A fourth origin for the ‘operculum’

The operculum
is the gill cover in sturgeons, bony fish, ratfish and paddlebills, all derived independently.

Here is the phylogenetic sequence in the LRT that gave us
paddlebills (Fig 1) which continue to be derived from basking sharks. Toward the bottom of the graphic are taxa the LRT recovered as paddlebill descendants. These retained the short snout of juvenile paddlebills (neotony) yet developed an even more elaborate set of operculum + ventral branchial cowl flaps.

Convergence, like this example, tends to multiply the degree of difficulty one faces in phylogenetic analysis. That may be why this effort has taken so long.

Figure 1. Cetorhinus, Polyodon, Chondrosteus and Strongylosteus in phylogenetic order demonstrating the fourth origin of the operculum. ” data-image-caption=”

Figure 1. Cetorhinus, Polyodon, Chondrosteus and Strongylosteus in phylogenetic order demonstrating the fourth origin of the operculum.

” data-medium-file=”https://pterosaurheresies.files.wordpress.com/2024/02/polyodon2chondrosteus588.jpg?w=94″ data-large-file=”https://pterosaurheresies.files.wordpress.com/2024/02/polyodon2chondrosteus588.jpg?w=322″ src=”https://pterosaurheresies.files.wordpress.com/2024/02/polyodon2chondrosteus588.jpg” alt=”Figure 1. Cetorhinus, Polyodon, Chondrosteus and Strongylosteus in phylogenetic order demonstrating the fourth origin of the operculum.” class=”wp-image-84134″ />

According to Wikipedia,
“The operculum is a series of bones found in bony fish and chimaeras that serves as a facial support structure and a protective covering for the gills; it is also used for respiration and feeding.”

Among bony fish
catfish have only one operculum. Coelacanths have one, sometimes two. Luganoia, a tiny fish nesting at the base of the coelcanths, has two with the large one overlapping the under one. I think the rest of fish that have an operculum have three. Tesselation enters the equation because Brachyacanthus has three sets of six PLUS the placoderm quadrate, takes over the duties in what might be called a fifth convergent origin for the operculum, all originating from different temple and cheek bones.

If I forgot any other exceptions I’ll add them later.

PS
Happy to announce images can now be uploaded and accessed in WordPress again without taking a detour or end-around. Other problems persist, but they should not affect what can be presented.

References
wiki/operculum (fish)