Decoupled and diverse: A morphological examination of the two-jaw system in Mormyridae (Osteoglossiformes)

The tongue-bite apparatus (TBA) is a secondary jaw system found in Osteoglossomorpha and, convergently, Salmonidae. It is thought to have similar prey-processing functions as the pharyngeal jaws, possibly allowing the oral jaws to evolve separately for more efficient prey capture. The TBA is incredibly morphologically diverse, but this diversity has not been well documented across Mormyridae, the most speciose family within Osteoglossomorpha. In this study, we examined shape diversity, levels of evolutionary integration and modularity, and rates of evolution of both the TBA and other associated components of the skull in Mormyridae, both across the family and in its two subfamilies, Petrocephalinae and Mormyrinae. Petrocephalinae, the sister group to the rest of Mormyrinae, is less species rich and has evolved independently from Mormyrinae for approximately 50 million years. This has resulted in disparity across the two groups, not only morphologically but also in terms of electric organ discharges. Results of this study support different models of modularity of the skull for these two subfamilies, with Petrocephalines having two modules as compared to Mormyrine's four modules. In both groups, the TBA makes up its own module, often evolving faster than other components of the skull. We propose that this is an example of a truly decoupled two-jaw system, making it unique compared to the pharyngeal and oral jaws of other well-studied groups, such as cichlids and labrids.