Operculum: origin, disappearance, reappearance and disappearance

The fish operculum first appeared
in the sturgeon, Acipenser, among tested taxa in the large reptile tree (LRT, 1870+ taxa; subset Fig. 2). More primitive taxa, from the hagfish (Myxine) to Thelodus, had several gill openings lined up without an operculum covering the serial openings.

The operculum first disappeared
and multiple gill openings reappeared in sharks and rays, like Manta and Loganellia.

The operculum first reappeared
in ratfish (Chimaera), then reappeared again in paddlefish (Polyodon) and again in bony fish (Gregorius).

The operculum finally disappeared
in placoderms, and again in moray eels (Gymnothorax) + gulper eels (Eurypharynx) and again in tetrapods (Greererpeton, Fig. 1).

Then concluded,
“So elephant sharks have lost the ability to make posterior branchial rays, but still carry the embryonic pattern for where they should form. It seems the common ancestor of Elasmobranchs and Holocephalans had the gills of a shark, rather than the gills of a chimaera,” according to Gillis et al. 2011 in Neil Shubin’s lab. This is confirmed by the LRT (Fig. 2).

The takeaway message is this:
Don’t get caught Pulling a Larry Martin! (= defining a clade based on a single trait or a dozen traits). You’ve just seen how one trait can appear and disappear and reappear and disappear. This is but one example of several hundred I could have shown. Only the last common ancestor method can define a clade. A last common ancestor can be hypothetically recovered in phylogenetic analysis using 150 to 250 multi-state character traits and as many taxa as you have time to do. Convergence and reversal are out there, so beware!

References
Gillis JA, Rawlinson KA, Bell J, Lyon WS, Baker CVH and Shubin NH 2011. Holocephalan embryos provide evidence for gill arch appendage reduction and opercular evolution in cartilaginous fishes. Proceedings of the National Academy of Sciences USA 108:1507–1512.

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