Things were not quite right in the catfish-placoderm clade,
so a critical examination of the traits and scores was due.
As longtime readers know,
every new taxon added to the LRT is a new experience, scored to the best of my nascent ability each time. When the first few taxa were scored, I had little to no experience with any fish. Now, with a substantial taxon list, comparisons can be reexamined that were overlooked or not present before.
Because evolution works gradually,
bones and proportions that appear on one taxon should also appear on closely related taxa. Here tetrapod labels were put on all fish skull bones, so the traditional published fish skull bone labels were not as helpful as they will be once other workers adopt this several times earlier proposed nomenclature standard.
Figure 1. Subset of the LRT focusing on the branch of the Osteichthys that includes placoderms and their relatives.
The basic tree topology in this clade has not changed.
A few of the taxa have been rescored (Fig. 1).
Figure 3. Menaspis armatas in situ. Colors added to bones and skin. White area above restores the displaced mandibles relative to one another.
A former odd Permian ‘placoderm’ with barbels, Menaspis (Fig. 2), moved over to the Siluriformes (catfish clade). That only makes sense.
Menaspis armata (Ewald 1848; Late Permian; > 15cm long) was described as the ‘last known arthrodire placoderm’. Here it nests with the catfish, Clarias and Wuttagoonaspis. The former skull spine is the displaced mandible. The former ‘horns’ are barbels. The orbit is somewhere under the barbels. The entire ventral half of the skull is missing here on the counterplate. This is a ventral view of the dorsal skull plates.
Figure 3. Tiny unnamed arthrodire, ANU V244-3 in various views. The upper left image lacks jaws. The jaws are upper right. Palatal view at middle right.
A tiny arthrodire ANU 244 now nests basal to the open water predatory clade of large to giant placoderms.
ANU V244 (Hu, Lu and Young 2017; Early Devonian) is a tiny basal arthrodire. The authors provided several views of the skull, even dividing it in half to show upper and lower elements separately (Fig. 3). The authors followed tradition in the proposal that placoderms were basal to gnathostomes not realizing placoderms have lost the maxilla, like their sisters, the catfish.
Several cheek bones on other placoderms were re-identified following this holistic look at several taxa all at once. Each specimen contributed to the understanding of the clade. Placoderms are highly derived leaving no descendants in the Mesozoic or thereafter. Traditional cladograms nesting placoderms basal to sharks and bony fish are in error, according to the LRT, which tests a wide gamut without prejudice.
On that note: the traditional ptyctodontid ‘placoderms’, Astroptyctodus and Campbellodus (Fig. 4), still nest outside the clade that includes the other placoderms.
Figure 4. Cheirodus and Campbellodus to scale. These two nest together in there LRT.
The radiation of catfish and placoderms
must have happened deep in the Silurian with late survivors among the tested taxa.
Placoderms developed internal fertilization
with claspers and live birth of a few large young, convergent with sharks and manta rays.
On the other hand, catfish retained external fertilization
with thousands of eggs produced by a single female through several spawning periods. Typically 10% develop and survive. The first few spawnings produce none or fewer than five eggs.
Ewald J 1848. Über Menaspis, eine neue fossile Fischgattung. Berichte Über die zur Bekanntmachung Geeigneten Verhandlungen der Königlich-Preussischen Akademie der Wissenschaften zur Berlin 1848:33-35.
Hu Y, Lu J and Young GC 2017. New findings in a 400 million-year-old Devonian placoderm shed light on jaw structure and function in basal gnathostomes. Nature Scientific Reports 7: 7813 DOI:10.1038/s41598-017-07674-y