Updated Nov 21, 2019
with new bone identities for Entelognathus.
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.
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.
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.
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.