The black swallower (Chiasmodon) enters the LRT

The snaketooth fish
(= black swallower, genus: Chiasmodon niger; Figs. 1, 2; Johnson 1864, up to 25cm) is a deep sea fish famous for having a giant stomach. It is capable of swallowing, but not necessarily digesting before decomposition, prey larger than itself and up to 10x its mass. Videos and images (below) tell this most of this tale. The LRT (Fig. 4) tells the rest.

Figure 1. Chiasmodon is what happens to a lizardfish that gets used to deeper seas.

Figure 1. Chiasmodon is what happens to a lizardfish that gets used to deeper seas. Photo credit: https://www.animalsphotos.xyz/black-swallower-fish-facts/

Basically the black swallower is a deep sea lizardfish,
like Trachinocephalus (Fig. 3). In the large reptile tree (LRT, 1586 taxa) both of these taxa nest with Cheirolepis among the new basal Sarcopterygii, phylogenetically appearing prior to the advent of lobe fin development. Scales are absent, distinct from the related lizardfish.

Figure 2. Chiasmodon from Gregory 1938, here colorized. Compared to the lizardfish, Trachinocephalus, in figure 3.

Figure 2. Chiasmodon from Gregory 1938, here colorized. Compared to the lizardfish, Trachinocephalus, in figure 3.

Traditionally
the black swallower nests with the lizardfish, which is confirmed by the LRT.  However both are traditionally considered Perciformes due to taxon exclusion.  The LRT does not nest these taxa with the perch, Perca (subset Fig. 4) due to a wider gamut of included taxa. That’s the value of minimizing bias in taxon selection.

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

Figure 3. The lizardfish, Trachinocephalus with colors added. Diagram from Gregory 1936. This taxon nests with Devonian Cheirolepis, a basal ray-fin fish.

The value of seeing skulls side-by-side
(Figs. 2, 3) makes it easy to appreciate the phylogenetic proximity of these two.

Figure 2. Updated subset of the LRT focusing on basal vertebrates (fish). Arrow points to Hybodus. This tree does not agree with previous fish tree topologies.

Figure 4. Updated subset of the LRT focusing on basal vertebrates (fish). Arrow points to Hybodus for an earlier post. This tree does not agree with previous fish tree topologies.


References
Johnson JY 1864. Description of three new genera of marine fishes obtained at Madeira. Proceedings of the Zoological Society of London, B 1863(3): 403-410.

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