The origin of gnathostomes in the LRT — updated

Recent papers have shed light on the origin(s) of jaws
in vertebrates transitioning to the clade Gnathostomata. Freshman mistakes were made earlier in the large reptile tree (LRT, 2164 taxa) which recovered a dual origins of jaws: one in tiny placoderms, like Shenacanthus (Fig 5), and another between sturgeons and Chondrosteus (Fig 6). After the addition of new taxa and recent housekeeping in the LRT, that hypothesis is no longer valid. Corrections are part of the process as new taxa shed new new light.

Now sturgeons nest much more basally in the LRT, far from the origin of jaws. Chondrosteus nests much later, following placoderms.

Here are the current steps in broad brush strokes
for the origin of jaws in the LRT (Figs 1–6) starting with a flat, jawless placoderm, Qilinyu, from the Late Silurian. That’s far after the origin of jaws prior to the Early Silurian. So this was a late surviving taxon from an earlier radiation.

Figure 1. Jawless placoderm Qilinyu in palatal view. Animated Meckel’s cartilage here.

Late Devonian Bothriolepis
was also jawless (Fig 2), but had a mobile upper lip created by the (pink) nasals and bordered by a (yellow) toothed premaxilla. Perhaps these were used for scraping benthic sediments or corals.

Figure 2. Bothriolepis palate view animated.

Fragile mobile mandibles
appear with tiny Latest Silurian Bianchengichthys (Fig 3). This was another late survivor of an earlier Early Silurian genesis of more substantial jaws (Figs 4, 5).

Figure 3. Tiny Bianchenichthys documents the first appearance of jaws in the gnathostome lineage of placoderms, distinct from the convergent ptyctodont origin of jaws in figure 3.

It is either ironic or unfortunate
that the phylogenetic acquisition of jaws does not chronologically follow fossil examples from rare sediments. Bearing that in mind, toothless Coccostesus (Fig 4) from the Middle Devonian represents the next stage in mandible evolution with sharp, strong biting elements.

Figure 4. Austroptyctodus animated. Here the bones are labeled according to Coccosteus (above), the outgroup taxon of the Ptyctodontidae. Note the fusion of nasal elements (pink) with palatine elements (indigo). In front of the lateral view is a central nasal bone in anterior view flanked by two hypothetical nares as in Coccosteus at the top of the image.

Coccosteus ancestors gave rise to famous giant predatory placoderms
along with smaller, less famous ptyctodontids, like Austroptyctodus (Fig 4) This clade is characterized by a narrower skull with fewer facial and temporal bones.

Figure 5. Tiny Early Silurian Shenacanthus documents the earliest appearance of gnathostome jaws.

Stepping back to the Early Silurian
is wasp-sized Shenacanthus. It had a dentary, but lacked a premaxilla and maxilla. The upper tooth plate (palatine) erupted from the arching lacrimal, medial to the postorbital (amber), as in Coccosteus, the ANU V244 specimen and probably Entelognathus.

Figure 6. The Early Silurian ‘soft placoderm’, Loganellia is a transitional taxon from placoderms to sharks.

By the Early Silurian gnathostomes were radiating.
One of these was Loganellia (Fig 6), best described as ‘soft placoderm’ transitional to basal sharks, like Chondrosteus (Fig 7) and acanthodians like Climatius.

Figure 7. Chondrosteus revised and animated. This toothless taxon is derived from soft placoderms like Loganellia (Fig 6). Apparently all teeth were absent in this Early Jurassic late survivor of an Early Silurian radiation.

Housekeeping continues
on the LRT. Traditional problems are finding new solutions by trait analysis.

References
Li Q et al. 2021. A new Silurian fish close to the common ancestor of modern gnathostomes. Current Biology 31:3613–3620. doi.org/10.1016/j.cub.2021.05.053
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.
Zhu M et al. 2016. A Silurian maxillate placoderm illuminates jaw evolution. Science 354.6310 (2016): 334-336.

wiki/Bianchengichthys
wiki/Gnathostomata