A pseudo-operculum in sturgeons

Short one today.
because sharks are getting a makeover and review.

Shark-like
Chondrosteus (Fig. 1) is the phylogenetically first taxon we see with an operculum, but no preoperculum.

The operculum continues and expands
with the other shark-like taxon, the paddlefish, Polyodon. In neither taxa are these bones well-ossified, as in the bony fish clade, Osteichthyes (Fig. 4). In sharks the operculum is lost only to be regained eons later by reversal in bony fish. The ratfish, Chimaera, also redevelops an operculum.

Sturgeons appear to have an operculum, too,
(Fig. 2) but it is created by the hyomandibular, the dark green bone (Fig. 2) that supports the pro to-mandible. As you’ll note, Chondrosteus developed a separate hyomandibular (Fig. 1, dark green) and operculum (Fig. 1, lavender).

The basalmost living bony fish,
the moray eel, Gymnothorax (Fig.  3), doesn’t have much of an operculum.

The basal bony fish
Calamopleurus (Fig. 4) has an well-ossified preoperculum and operculum. Note the many shared traits with Gymnothorax (Fig. 3).

The other clade of bony fish,
represented by the bowfin, Amia (Fig. 5), likewise have a well-developed preoperculum and operculum.