is the extant swordfish, a large, fast, open seas predator.
The swordfish lacks facial bones
the barracuda retains as vestiges rimming the orbit. That’s why it does not nest with the barracuda, but between the barracuda and the clade of halibut + electric eel, which also lacks those post-orbital bones… given the present paltry number of fish presently tested by the large reptile tree (LRT, 1490 taxa, subset Fig. 6).
Even tiny post-hatchling swordfish
(Fig. 3) have a large sword-like rostrum… and a matching mandible that shortens relative to the rostrum with maturity.
The swordfish vertebral column
(Figs. 1, 4) has far fewer vertebrae than in deep ancestor taxa, like Amia. Eocene swordfish ancestors, like Blochius (Fig. 1), already have fewer, but more elongate vertebrae and also lack pelvic fins.
Xiphias gladius (Linneaus 1758; Gregory and Conrad 1937; up to 4.5m in length) is the extant swordfish, derived from a sister to the barracuda, Sphyraena. 1cm long hatchlings more closely resemble little barracudas,,, then they look like little sailfish… then they reduce the long dorsal fin, keeping the portion just posterior to the skull.
The sword-like rostrum
is not used to spear, but to slice and maim smaller fish traveling in dense schools. Larval swordfish feed on zooplankton including other fish larvae. Juvenile swordfish eat squid, fishes, and pelagic crustaceans.
Egg production in a phylogenetic and size context:
Small Amia females produce 2000–5000 eggs.
Larger Sphyaena females produce 5000–30,000 eggs.
Even larger Xiphia females produce up to 29 million eggs.
An oil producing gland adds to speed.
Videler et al. 2016 report, “the discovery of a complex organ consisting of an oil-producing gland connected to capillaries that communicate with oil-excreting pores in the skin of the head. The capillary vessels transport oil to abundant tiny circular pores that are surrounded by denticles. The oil is distributed from the pores over the front part of the head. The oil inside the gland is identical to that found on the skin and is a mixture of methyl esters. We hypothesize that the oil layer, in combination with the denticles, creates a super-hydrophobic layer that reduces streamwise friction drag and increases swimming efficiency.” (Fig. 7).
it’s worthwhile to compare the Jurassic ichthyosaur, Eurhinosaurus (Fig. 8) with the recent swordfish, Xiphias. The ichthyosaur does not have the oil gland, does not lose teeth from the ‘sword’, does not reduce the number of vertebrae, does not lose the pelvic fins, and does not have deep caudal region. Don’t expect Eurhinosaurus to compete for speed with the swordfish.
Gregory WK and Conrad GM 1937. The comparative anatomy of the swordfish (Xiphias) and the sailfish (Istiophorus). The American Museum Novitates, 952:1-25.
Linnaeus C 1758. Systema naturæ per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata.
Linneaus C von 1766. Systema naturæ per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio duodecima, reformata. pp. 1–532. Holmiæ. (Salvius)