What shapes the oral jaws? Accommodation of complex dentition correlates with premaxillary but not mandibular shape

Teeth are integrated into the vertebrate oral jaws to provide a functional unit for feeding, however little is known about how this integration occurs during growth and development. The purpose of this study is to identify the ontogenetic changes in oral jaw shape that are associated with the transition of the oral dentition from unicuspid teeth to multicuspid teeth. Here, we compare the shape of the occluding upper (premaxilla) and lower (mandible) jaws of the toothed Mexican tetra (Astyanax mexicanus) and the toothless (oral teeth present, pharyngeal teeth absent) zebrafish (Danio rerio) over development. Gross morphology combined with morphometric analyses were used to analyse shape changes of the occluding oral jaws in each species. Histological analyses were also used to examine the development of the mandibular symphysis.

The occluding edge of the premaxilla is the first region to ossify in the Mexican tetra, but the last to ossify in zebrafish. Morphometric analyses revealed that the early shape of the premaxillae (in fish younger than 8mm SL) is the same in each species but that the premaxilla shape changes significantly at larger sizes. These changes are apparent in the tooth bearing region of the Mexican tetra. The rostral region of the mandible also houses teeth, however ossification and shape in this region were surprisingly similar between species despite differences in the presence of oral dentition. Furthermore, we found that the mandibular symphysis of the Mexican tetra is composed of interdigitating bone, while the symphyseal region of the zebrafish is composed of fibrous connective tissue.

These differences in the jaw skeleton have likely evolved due to different feeding strategies utilised by each species. Our results show that premaxillae shape correlates strongly with the development of complex dentitions unlike in the mandible. This study provides important insights into the relationship between jaw and tooth development in bony fishes and suggests that these mechanisms may be similar amongst vertebrates.