Trichiurus, the largehead hairtail, another cutlassfish, enters the LRT

Today’s fish was made infamous
by Ursula’s evil cohorts, Flotsam and Jetsam (Fig 1) in the decades old Disney cartoon, “The Little Mermaid” (1989).

Figure 1. Cartoon characters Flotsam and Jetsam appear to be based on Trichiurus. Compare to figure 2.

More importantly,
Trichiurus (Fig 2) comes in as a welcome addition to the large reptile tree (LRT, 2282 taxa) because it helped clarify some of the skull bones in its sister that still retained a tiny diphycercal tail, Aphanopus (Fig 3). Apparently both lack a parietal. That came as a surprise.

Figure 2. Trichiurus lepturus from Gregory 1933. Colors added here.

Both of these eel-like marine taxa
are derived from the tiny (9cm, Early Eocene) muskie and barracuda ancestor, Ductor in the LRT. Note the long teeth and prognathic mandible.

Related needlefish, like Saurichthys, go back to the Early Triassic, which moves their radiation much further back in deep time.

Figure 3. Newly revised Aphanopus skull.

Trichiurus lepturus
(Linneaus 1758, 2.3m) is the largehad hairtail, a type of cutlass fish. The pelvic fins are gone. So is the caudal fin.

Figure 4. Meter-long Aphanopus, the black scabbard fish, has a long, eel-like torso tipped with a tiny diphycercal tail. This is a descendant of the Ductor clade that includes barracuda and muskie.

Aphanopus carbo
(Lowe 1839, up to 1.1m) is the extant black scabbardfish, close to the eel-like cutlassfish. The body is extremely elongate with an odd little diphycercal tail. The pelvic fins are vestiges in juveniles, absent in adults. The premaxilla have fangs and are ventrally concave.

References
Linnaeus C 1758. Systema naturæ per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata.
Lowe RT 1839. A supplement to a synopsis of the fishes of Madeira. Proceedings of the Zoological Society of London, 7: 76–92.

wiki/Black_scabbardfish
wiki/Trichiurus