Protacanthopterygii - Systematics

Systematics

As mentioned initially, one of the closest relatives of the Protacanthopterygii are the Otocephala, which include such different fishes as carp, catfish and herrings in superorders Clupeomorpha and Ostariophysi. While the core of the present superorder has always consisted of three orders, additional taxa are likely to belong here but are more disputed.

In recent times, there is a trend to split the Osmeriformes in two, as their supposed suborders do not seem to be particularly closely related among the Protacanthopterygii. The resultant order Argentiniformes contains a number of peculiar deep sea forms. And as it seems, the superorders Cyclosquamata and Stenopterygii which contain some bizarre apomorphic forms are so closely related to the "traditional" Protacanthopterygii as to be included therein. Alternatively, the unranked clade name Euteleostei has been proposed for the whole group, but that would require splitting up the Protacanthopterygii lest they become paraphyletic. Thus, up to seven orders might be included here:

Protacanthopterygii sensu stricto

• Argentiniformes – barreleyes, slickheads and allies
• Esociformes – pikes and mudminnows
• Osmeriformes – galaxiids, true smelts and allies
• Salmoniformes – salmons, trouts and allies

"Stenopterygii"

• Ateleopodiformes – jellynose fishes
• Stomiiformes – dragonfishes, marine hatchetfishes and allies

"Cyclosquamata"

• Aulopiformes – lancetfishes, lizardfishes and alies

The placement of the Myctophiformes, traditionally held to be close relatives of the Aulopiformes and separated in a superorder "Scopelomorpha", as well as the Lampriformes (the monotypic superorder Lampridiomorpha, and putative relatives of the "Stenopterygii") consequently warrants further study. However, these two seem to be somewhat more advanced and closer to the Paracanthopterygii.

More dubiously, the Cetomimiformes and "Ctenothrissiformes" are sometimes placed here. While they are indeed of uncertain relationships, the former are widely assumed to belong to superorder Acanthopterygii and be closely related to the Stephanoberyciformes. Thus, it is rather unlikely that they are correctly placed here. As regards the latter, it is an entirely fossil group and may well be an invalid paraphyletic assemblage of ancient Teleostei, and in part may indeed belong in the Protacanthopterygii sensu lato.

The supposed family "Macristiidae" was sometimes considered the only extant member of the "Ctenothrissiformes". But actually these are larvae of certain Aulopiformes, as was already suspected when the first "macristiid" was scientifically studied. They were subsequently split as a distinct family and placed in the Osmeriformes. This is interesting in the light of the modern view that these two orders are not nearly as distantly related as they were believed throughout most of the 20th century.