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A splitting, disappearing postorbital in lazy predatory weakly electric river fish

Thursday, November 9, 2023 16:08

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A 3m long extant Amazon fish,
Arapaima, has large cheek bones (= circumorbital ring = multiple postorbitals, colored amber in Fig 1). These lateral plates extend to the posterior margin of the skull, as in Amia, the related and smaller bowfin.

Figure 1. Extant Arapaima has large postorbitals = circumorbitals. This is the starting point today.

A related, 1.5m African fish with a weak electrical signal,
Mormyrops, (“Cornish Jack’) reduces the circumorbital ring (colored amber in Fig 2). In this diagram from Gregory 1933 the ‘scale bone’ (also colored amber) is scored here as the posterior postorbital. The middle portion has eroded and disappeared. Compare to Arapaima in figure 1.

Figure 2. Extant Mormyrops with ‘scale bone’ placed in the posterior portion of the postorbital position of Arapaima in figure 1. Note the reduced surface exposure of the postfrontal (orange).

Another related extant African fish with a weak electrical signal,
Gymnarchus, also has a scale bone (colored amber in Fig 3) that fits over a lateral hollow on the posterior margin of the skull. This helps identify the slender line of postorbital bones extending between the eyeball and scale bone as remnants of a once larger, more robust set of cheek bones still present in Arapaima (Fig 1).

Figure 3. Extant Gymnarchus with ‘scale bone’ placed in the posterior portion of the postorbital position of Arapaima in figure 1 and Mormyrops in figure 2. Note the lack of any surface exposure of the postfrontal (orange) in this taxon.

In unrelated fish taxa, what Gregory 1933 also called the ‘scale bone,’
is homologous with the tetrapod supratemporal. Until today I followed that paradigm. Then I realized it was homologous with a detached posterior postorbital in these three taxa.

This is a minor example of the sort of barriers to understanding fish phylogeny that one must overcome to correctly nest related taxa. Hope this insight helps your efforts.

References
wiki/Cornish_jack
wiki/Mormyridae
wiki/Gymnarchus

Source: https://pterosaurheresies.wordpress.com/2023/11/10/a-splitting-disappearing-postorbital-in-lazy-predatory-weakly-electric-river-fish/

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