Placoderms evolve from ordinary fish

Everyone thought placederms were exotic primitive fish
that evolved long before regular fish evolved. That turns out to be not true when more taxa are added.

Eurynotus crenatus (Traquair 1880)
is a largely overlooked bony fish that links Cheirodus and catfish to placoderms (Fig. 1) in the large reptile tree (LRT 1696+ taxa; subset Figs. 2,4).

Figure 1. Eurynotus crenatus nests between Cheirodus and basal placoderms like Coccosteus and Autroptycodus in the LRT. Friedman et al. provided the jugal and mandible of Eurynotus, somewhat distinct from the Watson 1928? engraving at right. Note how the large jugal of Eurynotus splits to form a posterior squamosal.

Figure 1. Eurynotus crenatus nests between Cheirodus and basal placoderms like Coccosteus and Autroptycodus in the LRT. Friedman et al. provided the jugal and mandible of Eurynotus, somewhat distinct from the Watson 1928? engraving at right. Note how the large jugal of Eurynotus splits to form a posterior squamosal.

Eurynotus data is hard to find.
Recently Friedman et al. 2018 discussed the durophagus (hard-shelled prey) feeding strategies of Eurynotus. Otherwise only the Traquair 1880 (Fig. 1) provides Eurynotus data for the LRT. (Thanks to Dr. Friedman for providing a PDF!)

Figure 2. Subset of the LRT focusing on catfish + placoderm clade.

Figure 2. Subset of the LRT focusing on catfish + placoderm clade.

Friedman et al. reported, “Several lineages of durophagous placoderms appeared during the Devonian. The most diverse of these, the ratfish-like ptyctodonts. Isolated dental plates represent the most common remains of ptyctodonts, which have greatly reduced cranial and thoracic armour relative to other placoderms.” 

Unfortunately,
Friedman et al. had it backwards due to taxon exclusion. They did not understand that ptyctodonts (Fig. 1) were placoderm-mimics, sisters to Cheirodus and closer to catfish in the LRT.

Figure 3. Eurynotus nests between Cheirodus and ptyctodont placoderms in the LRT.

Figure 3. Eurynotus nests between Cheirodus and ptyctodont placoderms in the LRT.

Eurynotus crenatus (Agassiz 1835; Traquair 1880; Friedman et al. 2018; Early Carboniferous, 310 mya) is widely considered a relative of Cheirodus and a member for the Lepidoid family of ganoid scale fish. Here Cheirodus and Eurynotus respectively are transitional taxa leading to the ptycodontid placoderm mimics (Austroptyctodusand other placoderms, like Coccosteus,  Note the absence of marginal teeth and the absence or fusion of the maxilla with the large jugal. These are traits retained by all later placoderms and their extant relatives, the catfish.

Figure x. Newly revised fish subset of the LRT

Figure 4. Newly revised fish subset of the LRT

And now a brief word for
Vertebrate Palaeontology Researcher in Residence Darren Naish.

Quoting from his ResearchGate.net page:  “I am currently working on a giant volume (over 1200 pp) titled THE VERTEBRATE FOSSIL RECORD.” That’s good. I support any wide-gamut look at vertebrate evolution. Naish started TVFR several years ago. He is currently seeking donations through Patreon where he includes sample pages (generously illustrated) along with his goals.

I have several concerns with this large project:

  1. Science textbooks, by their very nature, start to become out-dated the moment they are published. Nowadays its better to let others know of your hypotheses online. Those can be updated constantly. Online publication also guarantees worldwide distribution for free, both for the publisher and the reader.
  2. No indication of a wide gamut phylogenetic analysis appears to be at the core of this  project. In an open letter here, along with several private emails I have encouraged Naish to do this first, so he won’t be at the mercy of the innumerable academically published papers he will cite that suffer from taxon exclusion.
  3. He is sending his chapters out for review, which is standard practice in academic circles. Unfortunately that system perpetuates earlier textbook errors, leaving no room for repairs or novel hypotheses. In order to get a good review or a good grade in academia you have to repeat what the professor professes or your colleagues think is safe. We’ve seen where this goes horribly awry several times, but the go-to example can be found here.
  4. Naish knows how to build a phylogenetic analysis, but if the results someday mirror those in the Large Reptile Tree and its subtrees, that might be hard for him to accept—especially after all the bad press he has given ReptileEvolution.com. So, unless his wide-gamut, comprehensive cladogram goes in another direction, Naish has painted himself into a corner. After all, he has the PhD and I do not. This is more than ironic, because I started as a journalism student and retired as a scientist, while Naish started as a scientist and, if not careful, will end up being a journalist recounting the work of others in his TVFR.
  5. It’s never too late, Darren. Do the right thing and build your wide-gamut phylogenetic analysis. Then you will have the authority to posit your hypothesis—after running the experiment that must be run.

References
Agassiz JLR 1835. Recherches sur les Poissons fossiles, 5 volumes. Imprimerie de Petitpierre et Prince, Neuchaatel, 1420 pp.
Agassiz JLR 1835. On the fossil fishes of Scotland. Report of the British Association for the Advancement of Science, British Association for the Advancement of Science, Edinburgh.
Friedman M, Pierce SE, Coates M and Giles S 2018. Feeding structures in the ray-finned fish Eurynotus crenatus (Actinopterygii: Eurynotiformes) implicationsfor trophic diversification among Carboniferous actiniopterygians. Earth and Environmental Science Transactions of the Royal Society of Edinburgh, 109, 33–47.
Traquair RH 1880. X.—On the structure and affinities of the Platysomidae pp. 343 F.R.S.E., Keeper of the Natural History Collections in the Museum of Science and Art, Edinburgh.
Watson DMS 1928. On some points in the structure of palæoniscid and allied fish. Proceedings of the Zoological Society of London 1928, 49–70.

wiki/Eurynotus (not yet written)

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