Robustichthys: another nail in the coffin of the traditional clade ‘Holostei’

Updated May 3, 2020 with a new nesting for Robustichthys
with the armored catfish, Hoplosternum (Fig. 1).

Figure 1. Hoplosternum in vivo. You can see the armor/bone beneath its shiny skin.

Figure 1. Hoplosternum in vivo. You can see the armor/bone beneath its shiny skin. A sister to Robustichthys in the LRT.

We looked at the breakup of the traditional clade ‘Holostei’
earlier here. Today’s new taxon (Fig. 1) does not repair that breakup.

Figure 1. Robustichthys in situ.

Figure 1. Robustichthys in situ.

Described as “the largest holostean of the Middle Triassic,”
Robustichthys luopingensis (Xu et al. 2014, Xu 2019; Figs. 1, 2) does not nest with other traditional holosteans in the large reptile tree (LRT, 1548/1680 taxa). Rather it nests with Pholidophorus (Fig. 3), a tuna-like fish from the Late Triassic. The skulls are nearly identical (Figs. 2, 3), more so that the two skulls attributed to Pholidophorus (Fig. 3). Adding colors to match tetrapod patterns (Fig. 2) is a practice I have encouraged all paleoichthyologists to adopt.

Figure y. Hoplosternum skull with bones identified as homologs to those in Robustichthys.

Figure y. Hoplosternum skull with bones identified as homologs to those in Robustichthys.

Figure z. Skull of Robustichthys and reconstruction of same. Note resemblance to Hoplosternum (Fig. y), distinct from diagram in Xu 2019.

Figure z. Skull of Robustichthys and reconstruction of same. Note resemblance to Hoplosternum (Fig. y), distinct from diagram in Xu 2019.

Earlier the LRT nested
three traditional extant ‘holosteans’, Amia, the bowfin, the distinctly different Lepisosteus, the gar, and Pholidophorus (Fig. 3), from the Late Triassic, several nodes apart from one another (Fig. 4), creating a polyphyletic clade ‘Holostei’. Robustichthys nests with Pholidophorus. So traditional traits that describe members of the traditional clade ‘Holostei’ are convergent among ray-fin fish in the LRT, which tests skull and skeleton traits.

Xu 2019 only mentions Pholidophorus (while pulling a Larry Martin),
“Recently, Arratia (2013) described that the symplectic also articulates with the lower jaw in the pholidophorid Pholidophorus gervasutti, but this condition, unknown in other early teleosts, probably represents another convergent evolution.” Xu did not include the LRT sisters and cousins, Pholidophorus, Strunius, and Thunnus, among several other taxa that split gars from bowfins in the LRT. Neither did any of the prior workers who produced cladograms included in Xu 2019. So, taxon exclusion is once again the problem here. The only way to test convergence in evolution is to test it, not dismiss it, as Xu did.

What is a Symplectic?
“relating to or being a bone between the hyomandibular and the quadrate in the mandibular suspensorium of many fishes that unites the other bones of the suspensorium.”

Figure 4. Pholidophorus holotype from Arratia 2013, overlay drawing from Agassiz 1845.

Figure 4. Pholidophorus holotype from Arratia 2013, overlay drawing from Agassiz 1845.

Robustichthys luopingensis (Xu et al. 2014; Xu 2019; Middle Triassic) was described as the largest holstean fish of the Middle Triassic, but here the clade Holostei is polyphyletic and Robustichthys nests alongside the armored catfish, Hoplosternum. The long frontal extends posterior to the orbit. The expanded jugal is split up. The maxilla is absent, as in all catfish.

Figure 2. Subset of the LRT focusing on basal vertebrate (fish).

Figure 2. Subset of the LRT focusing on basal vertebrate (fish).

You may wonder
how the LRT is able to recover so many novel solutions that fall outside mainstream hypotheses. Evidently students and professors follow textbooks, the textbooks the professors write and update. The LRT confirms and/or refutes textbooks as it tests every possible combination of the 1548 taxa now employed. It does not matter that the multi-state character count is only a fraction of the taxon count. It does not matter if I do not see the specimen in person and use a professionally rendered drawing (Fig. 3).

What matters is taxon inclusion.
You can’t tell who is related to who else unless you invite them all to participate. That is the number one issue separating this growing online hypothesis of interrelationships and all others.

What matters is lumping and separating all tested taxa
to get a completely resolved tree and making sure that all sister taxa document a gradual accumulation of derived traits (which could include losses of certain bones or bone processes.

Reporting results that differ from the mainstream is not a crime
and does not injure reputations, in the long run. The competition is fierce for discoveries and those who invest heavily into their PhDs and the papers they write fear they have the most to lose. Contra this hypothesis, I’ve never seen a PhD pilloried for making a mistake (unless it was in a tent out in the field with a female student). So all you PhDs out there… relax. If you don’t want to fix the problems in our field of study by including a wider gamut of taxa, let the LRT do it.

As you’ll find out someday, some traditions and paradigms are wrong.
Don’t trust authority. Don’t trust the LRT. Find out for yourself which hypotheses are wrong and right by running your own tests. Let me know if you find a different tree topology than the LRT.


References
Xu G-H 2019. Osteology and phylogeny of Robustichthys luopingensis, the largest holostean fish in the Middle Triassic. PeerJ 7:e7184 DOI 10.7717/peerj.7184
Xu G-H, Zhao L-J and Coates MI 2014. The oldest ionoscopiform from China sheds new light on the early evolution of halecomorph fishes. Biology Letters 10(5):20140204
DOI 10.1098/rsbl.2014.0204.

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