How Early Devonian Brindabellaspis became a sister to coeval Diandongpetalichthys

Earlier
the large reptile tree (LRT, 2173 taxa) nested the strange, long-nose Early Devonian ‘placoderm’, Brindabellaspis (Young 1980, Fig 1) with Tiktaalik, a superficially similar, flat-head lobe-fin from the Late Devonian. Back then, in my freshman naiveté, I colored the skull bones following a tetrapod pattern. I even added a hypothetical mandible. That seemed appropriate at the the time, especially given the Middle Devonian tetrapod tracks, but that was a mistake.

Like all fish,
some skull bones split while others fuse in patterns that can recall tetrapod homologies. Other times they go their own way. It took time to understand the code, the pattern, the exceptions. After 60 days of housekeeping 414 fish taxa the new tree topology is once again fully resolved.

Recent housekeeping
has rescored many taxa, including Brindabellaspis. A new restoration (Fig 1) fills in the missing parts in a basal, jawless placoderm pattern. Only fossil data was scored. DGS colors seem to identify bones faster and easier than abbreviated labels and arrows do when it comes to comparative anatomy and recognizing synapomorphies.

In the end the best match for Brindabellaspis
was with the much smaller and much simpler, but coeval Diandongpetalichthys (Fig 1), a taxon that gives us a peek beneath the skull to view the oral and branchial cavity. This skull is essentially an atrium without a floor prior to the invention of jaws. Note the primitive placement of the orbits, visible both dorsally and ventrally, right on the edge, as in finless precursors, like Drepanaspis and Poraspis. You’ll note the gradual lengthening of the rostrum takes place in oly this clade. The other placoderms don’t do this.

Brindabellaspis stensioi
(Young 1980; King B, Young GC and Long JA 2018; Early Devonian) was originally considered a strange sort of long-rostrum placoderm. Here it nests with Diandongichthys, another flat-head placoderm. The wrap-around nasal and jugal are restored here.

Diandongpetalichthys liaojiaoshanensis
(Zhu et al. 1991; Early Devonian) is a petalichthid placoderm close to Lunaspis. The postorbital is restricted to the lower orbit. Jaws are lacking.

Rarely
do reference lists include authors whose names begin only with Y and Z. Today is one of those rare days.

References
Young GC 1980. A new Early Devonian placoderm from New South Wales, Australia, with a discussion of placoderm phylogeny. Palaeontogr. Abt. A Palaeozool-Stratigr. 167: 10–76.
Zhu M 1991. New information on Diandongpetalichthys (Placodermi: Petalichthyida). Early vertebrates and related problems of evolutionary biology. Beijing: Science Press. 179–192.
Zhu et al. 2012. An antiarch placoderm shows that pelvic girdles arose at the root of jawed vertebrates. Biology Letters Palaeontology 8:453–456.

wiki/Lunaspis
wiki/Shearsbyaspis – not yet posted
wiki/Wuttagoonaspis
wiki/Diandongpetalichthys
wiki/Brindabellaspis