The origin of the vertebrate jaw: neoclassical ideas versus newer, development-based ideas.

Here I consider ways to test two hypotheses of the origin of jawed vertebrates: my neoclassical hypothesis, which derived from comparative morphology; and the heterotopic hypothesis, which derives from modern developmental findings. The heterotopic hypothesis, unlike the neoclassical hypothesis, says that major developmental changes had to occur before the upper jaws could evolve: a caudal shift in the expression domains of genes that pattern oral structures; and the loss of ancestral, lamprey-like upper lips. To test whether these did occur, I propose studies on the development of chondrichthyans (sharks and chimaeroid ratfishes), an understudied group that is likely to retain primitive features of the jaw region. The heterotopic hypothesis says no gnathostome retains the upper lip that is so prominent in larval lampreys, yet the neoclassical hypothesis identified such lips in sharks and ratfishes, and it predicts that their lip-skeleton likewise develops from premandibular neural crest. The development and innervation of upper-lip muscles in lampreys and chimaeroids can also be compared. The proposed studies can determine if the upper lips of chondrichthyans and lampreys are homologous (which would support the neoclassical hypothesis), or homoplasious (which would support the heterotopic hypothesis). Also, I argue that the evolution of the upper jaws (as lateral structures) was not linked to a nasohypophyseal complex (which is a median structure), although such a link is claimed by the heterotopic hypothesis. Finally, I update the neoclassical hypothesis to address recent evidence that the trabeculae of the lamprey skull are not homologous to the trabeculae cranii of gnathostomes.