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The neurocranium of a swordfish

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 :
- 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..
- 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.
- 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.
- 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.