooth pattern in early ontogeny of Polypterus senegalus is investigated using transparent preparation, histological sections, and SEM. During the attachment step of the yolk-sac larva the first dermal bones and teeth are formed. Teeth appear simultaneously in the areas of the maxillary, dentary, dermopalatine, prearticular, and coronoid 1 along with the first separate anlagen of these bones. A monostichous arrangement of primary teeth is established on the maxillary, dentary, and dermopalatine. Polystichous tooth arrangements do not occur before the early pterolarval phase, and then only in connection with bones of the palate and inner dental arcades. Especially pronounced is the influence of tooth formation on the structure of the parasphenoid that becomes much thickened by accretion of denticulate platelets, but we found neither evidence for a distinct vomeral contribution to the parasphenoid, nor a composite origin of the ectopterygoid in ontogeny. First replacement teeth are found in association with the maxillary and dentary as early as the late apterolarval phase. Primary teeth are of a single general type, whereas from the pterolarval phase onward three tooth types can be distinguished that are restricted to certain tooth bearing bones. Relatively late in ontogeny, dermo-metapterygoid and entopterygoid become formed and colonised by teeth, whereas first branchial teeth and tooth plates appear earlier during the first phase of extrinsic larval feeding. Characteristics of development of the dentition are discussed in comparison with character states of other better known fossil and recent taxa among Actinopterygii and Sarcopterygii. Compared to the assumed basic pattern of actinopterygian fishes, Polypteriformes show a derived condition with respect to structure, arrangement, replacement, and differentiation of teeth, which arises in sequence during larval development. This also corresponds to observed changes of feeding behaviour and functional demands during larval life.