Placoderm Entelognathus skull bones re-identified with tetrapod homologies

Images repaired May 18, 2017 after studying photos of the specimen, comparing related taxa and dispensing with false paradigms. Click here for more details. 

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 understood by the original authors, who identified the bones with traditional placoderm names. (Ala, placoderms, bony fish and sacropterygians, including tetrapods, have different names for the same bone). 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. And this was my first day studying placoderms.

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.

Figure 1. Entelognathus drawings from Zhu et al. 2013, with colors and homologous tetrapod bone. abbreviations added.

Figure 1. Entelognathus drawings from Zhu et al. 2013, with colors and homologous tetrapod bone. abbreviations added. This revised image adds a small triangular frontal between the anterior processes of the parietal and the rest of the bones follow suit. 

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 triangular frontal was overlooked
  6. The pineal plate is a pair of parietals
  7. The central plate is a pair of postparietals
  8. The marginal plate is the supratemporal
  9. The anterior paranuchal plate is the tabular
  10. The opercular is the quadratojugal
Figure 2. Cheirolepis skull (left) with skull bones colorized as in Osteolepis (right) and Enteognathus, figure 1. Colors make bone identification much easier. Note the post opercular bone differences between Osteolepis and Cheirolepis indicating separate and convergent derivation, based on present data.

Figure 2. Cheirolepis skull (left) with skull bones colorized as in Osteolepis (right) and Enteognathus, figure 1. Colors make bone identification much easier. Note the post opercular bone differences between Osteolepis and Cheirolepis indicating separate and convergent derivation, based on present data.

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 postparietal 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 what? 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, making a case in point. A pineal opening is not present in the pineal plate (fused parietals) of Enteleognathus. This is further evidence that the pineal opening migrated from the frontals to the parietals over tens of millions of years. More on that tomorrow.

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 alwaybranches outside the radiation of living jawed vertebrates, meaning that key components othe osteichthyan face are no longer unique innovations of that group. Second, acanthodians — that pivotal assortment of extinct shark-like fishes — are shifted, en masse, tthe 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 bonarmor othe sort common to both placoderms anbony 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 the present 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, which was one source of the earlier problems I had, no hopefully settled.

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

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3 thoughts on “Placoderm Entelognathus skull bones re-identified with tetrapod homologies

  1. Homology with actino- and sarcopterygian skull bones is a major point of the paper. Read the supplementary information! It even contains a table showing these homologies, and pictures that color-code this!!!

    You don’t even get things right. The “central plate” is the fused postparietals; that’s why it doesn’t contain the pineal foramen.

  2. Dave, thanks for the insight. This was my first day with placoderms and I was confused by the SuppData with color-coded homologies. Also came to a fork in the road with regard to the splitting or not splitting of the Osteolepis postparietals and finally took the right fork. How I worked the rest out was to go to the fossil itself, and I found a few errors and possible errors in the drawings. This will be the subject of a future blog. The cussing, deleted from your reply, is going to put your replies in my spam folder. Take the grunt out and just throw the ball.

  3. I agree, though, that the authors shouldn’t have hidden the anatomy so deep in the supp. inf.; it only starts on p. 53, after the character list, the matrix (which is given in a rather useless format) and the apomorphy list… this is important stuff!

    And so, our love-hate relationship with Nature continues. On the one hand, we abhor the extended abstracts that force us to call the actual paper “supplementary information”; on the other, we need the impact factor…

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