Entelognathus: revisions

Yesterday we looked at Entelognathus (Figs. 1-3; Zhu et al. 2013), a Silurian placoderm transitional to bony fish. That was my first placoderm and I made some errors that have since been corrected. Those errors were corrected when I realized the frontal (pineal in placoderms and Cheirolepis) originated as a tiny median (purple) triangle that included the pineal opening. I was also confused by the splitting of the parietal in Osteolepis, which I thought gave rise to the parietal/postparietal split, but instead that is an autapomorphy arising only in certain Osteolepis specimens. Further confusion comes from the fusion of bones, the splitting of bones and the different names given to the same bone in Silurian to Devonian taxa. Because of this, today and today only I will call the bones by the colors provided by Zhu et al. A key to their various names is provided (Fig. 1).

I was also surprised
to see that Zhu et al. 2013 found no trace of a purple/orange division in Entelognathus (Fig. 1f). This is odd for a transitional taxon, but still possible. Worth looking into. Equally odd, Zhu et al. did not color the purple bone consistently (Fig. 1).

The pineal opening drift
from the purple to the orange bones attends the lengthening of the rostrum and perhaps the brain and olfactory regions. The purple bone invades the paired orange bones and at the posterior tip of the parietal is the pineal opening. So the purple bone more or less delivers the pineal opening more or less in the middle of the orange bones.

Figure 1. From Zhu et al. 2013 SuppData showing placoderm and other basal vertebrate skull roofs. Note: Entelognathus is the only taxon without frontals, which I found in the photos of the fossil, figure 2.

Figure 1. From Zhu et al. 2013 SuppData showing placoderm and other basal vertebrate skull roofs. Note: Entelognathus is the only taxon without a frontal/parietal split, which I found in the photos of the fossil, figure 2 and corrected at the tip of the long arrow.

I traced bone sutures on photos of the specimen
and found that purple/orange division. So now Entelognathus has a complete set of skull roofing bones from the nasal to the frontal to the parietal and post parietal. I may have even seen where the yellow green intertemporal splits from the orange parietal.

Figure 2. Entelognathus fossil. Scale bar = 1 cm. Here the frontal/parietal division is shown.

Figure 2. Entelognathus fossil. Scale bar = 1 cm. Here the frontal/parietal division is shown. Rather than a median uture, one finds a medial ridge.

I hope to never do another fish.
But happy that I was able to resolve some earlier questions and move on. Feelings aside, mistakes that go on unnoticed are worse than mistakes you, or others, find and correct.

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

Figure e. Entelognathus drawings from Zhu et al. 2013, with colors and homologous tetrapod bone. abbreviations added. Corrected from an earlier version.

References
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.

Cheirolepis fossil images
wiki/Cheirolepis
wiki/Eusthenopteron
wiki/Entelognathus

 

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One thought on “Entelognathus: revisions

  1. Equally odd, Zhu et al. did not color the purple bone consistently (Fig. 1).

    In the cases where they didn’t color anything purple, they evidently thought the pineal bone either wasn’t there or couldn’t be identified with sufficient confidence. Obviously that’s where it gets subjective; I, for one, don’t have an opinion on whether that wedge-shaped bone in Romundina is the pineal or a postrostral (or, who knows, a fusion of both).

    I’m also not convinced that the pineal is homologous to the frontals. Eusthenopteron has both a median bone that could be the pineal and a series of bone pairs lateral to it that could be the frontals and/or nasals and/or who knows what. (Traditionally, at least sometimes, the median bone has been identified as “fused frontals”; AFAIK, there’s no evidence either way.)

    I traced bone sutures on photos of the specimen
    and found that purple/orange division.

    Then please show us the suture. All you’re showing us are small pictures where the border between purple and orange completely hides the supposed suture under itself.

    The paper’s figure 2d, if you look at it at 300% or 400% magnification, shows clearly that the suture not only isn’t there, but would cut right through an ossification center. The neat thing about ridges as bone ornamentation (except in ontogenetically old individuals sometimes) is that they are parallel to the directions of growth of each bone; where they meet in a star shape, the bone is oldest; where the ridges from different such stars meet, you have a suture (or a very recently fused one) even if preservation is such that you can’t actually see the suture itself. You can see this very well in fig. 5b of the paper and in supp. fig. 13c, and perhaps best of all in supp. fig. 16.

    That’s also how you can tell the postparietals (“centrals”) must have fused early in the ontogeny of Entelognathus: their ornamentation forms a single star, not two. This is beautifully visible in fig. 5b and supp. fig. 13c, 16.

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