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Victor Bach

Acanthodian Origins of Chondrichthyes

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Eugnathostomata was originally the clade branching into the three major clades of "true" jawed vertebrates: Osteichthyes (aka bony fish), Chondrichthyes (aka cartilaginous fish) and Acanthodii (aka "spiny sharks"). The latter of the three had always been a complex group to classify, since it shared features with both Osteichthyans and Chondrichthyans. Acanthodians had cartilaginous skeletons, an epidermis covered with small platelets (reminiscent of ganoid scales in extant Holosteans) and also fins with bony bases and large structural dentine spines (which gave them the nickname "spiny sharks"). At the beginning of this decade, closer inspection of certain fossils like Acanthodes' cranium where hinting at the possibility for Acanthodii to be ancestral to both Osteichthyes and Chondrichthyes. However, in 2013 the discovery of Entelognathus (a placoderm with bony fish jaw features) led to the revision of this clade, eventually leading to the conclusion that it was paraphyletic. Acanthodians are now believed to be stem Chondrichthyans and the clade from which sharks, rays and chimaeras evolved from.

Chondrichthyans have been hypothesized to appear as early as the Ordovician, due to findings of isolated scales made of dentine and bone with a similar structure to that of cartilaginous fish, but these could also be remains of jawless fish like Thelodonts. The earliest "confirmed" Chondrichthyan fossil so far has been Kannathalepis from the early Silurian, but its remains consist mainly of scales found in the Canadian Arctic, which make classification of the specimen difficult. Doliodus was also originally labelled as a "conventionally defined Chondrichthyan" from the early Devonian due its shared features with sharks like expanded paired coracoids, successional replacement teeth and large basals and elongated radials on the pectoral fins. However, more recent papers have also pointed out similarities in its dermal skeleton with Acanthodians, which has left its exact classification unclear. Both Kannathalepis and Doliodus could potentially be classified as "spiny sharks", but Acanthodians were not sharks (not exactly). They were an ancient group of fish with physical adaptations that would lead to the evolution of Chondrichthyans; ancient relics of the evolutionary steps that produced some of the oceans' most successful predators.

The line that separates Acanthodians from Chondrichthyans is getting more and more difficult to see as new discoveries keep providing new insights. This differentiation may as well not exist as more "Acanthodian fish" are being found to be nested among "conventionally defined Chondrichthyans" and vice-versa. Acanthodii should perhaps simply be seen as the clade that regroups all cartilaginous fish taxa and their "Acanthodian" ancestors and relatives, or it may be replaced completely by a new less confusing clade. Either way, extinct Acanthodians seem to guard the secrets of how Chondrichthyes progressively lost their bony skeletons to become the known extant cartilaginous fish.

The phylogeny of these ancient fish is more than likely to be subject to constant changes and revisions as we learn more about the evolution of predominantly cartilaginous skeletons in fish, therefore I encourage anyone to share any news, papers, videos or other information that may help better build their clades in the PEP. I attached a couple images on the newly understood evolution and phylogeny of Acanthodians as well as a paper on Doliodus for those further interested in this topic.

entelognathus 3 by gogosardina.jpg

Relationships among early gnathostomes.jpg

Maisey_et_al._(2017)_Doliodus,_Bridging_the_"Acanthodian"-Chondrichthyan_Divide.pdf

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