Protacanthopterygii: A Brief History of a Vague Idea

There are some taxon names whose concepts are rock-solid, that have been universally recognised since their inception almost without variation. There are some taxon names that are coined, potentially linger through one or two subsequent uses, then disappear into the mists of history never to be used again. And then there are some taxon names that are used regularly but whose actual concept shifts wildly over time: names that seem to be used not so much for their own sake as because authors seem to think they need to be in there somewhere. Witness today's subject, the Protacanthopterygii.

Brown salmon Salmo trutta, photographed by Eric Engbretson, about as close to a definitive 'protacanthopterygian' as you're going to get.

The Protacanthopterygii has widely been recognised as a major group of ray-finned fishes since the name was established by Greenwood et al. (1966). Using the modern parlance, Greenwood et al.'s Protacanthopterygii was an explicitly paraphyletic group of euteleost fishes that could be recognised as branching off the lineage leading to the Acanthopterygii and Paracanthopterygii but lacked the full suite of characteristics of the latter group. As such, many of the characters listed by Greenwood et al. as diagnostic of the Protacanthopterygii were expressed in the form of trends: "widespread trend toward the development of premaxillary processes", for instance, or "hyoid and branchiostegal skeleton approaching paracanthopterygian and acanthopterygian form". We also get a number of references to majority rather than universal features: "glossohyal teeth usually prominent", or "few species with opercular spines or serrations". Greenwood et al. included the bulk of their Protacanthopterygii in the order Salmoniformes, but recognised this order in a much broader sense than modern authors. As well as the Salmonidae itself, their Salmoniformes included taxa that would now be placed in the orders Galaxiiformes, Esociformes, Myctophiformes, Aulopiformes and Stomiiformes, among others. Greenwood et al.'s Protacanthopterygii was also supposed to include the orders Cetomimiformes, Gonorynchiformes and Ctenothrissiformes. Their concept of Cetomimiformes is now recognised as polyphyletic and neither Cetomimiformes and Gonorynchiformes include any taxa closely related to Salmonidae; the case of Ctenothrissiformes has been discussed on this site previously.

Northern pike Esox lucius, copyright Jik jik.

In the intervening years, of course, the philosophy of systematics has shifted to prioritising the recognition of monophyletic taxa, requiring the dissolution of the original Protacanthopterygii. Unfortunately, calculating basal euteleost relationships has not proven an easy task. As a result, authors have differed considerably on exactly which fishes should be regarded as 'protacanthopterygians'. About the only constant factor in all circumscriptions of the taxon has been the inclusion of the Salmonidae, the salmons, trouts and the like. Indeed, the most extreme restriction of the Protacanthopterygii would treat it as including this family alone.

Recent molecular studies have agreed on the recognition of a clade uniting the Salmonidae with the Esociformes. The Esociformes is a small order of a bit over a dozen species of freshwater fish found in the Holarctic region, uniting the pikes of the genus Esox with the mudminnows of the Umbridae. Betancur-R et al. (2017) recognised Protacanthopterygii as the name for a clade uniting the Salmonidae, Esociformes, Argentiniformes (a marine order including herring smelts, barreleyes and the like) and Galaxiidae (whitebaits). However, other studies have not supported this clade.

Spotted galaxias Galaxias truttaceus, copyright Nathan Litjens, an Australian member of the whitebait family. Though galaxiids are rather salmon-like in overall appearance, it remains an open question whether this resemblance indicates any sort of direct relationship or just a shared hold-over from some ancestral neoteleost.

Considering the difficulty in defining it, one might question why the concept of a 'Protacanthopterygii' persists at all. Really, there doesn't seem to be much reason for it other than that the Greenwood et al. (1966) classification was long the base standard for teleost classifications, leaving subsequent authors loathe to discard any taxon recognised therein lightly. It might, in theory, be possible to rescue the Protacanthopterygii concept by phylogenetic definition: for instance, as those species more closely related to Salmo than Perca (indeed, I would not be surprised to learn this has already been done). But considering that the uncertain composition of the resulting clade would reduce the practicality of its recognition, I don't think I would be weeping too much if someone would just take the Protacanthopterygii concept out the back and shoot it.

REFERENCES

Betancur-R., R., E. O. Wiley, G. Arratia, A. Acero, N. Bailly, M. Miya, G. Lecointre & G. Ortí. 2017. Phylogenetic classification of bony fishes. BMC Evolutionary Biology 17: 162.

Greenwood, P. H., D. E. Rosen, S. H. Weitzman & G. S. Myers. 1966. Phyletic studies of teleostean fishes, with a provisional classification of living forms. Bulletin of the American Museum of Natural History 131 (4): 339–456.