Mapping the rabbitfish (Chimaera) skull

Updated December 14, 2020
because so many more chondrichthyes and chimaera have been added to the LRT.

Chimaera monstrosa (Linneaus 1758; Figs. 1–3), the extant rabbitfish, a type of ratfish, nests between sharks and sturgeons in the large reptile tree (LRT, 1467 taxa).

The fish’s quadrate
shifts anteriorly to below the orbit. The mouth is largely ventrally oriented. The teeth are transformed and fused to plates and beaks. The large pectoral fins provide propulsion while the tail is reduced to a whip.

Figure 1. Chimaera monstrosa in vivo.

Figure 1. Chimaera monstrosa in vivo.

No one, it seems, has attempted to map the skull before.
The rabbitfish skull is an apparently suture-less cartilaginous shape, almost as if it had been 3D printed (Fig. 2). Check out the academic literature and you’ll see skull shape outlines, and that’s about it. Not many workers call ratfish their specialty. Figuring out where the likely boundaries of the bones are is best left to phylogenetic bracketing (comparisons with putative and traditional sister taxa). Digital graphic segregation (coloring the bones) seems to help, especially in the presentation.

Figure 2. Ratfish skull with 'bones' (actually precursor cartilage' colored and labeled.

Figure 2. Ratfish skull with ‘bones’ (actually precursor cartilage’ colored and labeled.

Most ratfish
have a extended soft rostrum (Fig. 3). This one does not (or it was cut off (Fig. 2).

Figure y. Basal Gnathostomata with the addition of Rhinochimaera.

Figure 3. Basal Gnathostomata with the addition of Rhinochimaera.

Most workers
recognize the relationship of chimaeras with sharks and rays, but apparently none add sturgeons and spoonbills to that list, despite the fact that chimaeras also have gill covers and similar morphologies overall (Fig. 3). As we learned earlier here, acanthodians are not related to sharks.

Figure 5. Shark skull evolution according to the LRT. Compare to figure 1.

Figure 4. Shark skull evolution according to the LRT. Compare to figure 1.

PS.
For those interested in the controversial basal bird Ambopteryx,
a closeup of the radius in question is presented here. I can’t think of anything in science more exciting than to be published in Nature. I can’t think of anything more haunting than to have a mistake published in Nature.


References
Linnaeus C 1758. Systema naturæ per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata.

wiki/Chimaera

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