Early Devonian tetrapodomorph: Brindabellaspis

Short summary:
Another taxon misdirected due to taxon exclusion.

King et al. 2018 acid-prepared and redescribed
an odd-sort of Early Devonian ‘placoderm’ unlike any other, Brindabellaspis stensioi (Young 1980). All prior authors considered this taxon a placoderm distinct from all others and perhaps close to the base of the clade.

King et al. report, 
“As interpreted, the skull roof pattern of Brindabellaspis is unique.”

That’s a red flag. The authors restricted their search to just placoderms. Expand the taxon list to properly nest this taxon. In the large reptile tree (LRT, 1758+ taxa) Brindabellaspis nested with Elpistostege (Fig. 2), a Late Devonian tetrapodomorph. Force-shifting Brindabellaspis to the placoderms adds 17 steps based only on traits from the top of the cranium,

“The new specimens of Brindabellaspis provide information on the anterior part of the skull, demonstrating the presence of an elongate premedian plate, and the dermal bone pattern anterior to the orbits. This places the centre of the orbits in the posterior half of the skull roof length.”

Another red flag. No placoderms have an elongate preorbital region.

“Possibly both incurrent and excurrent nasal openings were dorsal in position for Brindabellaspis, because the floor of the orbit and preorbital space probably occluded any ventral passage.”

A third red flag. No placoderms have that narial arrangement. The authors are guessing without the benefit of phylogenetic bracketing. Here the naris is restored near the jawline, as in LRT relatives like Koilops and Elpistostege (Fig. 2).

Taxon exclusion by King et al. 2020
results in a lack of understanding for the clade itself and outgroups for the Placodermi. Add taxa without bias to solve this problem.

When you add taxa,
you discover Brindabellaspis is not a placoderm after all. No wonder it had a unique skull roof pattern. How many times have workers attempted to force a new taxon into the wrong clade? Pterosaurs. Caseasaurs. Vancleavea. Cartorhynchus. Mesosaurs. Mysticetes. Turtles. Snakes. The list goes on and on. Taxon exclusion is the number one problem in paleontology over the past many decades. Apparently no one wants to test competing candidates with a wide gamut of taxa, like the LRT. Workers prefer to cherry-pick the ones they want to test and end up perplexed.

In the LRT Brindabellaspis nests as the earliest known tetrapodomorph,
a clade previously and traditionally restricted to Late Devonian fossils and Middle Devonian tracks. That Brindabellaspis is not the most primitive tetrapodomorph in the LRT provides one more clue that the genesis of many fish clades, including the Tetrapodomorpha, preceded the Early Devonian.

Fish workers applying fish nomenclature to skull bones
did not acknowledge the tetrapod homologs they were dealing with. This is an academic tradition sealed in textbooks and taught by professors to their eager students. Let us all agree to start using tetrapod homologs for all vertebrate skulls and skeletons, wherever possible. That will help the next generation of paleontologists understand the homologs present in all vertebrates.

Recovery of Brindabellaspis,
not at the base of the tetrapodomorpha, but as it’s earliest known representative, appears to be a novel hypothesis of interrelationships. If others have presented this hypothesis previously let me know so I can promote that citation.

References
King B, Young GC and Long JA 2018. New information on Brindabellaspis
stensioi Young, 1980, highlights morphological disparity in Early Devonian placoderms. R. Soc. open sci. 5: 180094. http://dx.doi.org/10.1098/rsos.180094
Young GC 1980. A new Early Devonian placoderm from New South Wales, Australia, with a discussion of placoderm phylogeny. Palaeontographica Abt. A Palaeozool-Stratigr. 167, 10–76.

wiki/Brindabellaspis 
wiki/Elpistostegalia
wiki/Tetrapodomorpha