The stem osteichthyan Andreolepis and the origin of tooth replacement

The teeth of gnathostomes (jawed vertebrates) show rigidly patterned, unidirectional replacement that may or may not be associated with a shedding mechanism. These mechanisms, which are critical for the maintenance of the dentition, are incongruently distributed among extant gnathostomes. Although a permanent tooth-generating dental lamina is present in all chondrichthyans, many tetrapods and some teleosts, it is absent in the non-teleost actinopterygians. Tooth-shedding by basal hard tissue resorption occurs in most osteichthyans (including tetrapods) but not in chondrichthyans. Here we report a three-dimensional virtual dissection of the dentition of a 424-million-year-old stem osteichthyan, Andreolepis hedei , using propagation phase-contrast synchrotron microtomography, with a reconstruction of its growth history. Andreolepis , close to the common ancestor of all extant osteichthyans, shed its teeth by basal resorption but probably lacked a permanent dental lamina. This is the earliest documented instance of resorptive tooth shedding and may represent the primitive osteichthyan mode of tooth replacement. The extinct Andreolepis , an early fish that is close to the common ancestor of all bony fish and land vertebrates, shed its teeth by basal resportion—the earliest example of this mode of tooth replacement. New teeth for old When a small child sheds a milk tooth, the part that is shed is the crown, and the roots are resorbed. This seems to be the primitive pattern for all bony fishes (of which we constitute a highly derived offshoot), according to Per Ahlberg and colleagues, who have been examining tooth replacement in the fossil fish Andreolepis hedei which, at 424 million years old, is close to the ancestry of all bony fish. Andreolepis shed its teeth by basal resorption, the earliest documented instance of this phenomenon, which may represent the primitive osteichthyan mode of tooth replacement.