Multiple origins for the preopercular

Yesterday we looked at the origin of the quadratojugal in tetrapods arising from the split of the long, deep maxilla in Gogonasus (Fig. 1). But wait! There’s more!

Figure 1. Gogonasus skull demonstrating the genesis of the split between the toothy maxilla and the toothless quadratojugal.

Figure 1. Gogonasus skull demonstrating the genesis of the split between the toothy maxilla and the toothless quadratojugal.

Gogonasus also gives us
the origin of the preopercular arising from an anterior/posterior split of the squamosal (Fig. 1). This type of preopercular shrinks and disappears in Late Carboniferous tetrapods.

Figure 1. The lizardfish, Trachinocephalus with colors added. Diagram from Gregory 1936. This taxon nests with Devonian Cheirolepis, a basal ray-fin fish.

Figure 2. The lizardfish, Trachinocephalus with colors added. Diagram from Gregory 1936. This taxon nests with Devonian Cheirolepis, a basal ray-fin fish.

By convergence,
the preopercular appears in Trachinocephalus (Fig. 2) and in no other taxa derived from that clade. Since this is an extant taxon with a convergent loss of cheek/facial bones it would be good to find out what happened in fossil ancestors here.

Figure 3. Pholidophorus in situ and two skulls attributed to this genus. Compare the one on the left to figure 2. No tested fish in the LRT is closer to Robustichthys than Pholidophorus.

Figure 3. Pholidophorus in situ and two skulls attributed to this genus. Compare the one on the left to figure 2. No tested fish in the LRT is closer to Robustichthys than Pholidophorus.

By a third convergence
the preopercular has yet another genesis in Pholidophorus (Fig. 3), a trait that continues in all descendant taxa among the remaining ray-fin fish. Is this the same preopercular as in Trachinocephalus? At this point, no. However just a few fossil added intervening taxa would solve this issue.

Figure 4. Skull of Lepisosteus in dorsal and lateral views. Several cheek bones were lost earlier and refilled here creating another solid cheek without a lateral temporal fenestra

Figure 4. Skull of Lepisosteus in dorsal and lateral views. Several cheek bones were lost earlier and refilled here creating another solid cheek without a lateral temporal fenestra

Note
in Lepisosteus (Fig. 4) the preopercular (light green) has evolved to be like those in pipefish and sea horses: completely horizontal, ventral to the jugal, with the anterior end following the jaw joint (quadrate) as it migrates anterior to the orbit.

Figure 2. Subset of the LRT focusing on basal lobefin fish and kin.

Figure 5. Subset of the LRT focusing on basal lobefin fish and kin.

Catfish
also develop a preopercular (Fig. 5).

Figure 1. Synodontis skull with bones colored as homologs to tetrapod bones. Note the wrapping of the postparietals around the dorsal fin and the posterior extension of the pectoral girdle (cleithrum) to protect the  flank. 

Figure 1. Synodontis skull with bones colored as homologs to tetrapod bones. Note the wrapping of the postparietals around the dorsal fin and the posterior extension of the pectoral girdle (cleithrum) to protect the  flank.

Bottom line:
traits can converge. The same new bone can appear more than once. Or disappear more than once.

 

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