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A postfrontal looking and acting exactly like a hyomandibular

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I was walking around with a cartoon question mark over my head
for the longest time whenever I studied Jagorina (Fig 2) and Gemuendina (Fig 3), two ray-like traditional placoderms. I wondered, ‘what were the transitional taxa that connected them with more traditional placoderms?’ It seemed they tended to stand apart from all the others.

In evolution no taxon should ever stand alone or apart.

Once again,
it took a plesiomorphic Bauplan to understand an otherwise cryptic derived taxon. This time the largely traditional, but notably ultrawide-skull placoderms, Stenosteus (Fig 2) and Titanicthys (Fig 1) provided that transitional Bauplan. Both seem to have been weak-jawed bottom feeders sifting and filtering quantities of sand for randomly buried prey items.

Figure 1. Late Devonian Titanichthys compared to scale with Late Silurian Entelognathus. See figure 2. ” data-image-caption=”

Figure 1. Late Devonian Titanichthys compared to scale with Late Silurian Entelognathus. See figure 2.

” data-medium-file=”https://pterosaurheresies.files.wordpress.com/2023/11/titanichthys-cleveland_skull588-1.gif?w=212″ data-large-file=”https://pterosaurheresies.files.wordpress.com/2023/11/titanichthys-cleveland_skull588-1.gif?w=584″ src=”https://pterosaurheresies.files.wordpress.com/2023/11/titanichthys-cleveland_skull588-1.gif” alt=”Figure 1. Late Devonian Titanichthys compared to scale with Late Silurian Entelognathus. See figure 2.” class=”wp-image-82599″ />

Figure 1. Late Devonian Titanichthys compared to scale with Late Silurian Entelognathus. See figure 2 for more closely related taxa. Note the large red quadrate, the jaw joint, and its relationship to the jaw joint anchor, the hyomandibular (dark green), a bone that started externally, but soon moved internally to do its job.

Starting with Titanichthys
(Fig 1) note the proximity of the postfrontal (orange) to the fused postorbital-preoperuclar-lacrimal (amber-light yellow-tan) compared to the more posterior hyomandibular (dark green in Fig 1). Also note the medial connection of the postfrontal to the two small skull bones, the intertemporal and supratemporal (yellow-green and green).

In almost all gnathostomes the hyomandibular connects to the jaw joint (the red quadrate) with an internal process (not shown here).

If you’re looking for teeth, these taxa probable had a premaxilla pasted beneath the tiny nasals, but the maxilla had not yet appeared.

Figure 2. Jagorina (left) and Stenosteus (right). Colors added here. Note the fusion of the parietals, the loss of the frontals and nasals and the connection of the bar-like postfrontal to the fused preorbital-preopercular-lacrimal in Jagornia. That same connection is not made in Stenosteus, which has a more primitive skull. ” data-image-caption=”

Figure 2. Jagorina (left) and Stenosteus (right). Colors added here. Note the fusion of the parietals, the loss of the frontals and nasals and the connection of the bar-like postfrontal to the fused preorbital-preopercular-lacrimal in Jagornia. That same connection is not made in Stenosteus, which has a more primitive skull.

” data-medium-file=”https://pterosaurheresies.files.wordpress.com/2023/12/jagorina_stenosteus_skull588.gif?w=300″ data-large-file=”https://pterosaurheresies.files.wordpress.com/2023/12/jagorina_stenosteus_skull588.gif?w=584″ src=”https://pterosaurheresies.files.wordpress.com/2023/12/jagorina_stenosteus_skull588.gif” alt=”Figure 2. Jagorina (left) and Stenosteus (right). Colors added here. Note the fusion of the parietals, the loss of the frontals and nasals and the connection of the bar-like postfrontal to the fused preorbital-preopercular-lacrimal in Jagornia. That same connection is not made in Stenosteus, which has a more primitive skull. ” class=”wp-image-82693″ />

Figure 2. Jagorina (left) and Stenosteus (right). Colors added here. Note the fusion of the parietals, the loss of the frontals and nasals and the connection of the bar-like postfrontal to the fused preorbital-preopercular-lacrimal in Jagornia. That same connection is not made in Stenosteus, which has a more primitive skull. The postfrontal was identified here based on its geographic relationship with other bones, not by what it does in this case, which is different from nearly all other vertebrates.

Compare the Titanichthys setup with the similar, but different elements
in the more derived wide-skull of Jagornia (Fig 2 left). Also compare Titanichthys (Fig 1) to the more primitive Stenosteus (Fig 2 right), which is narrower and closer to Coccosteus Fig 2 lower right corner. In Jagornia we see the fusion of the parietal elements, the loss of the frontals + nasals and the retained connection of the now bar-like postfrontal to the fused preorbital-preopercular-lacrimal, now also reduced to a curved bar forming the palate.

The same postfrontal connection is made in Gemuendina (Fig 3), a ray-like relative of Jagorina. The quadrate (red) is reduced to a vestige in both. The hyomandibular remains a broad surface plate in both alongside the tabulars (light red).

Figure 3. The ray-like placoderm Gemuendina skull in situ. Colors added here. Compare to Jagorina in figure 2. Note the plate-like hyomandibular (dark green) not associated with the mandible, which is anchored here by the postfrontal (orange). ” data-image-caption=”

Figure 3. The ray-like placoderm Gemuendina skull in situ. Colors added here. Compare to Jagorina in figure 2. Note the plate-like hyomandibular (dark green) not associated with the mandible, which is anchored here by the postfrontal (orange).

” data-medium-file=”https://pterosaurheresies.files.wordpress.com/2023/12/gemuendina-stuertzi_kgm-1983_306.jpg?w=187″ data-large-file=”https://pterosaurheresies.files.wordpress.com/2023/12/gemuendina-stuertzi_kgm-1983_306.jpg?w=584″ src=”https://pterosaurheresies.files.wordpress.com/2023/12/gemuendina-stuertzi_kgm-1983_306.jpg” alt=”Figure 3. The ray-like placoderm Gemuendina skull in situ. Colors added here. Compare to Jagorina in figure 2. Note the plate-like hyomandibular (dark green) not associated with the mandible, which is anchored here by the postfrontal (orange).” class=”wp-image-82695″ />

Figure 3. The ray-like placoderm Gemuendina skull in situ. Colors added here. Compare to Jagorina in figure 2. Note the plate-like hyomandibular (dark green) not associated with the mandible, which is anchored here by the postfrontal (orange).

Now that the cartoon question mark over my head has popped,
and the (hypothetical) solution is shared, it’s worthwhile to really appreciate the prevalence of convergence in vertebrates. Problems like this, especially when not recognized, lead to incorrect scoring in phylogenetic analysis. This is why the fish subset of the LRT is taking so long to complete. Hope this helps in your own studies.

If this is already taught at the uiniversity level, let me know.


Source: https://pterosaurheresies.wordpress.com/2023/12/02/a-postfrontal-looking-and-acting-exactly-like-a-hyomandibular/


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