Updated Nov 21, 2019 and August 25, 2022
with a larger cladogram and new bone identities for the skulls shown here. Adding taxa moves some placoderms away from catfish, toward the base of the vertebrates, others stay with catfish. One extant purported catfish, Phreatobius, nests instead with those placoderms. Adding more taxa solves all such phylogenetic problems.
FIgure 1. Clarias, the walking catfish is a living placoderm with skull bones colorized as homologs of those in Entelognathus (Fig. 2). Here the mandible shifts forward and the opercular shifts backwards relative to Entelongnathus in the Silurian.
This is so obvious at first glance
(Fig. 1) and confirmed by phylogenetic analysis in the large reptile tree (LRT, 1472 taxa; Fig. 4) that the relationship between Entelognathus, the placoderm (Fig. 2), and Clarias, the walking catfish (Figs. 1, 3), should have been discovered earlier. Please let me know if anyone has done so.
Figure 2. The placoderm, Entelognathus, is widely considered the outgroup to the crossopterygians, the stem tetrapods. Compare the skull bones to those of Polypterus (Fig. 1) and Tinirau (Fig. 3). The posterior is unknown.
The clade Placodermi
had been considered extinct by all paleontologists. Entelognathus is a recent addition to that clade and it has been shown to be related to gnathostomes (fish with jaws) and osteichthys (jawed fish with bones). Here Clarias nests closer to Entelongnathus than to any other tested fish.
Figure 3. Clarias batrachus, the walking catfish, in vivo. The pelvic fin is tiny. The single dorsal fin is elongate. The anal fin is also elongate. The skull is flat and provided with sensory barbels.
The former clade ‘Osteichthys’
is no longer monophyletic. Now traditional bony fish have at least three origins in the LRT.
Figure x. Subset of the LRT focusing on the placoderms that now nest at the base of the vertebrates.
Figure z. Subset of the LRT focusing on catfish and purported placoderms as on August 2022.
Earlier we looked at the connection
between jawless Thelodus and shark-like Squatina. Here (Fig. 4) the connection between armored Entelognathus and slimy Clarias presents yet another case of a living taxon we can study in order to understand a fossil clade, long thought to be extinct. The missing posterior of Entelognathus (Fig. 2), for instance, probably looks more like Clarias than a shark, as the artist originally guessed.
Entelognathus primordialis (Zhu et al. 2013; Late Ludlow, Late Silurian, 419 mya; IVPP V18620) is a genus of placoderm fish with dermal marginal jaw bones (premaxilla,
maxilla and dentary), features previously restricted to Osteichthyes (bony fish). Here all the tetrapod skull bones are identified on this fish (placoderm bones and bony fish bones often have different names). Two pairs of nares are present. Distinct from the original, purported sclerotic rings are circumorbital bones (prefrontal, postfrontal, jugal). The pineal plate is the parietal. The central plate is the postparietal. The rostral is the nasal. The marginal plate is the supratemporal. The anterior paranuchal plate is the tabular.
Clarias batrachus (Linneaus 1758, up to 50 cm in length) is the extant walking catfish. The skull bones are nearly identical to those in the placoderm, Entelognathus (above), with the exception that the jaw is shorter, the quadrate and jugal are rotated to follow the jaw joint and the operculum bones rotates toward the rear. The spiny pectoral fins keep it upright as it wriggles from pond to pond. No scales or bones appear on the surface. The teeth, if they are teeth, are more like toothbrush bristles.
Added fewer than 24 hours later:
I found a note in an article (RDMag.com, “Oldest ever fossil jawbone found”) about Qilinyu rostratra, a placoderm described in Science. According to the article (but not in the paper) “The fossil has bones halfway between an ancient placoderm jaw and a modern jaw in the fish, which was described in a Nature.com article as being 20 centimeters long and similar in appearance to a catfish.”
Added a few days later,
According to Young 2010, Huxley 1861 allied at least some placoderms with catfish.
References
Huxley TH 1861. Preliminary essay upon the systematic arrangement of the fishes of the Devonian epoch. Memoirs of the Geologoical Survey, United Kingdom10:1–40
Linnaeus C 1758. Systema naturæ per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata.
Young G 2010. Placoderms (Armored Fish): Dominant vertebrates of the Devonian Period. Annu. Rev. Earth Planet. Sci. 2010. 38:523–50
Zhu M, Yu X-B, Ahlberg PE, Choo B and 8 others 2013. A Silurian placoderm with osteichthyan-like marginal jaw bones. Nature. 502:188–193.
Zhu M, Ahlberg PE, Pan ZH, Zhu Y, Qiao T, Zhao WJ, Jia LT and Lu J 2016. A Silurian maxillate placoderm illuminates jaw evolution. Science 354(6310):334–336. PDF
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