Placoderm Entelognathus skull bones re-identified with tetrapod homologies

Barford 2013 wrote: 
“It may be hard to see, but you seem to share a family resemblance with Entelognathus primordialis. The fish, which lived 419 million years ago in an area that is now part of China, is the earliest known species with a modern jaw.” Here (Fig. 1) one can identify a complete set of homologous tetrapod skull bones overlooked by the original authors, who identified the bones with traditional placoderm names. And they made a mistake or two along the way, none of which negate their conclusions, but cement them.

I never thought I’d be featuring any placoderm fish in this blog
or in ReptileEvolution.com, but Entelognathus, as everyone already knows — and I just learned, is something very special. A major discovery.

Barford 2013 reported, “Palaeontologists have traditionally believed that the fishes’ features bore no relation to ours. They assumed that the placoderm face was lost to evolutionary history, and most thought that the last common ancestor of living jawed vertebrates had no distinct jawbones — that it was similar to a shark, with a skeleton made mostly of cartilage and at most a covering of little bony plates. The theory went that the bony fishes evolved later, independently developing large facial bones and inventing the ‘modern’ jaw. Such fishes went on to dominate the seas and ultimately gave rise to land vertebrates. [Entelognathus] has what looks like a bony fish’s jaw, even though it is older than the earliest known sharks and bony fishes.”

According to Wikipedia
Entelognathus primordialis (Zhu et al. 2013; Late Ludlow, Silurian, 419 mya; IVPP V18620) “is a genus of placoderm fish with dermal marginal jaw bones (premaxilla,
maxilla and dentary), features previously restricted to Osteichthyes (bony fish).”

More than that,
all of the skull bones find homologies in tetrapods and bony fish (Figs. 1, 2) when certain bones are correctly identified or homologized. It just takes a few colors here and there to make it all clear.

All of the bones in the skull of Entelognathus
find homologies with those in Cheirolepis (Whiteaves 1881; Fig. 2) and also with tetrapods. Entelognathus lived 59 million years before the appearance of tetrapods like Ichthyostega. and is someday going to be a part of the story behind those Middle Devonian footprints.

Here new labels and colors
repair original errors and indicate tetrapod homologies in Entelognathus (Zhu et al. 2013).

1. Three purported sclerotic bones are circumorbital bones (prefrontal, postfrontal, jugal)
2. The purported jugal is the dorsal half of the maxilla before these bones fused.
3. The purported quadratojugal is the posterior of the maxilla
4. The rostral is the nasal
5. The pineal plate is a set of fused frontals
6. The central plate is a set of fused parietals
7. The nuchal plate is the postparietal
8. The marginal plate is the intertemporal
9. The anterior paranuchal plate is the supratemporal
10. The posterior paranuchal plate is the tabular
11. The opercular is the quadratojugal

On the subject of nomenclature
Zhu et al. 2013 (SuppData) list the various names given to fish skull bones and their homologies in other fish clades. Some of the more confusing include:

1. The parietal in sarcopterygians is the frontal in actinopterygians and the preorbital in placoderms.
2. The post parietal in sarcopterygians is the parietal in actinopterygians and the central in placoderms.
3. The supratemporal in sarcopterygians is the intertemporal in actinopterygians and the marginal in placoderms.
4. The tabular in sarcopterygians is the supratemporal in actinopterygians and the anterior paranuchal in placoderms.
5. And there are others…

Where is the authority that can fix this problem? But if we fix it, then all prior literature will have to be translated. Either way, we’re hosed. Maybe we should just colorize homologous bones and leave it at that, as Zhu et al. did in their SuppData.

Entelognathus precedes Cheirolepis by 29 million years.
Preopercular and opercular bones do not appear in Entelognathus, but are present in Cheirolepis. So they are new bones in osteichythys.

The ‘al’ bone in Entelognathus (Fig. 1) is the cleithrum, supporting the pectoral fin.

The split (spiracle) between the skull roofing bones (intertemporal. supratemporal, tabular) and cheek bone (squamosal) do not appear in Entelognathus, but do so in Cheirolepis.

Sclerotic rings are not necessary in such small and well-protected eyes as in Entelognathus and if present, would have been very tiny and fragile.

Comparisons of the circumorbital bones in Entelognathus and Cheirolepis are strikingly similar down to the small post-orbit depression in the jugal in Entelognathus that becomes a notch in Cheirolepis.

Comparisons of the postopercular bones
of Cheirolepis and Osteolepis (Fig. 2) show little to no homology, suggesting a possible separate but convergent derivation.

Note some skull bones
later split apart at the median, while others fuse together. It’s their shapes and locations that identify them. “The large hexagonal central plate seems to have a single ossification centre, whereas most placoderms have paired centrals,” reports Zhu et al. A pineal opening is not present in the pineal plate (fused frontals) of Enteleognathus. This is further evidence that the pineal opening migrated from the frontals to the parietals over tens of millions of years.

Barford 2013 concludes
“There remains a chance that E. primordialis evolved its jaw independently from the bony fish, so that we did not inherit it, and the resemblance is an illusion.” I don’t agree with this conclusion. The evidence for homology elsewhere overwhelms any competing hypotheses.

Friedman and Brazeau (2013) also comment on this discovery.
“First, Entelognathus always branches outside the radiation of living jawed vertebrates, meaning that key components of the osteichthyan face are no longer unique innovations of that group. Second, acanthodians — that pivotal assortment of extinct shark-like fishes — are shifted, en masse, to the branch containing the cartilaginous fishes. This triggers a cascade of implications. If all acanthodians are early cartilaginous fishes, then their shark-like features are not generalities of jawed vertebrates, but specializations of the cartilaginous-fish branch. The most recent common ancestor of jawed vertebrates was thus probably clad in bony armor of the sort common to both placoderms and bony fishes. This inversion of a classic scenario in vertebrate evolution raises an obvious question: how did we get it so wrong?”

In summary
Even when someone gets it right, some of the details may still be correctable – and those corrections do not overturn the conclusion, but support it. As usual, I have not seen the fossil firsthand. I have not added Entelognathus to the LRT. I simply make comparisons to published figures of Cheirolepis, something the original authors failed to do.

Thanks to David M.
for directing me to the Entelognathus paper. : – )

Please let me know
if someone else has drawn the same insight in the last 4 years since the publication of Zhu et al. 2013. If so, I am unaware of it.

References
Barford E 2013. Ancient fish face shows roots of modern jaw. Nature News. online here.
Friedman M and Brazeau 2013. A jaw-dropping fossil fish. Nature 502:175-177. online here.
Whiteaves JF 1881. On some remarkable fossil fishes from the Devonian rocks of Scaumenac Bay, in the Province of Quebec. Annals and Magazine of Natural History. 8: 159–162.
Zhu M, Yu X-B, Ahlberg PE, Choo B and 8 others 2013. A Silurian placoderm with osteichthyan-like marginal jaw bones. Nature. 502:188–193.

wiki/Cheirolepis
wiki/Entelognathus