Odd looking Petrocephalus (Fig. 2) has an unexpected ancestry (Fig. 1) and even odder progeny (Fig. 3).
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.
(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.
de Lacepéde BG 1803. Histoire naturelle des poissons. Tome Cinquieme. 5(1-21):1-803 + index.