As we’ve seen
over the past several dozen fish additions to the large reptile tree (LRT, 1583 taxa) facial bones homologous with those of tetrapods often divide and fuse. We’ve already seen multipart jugals, lacrimals, nasals and squamosals in fish. This can be confusing and is probably the reason why fish facial bones are traditionally not labeled with tetrapod nomenclature. Expect homology arguments to last for decades, but here all fish facial bones are colored with tetrapod homologs.
The one bone that first appears by a split
of the maxilla into anterior toothy and posterior toothless portions is the quadratojugal. Phylogenetically the quadratojugal first appears on Gogonasus (Fig. 1). Prior taxa lack it. Later taxa have it. Even so, until the cladogram got figured out, this was puzzling.
Gogonasus andrewsae (Long 1985, Long et al. 1997; Late Devonian, 380 mya; NMV P221807; 30-40cm in length) is the best preserved specimen of its type. This is the crossopterygian transitional between rhizodontids, coelocanths and higher tetrapodomorphs. The maxilla splits in two creating the tetrapod quadratojugal. The squamosal splits in two creating the tetrapodomorph preopercular. A pineal opening appears, so does the tetrapodomorph choana. The lacrimal contacts the external naris. This is the crossopterygian that gave rise to tetrapodomorphs and tetrapods, rhizodontids and gulper eels.
Long JA 1985. A new osteolepidid fish from the Upper Devonian Gogo Formation of Western Australia, Recs. Western Australia Mueum 12: 361–377.
Long JA et al. 1997. Osteology and functional morphology of the osteolepiform fish Gogonasus Long, 1985, from the Upper Devonian Gogo Formation, Western Australia. Recs. W. A. Mus. Suppl. 57, 1–89.