Weird Petrocephalus enters the LRT between piranhas and elephantfish

Odd looking Petrocephalus (Fig. 2) has an unexpected ancestry (Fig. 1) and even odder progeny (Fig. 3).

Figure 1.  On the piranha, Serrasalmus, the premaxilla (yellow) does not extend to the tooth row. Instead the maxilla (green) carries all the upper teeth. Serrasalmus skeleton image courtesy of ©Steve Huskey and used with permission.
Figure 1.  On the piranha, Serrasalmus, the premaxilla (yellow) does not extend to the tooth row. Instead the maxilla (green) carries all the upper teeth. Serrasalmus skeleton image courtesy of ©Steve Huskey and used with permission.
Figure 2. Petrocephalus, a type of elephantfish without a trunk, nests between Serrasalmus (figure 1) and Gnathonemus (figure 3).
Figure 3. Gnathonemus, the elephant fish, for obvious reasons, has been traditionally considered a sister to Osteoglossum. Here it nests with Hippocampus, the seahorse. Image from Gregory 1938.

Evolution at work, starting with the piranaha, Serrasalmus:
In Petrocephalus the circumorbital ring is reduced.
In Gnathonemus the circumorbital ring is absent.

In Petrocephalus the jaws and teeth are much smaller.
In Gnathonemus the jaws are smaller on an extended, curved rostrum and the teeth are smaller still.

In Petrocephalus the posterior crest is reduced.
In Gnathonemus the posterior crest is smaller still.

In the LRT the relationship between the piranha and elephantfish
was already established. The addition of Petrocephalus (Fig. 2) makes it one of many transitional taxon between the piranha, Serrasalmus (Fig. 1), and the elephantfish, Gnathonemus (Fig. 3). The less lethal Brycon, the South American trout or Sabalo barracuda (Fig. 4), currently nests at the base of this clade. These are basal ray fin fish with an ancestry that probably extends back to the Devonian.

Figure 4. Brycon, the extant South American trout, is basal to the piranha and the mormyrids.
Figure 4. Brycon, the extant South American trout, is basal to the piranha and the mormyrids.

Petrocephalus bane
(Lacepède 1803; 20cm) is considered a mormyrid, like Mormyrops,and a species of elephant fish, despite lacking a ‘trunk’. Here it nests between Serrasalmus and Gnathonemus. The mandible is smaller than the orbit. Crests appears on the front and on the posterodorsal skull, but they do not show up outside the skin in vivo.

References
de Lacepéde BG 1803. Histoire naturelle des poissons. Tome Cinquieme. 5(1-21):1-803 + index.

wiki/Petrocephalus_bane

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