The neurocranium of a swordfish

The examination of the bones of a tuna or a common mackerel are good exercices to begin to study the osteology of the skull of acanthomorphs. Nevertheless, the neurocranium of many species within this group presents stunning pecularities around this general "scheme". The neurocranium of the swordfish -  Xiphias gladius  (Xiphiidae) - is one of these.

Dorsal view

Ventral view

Left lateral view

Occipital view

In Gregory & Conrad (1937)

Legends have been modified ; original figures can be consulted at this address. 

Dorsal view

Left lateral view

Detail of the left lateral view

The skull of a swordfish shows the same organisation as the skull of a tuna, but  with several modifications :

1. Some cephalic bones (nasals) and especially several bones of  the splanchnocranium  (premaxilla, maxilla) are not free as in the tuna or the mackerel, but are closely associated and fixed to the bones of the neurocranium. It is a case of reversion : a free and mobile premaxilla , not fixed to the cranium, is an innovation of teleostean fishes. But, in swordfish (Xiphiidae) and sailfishes  (Istiophoridae), this character-state is lost by the robust insertion of the premaxillae to the anteriormost bones of the neurocranium..
    
2. Similarly, the premaxilla of acanthomorphs possesses normaly a developped ascendant process ascendant  associated to a large important rostral cartilage and two independant deux ligaments connecting palatines to vomer. These character-states are absent in sworfish and sailfishes, once again there is  here reversion of character-states.
3. Basisphenoid is here only and partially ossified; this bone is present dorsally under the skull roof, but is reduced to fine and half ossified lamella of conjonctive tissue. 
4. Moreover, the two premaxillae are toothless and associated to form an elongated bill. This one is depressed, blade-like, in xiphiids while the bill istiophorids has a rounded section.

One of the consequences of this anterior extension of the premaxillae is that the vomer — the anteriormost bone in the neurocranium of the tuna or the mackerel — is in median region on the skull of the swordfish (see ventral view of the here presented skull of swordfish) . 

Sworfish was used to be classified with tunas in the scombroid group. Recent analyses, partially conducted by members of the team (Li et al., 2009 [1]), questioned this assemblage and proposed that xiphiids and istiophorids were closely related to dolphinfishes (Coryphaenidae), to jacks and pompanos  (Carangidae), to archerfishes   (Toxotidae) and flatfishes  (pleuronectiformes) within the Caranginomorpha.

References

Gregory, W.K. & G.M. Conrad (1937). The comparative anatomy of the swordfish (Xiphias) and the sailfish (Istiophorus). The American Museum Novitates, 952:1-25.