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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Poposaur mandibles

There’s still the question of Effigia’s mandible hanging out there.
The question is: “Is that a predentary or a dentary at the tip?” Fig. 1). Nesbitt (2007) says dentary. I say predentaries. Let’s look at the evidence.

To answer that,
I took a comparative survey of poposaur mandibles (Fig. 1), looking for evolutionary patterns and thereby strive to provide an update to the predentary/dentary question. Surprisingly, in the case of Effigia, when you add in the splenials, which neither Nesbitt nor I did before, the mandibular fenestra becomes substantially reduced. That may be similar to what one sees in Lotosaurus, in which the elements are not jumbled. And that provides more substance to the “predentary” argument. Other than Lotosaurus, the closest sister is Shuvosaurus, which is known from an incomplete mandible (Fig.1) showing similar patterns over the remaining portions. Shuvosaurus has something similar to what I saw in Daemonosaurus, that others consider something else. In any case, at some point, something interesting developed in front of the dentaries in certain phytodinosaurs.

The other question is,
when something similar to a predentary appears in front of the dentary, as in Sacisaurus (Figure 1), should it be considered a “beak” rather than a premaxilla? This bone may be paired, as it is in Sacisaurus, rather than a single median bone, as in the predentary of Heterodontosaurus (Fig. 1).

Figure 1. Poposaur (and kin) mandibles. Here are Daemonosaurus, Poposaurus, Pisanosaurus, Heterodontosaurus, Sacisaurus, Lotosaurus, Effigia and Shuvosaurus. The mandibles of Lotosaurus and Effigia appear to share a common heritage of design.  In Effigia the splenial reduces the mandibular fenestra helping to clarify the identify of the dentary and premaxilla (or beak).

Figure 1. Poposaur (and kin) mandibles. Here are Daemonosaurus, Poposaurus, Pisanosaurus, Heterodontosaurus, Sacisaurus, Lotosaurus, Effigia and Shuvosaurus. The mandibles of Lotosaurus, Shuvosaurus and Effigia appear to share a common heritage of design. In Effigia the splenial reduces the mandibular fenestra helping to clarify the identify of the dentary and premaxilla (or beak). The extension of the angular to the predentary is unique to this clade.

If all these other mandibles had a premaxilla or beak (or the possibility of one), is there any reason to suspect that Effigia did not?

The original reconstructions of the Effigia mandible
introduced us to the largest mandibular fenestra I have ever seen relative to the size of the jaw. The new reconstruction reduces the fenestra length and, no doubt, produces a stronger jaw with the splenial (lavendar to iris blue bone) laminated to the medial side and edges.

Typically the mandibular fenestra splits the surangular from the angular,
as it does in Heterodontosaurus. However, in Lotosaurus the mandibular fenestra develops largely below the dentary with very little surangular and angular exposure. In Shuvosaurus the same pattern could play out, but unfortunately the key parts are missing (perhaps due to a very large mandibular fenestra?). This is a different pattern than in ornithischians, saurischians and theropods. And this pattern is also different from rauisuchians. Among euarchosauriforms, only in aetosaurs does the very large mandibular fenestra develop largely below the dentary. In others, the fenestra develops midway or beneath the surangular and it doesn’t get to the size seen in Effigia and Lotosaurus.

One final point
The suture between the two premaxillae in Effigia is convoluted like a puzzle piece. In this way they are locking themselves together, convergent with the central or fused premaxilla of ornithischians, but homologous with the premaxilla in Lotosaurus and Shuvosaurus.

If I’m wrong, show me some data. At this  point, at least it’s worth talking about.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Ferigolo J and Langer MC 2006. “A Late Triassic dinosauriform from south Brazil and the origin of the ornithischian predentary bone”Historical Biology 19 (1): 1–11. online pdf.
Nesbitt SJ and Norell MA 2006. Extreme convergence in the body plans of an early suchian (Archosauria) and ornithomimid dinosaurs (Theropoda). Proceedings of the Royal Society B 273:1045–1048. online
Nesbitt S 2007. The anatomy of Effigia okeeffeae (Archosauria, Suchia), theropod-like convergence, and the distribution of related taxa. Bulletin of the American Museum of Natural History, 302: 84 pp. online pdf

AMNH Effigia webpage

wiki/Effigia