Enigmatic Niobrara fish, Martinichthys, enters the LRT with extant Arapaima

Martinichthys brevis (McClung 1926; Taverne 1999; KUVP 497; Late Cretaceous; Fig. 1) is a rare Niobrara fish here nesting with the slow-moving, extant, Amazon giant, Arapaima (Figs. 2, 3). Taverne 1999 considered this fish in the same clade as the distinctly different Pentanogmius, a taxon we looked at yesterday.

Figure 1. Two species assigned to the genus Martinichthys from the Niobrara. Colors and some restoration added here. Compare to previously overlooked Arapaima in figures 2 and 3. Scale bars are cm.

Not sure why Arapaima was overlooked in prior studies.
This may be so because adding living taxa to fossil cladograms does not happen very often. The large reptile tree (LRT, 1978 taxa) minimizes taxon exclusion by adding taxa, both extinct and extant, in order to minimize taxon exclusion.

Figure 2. Skull of Arapaima in two views. Compare to Martinichthys in figure 1.
Figure 2. Skull of Arapaima in two views. Compare to Martinichthys in figure 1.

The post-crania of Martinichthys
is not known except for a few vertebrae. Workers believe this may be due to poor ossification. Based on the similar size and morphology of the Arapaima skull, Martinichthys was likely about the same size and shape (Fig. 3).

Figure 3. Amia, Arapaima and several related taxa in the LRT.
Figure 3. Amia, Arapaima and several related taxa in the LRT to scale and to a common snout-tail length compared to the largest taxon in this graphic, Arapaima.

Once again, this appears to be a novel hypothesis of interrelationships.
If not, please provide a prior citation so I can promote it here.

References
McClung CE 1926. Martinichthys, a new genus of Cretaceous fish from Kansas, with descriptions of six new species. Proc. Amer. Philos. Soc. 65 no. 5, (suppl.) 20-26, 2 pls.
Taverne L 1999. Révision du genre Martinichthys, poisson marin (Teleostei, Tselfatiirormes) du Crétecé supérior du Kansas (États-Unis). Revision of the genus Martinichthys, marine fish (Teleostei, Tselfatiiformes) from the Late Cretaceous of Kansas (United States) Geobios 33(2):211-222.

oceans_of_kansas_martinichthys
wiki/Arapaima
wiki/Martinichthys
wiki/Tselfatiiformes

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