Manta cephalic lobes are derived from shark labial cartilages, former placoderm quadrates

Monday, January 29, 2024 7:39

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A few years ago,
Swenson et al 2018 reported they determined the origin of cephalic fins in cownose rays (Rhinoptera, Fig 1, red) were derived from an anterior subset of the pectoral fin modified for prey capture, detached and separated from the large lateral portion devoted to locomotion (cyan).

Figure 1. Ventral view of Rhinopterus from an old engraving. Colors added here. The cephalic fins (red) are not connected to the pectoral fins (cyan). Rather the lobes originate at the mouth corners, where the quadrate is/was. These are labial cartilages, as in other sharks, located lateral to the mouth.

That hypothesis is falsified here.
The cephalic lobes originate at the mouth corners, where the quadrate is/was. That homology was not recognized or identified by Swenson et al. In the present hypothesis, the placoderm quadrate becomes the shark labial cartilage (as we learned earlier here) and continues enlarging and becoming more complex and mobile in the ray cephalic fin/lobe. This occurred several times in rays by convergence, each time resulting in a different sort of cephalic lobe.just ‘Rays’ converged in body shape from several different origins according to the LRT.

The present hypothesis can be falsified. It awaits confirmation, refutation or modification by independent workers employing phenomic phylogenetic analyses.

Figure 2. The enigmatic structures surrounding the mouth of Harpacanthus now appear to be cephalic lobes derived from labial cartilages derived from placoderm quadrates.

The understanding that
cephalic lobes/fins in rays are derived from quadrate labial cartilages found in other sharks sheds light on similar structures found in Carboniferous Harpacanthus (Fig 2, Traquair 1886) appearing independently and by convergence due to a unique three-part substructure.

The traditional clade, Batoidea, was dismantled earlier by the LRT

Figure 3. The labial cartilages in the kitefin shark, Dalatias, are shown here in red.

Not all sharks retain labial cartilages.
Others, like Dalatias, the seafloor skimming kitefin shark, enlarge those former external quadrates.

References
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