The origin of jaws in the placoderm clade

If you ever wondered
how the under-slung parasagittal oral elements of jawless fish with a transverse hinge (Fig 1) evolved into the more familiar arc with paired lateral hinges (Figs 2–4), here’s a series of graphics that show a step-by-step evolution. This series applies only to the placoderm clade and all taxa that descended from placoderms.

As recovered earlier, stem tetrapod jaws developed in a different clade of fish. Thus the traditioinal clade, ‘Gnathostomata‘ is no longer monophyletic. See Birkenia for details.

Figure 1. Images from Lanzetti et al 2023, animated here. This is the oral apparatus of Rhinopteraspis after µCT scanning. These are not jaws.

Rhinopteraspis
(Fig 1) demonstrates the jawless condition in pre-placoderms. The mobile oral elements opened like a castle draw bridge. Note the transverse hinge of the multiple elements and the parasagittal orientation of the medial elements, unlike the jaws in extant vertebrates.

Figure 2. Qilinyu skull in ventral view with original identifications and revised here. The barely conjoined elements of the mandible that once were the tips of the ancestral oral elements shown in figure 1, now gone. The motion is the same. The axis of rotation has migrated to the lateral skull.

The tentative genesis of jaws in Late Silurian Qilinyu
(Fig 2) demonstrates the transition from parasagittal elements in jawless pre-gnathostomes to lateral elements in basal gnathostomes. Note the disappearance of all but the anterior tips of the former mobile oral elements (Fig 1), now tentatively conjoined, but still moving in the same limited arc. This is a very weak jaw.

Figure 3. In Bianchengichthys the jaws are still weak and fragile, but they arc across the margin of the mandible here as the nares rotate from ventral to anterior.

In phylogenetically miniaturized Bianchengichthys
(Fig 3) the jaw evolved into a greater arc that matched the rostral margin enabled by reduction of the nasal elements and migration of the once ventral nares. Whether the mandible elements were strongly conjoined or not cannot be determined from this tiny fractured fossil and its gracile mandible. Either way, the mandible was still fragile.

Figure 4 Skull of the small arthrodire placoderm, Coccosteus, demonstrating the evolution of robust mandibles without matching the upper jaw margins. Compare this ventral view with that of Qilinyu in figure 2.

Not all primitive placoderm jaws were gracile and weak.
Coccosteus (Fig 4) also had an under-slung jaw, like Qilinyu (Fig 2), but it was sharp and robust. Note the similar ventral narial openings in Coccosteus and Qilinyu. Are these excurrent nares? And why do they not appear in the anterior view of Coccosteus where the ventral nasal plate lacks the perforations seen in ventral view? Here we also see the origin of palatal teeth (blue arc in ventral view) recapitulating in arc and chain of gracile elements the earlier origin of the mandible.

Evolution is a fascinating subject. I’m glad you’re along for the ride.

Figure 5. Bothriolepis oral cavity animated. Here the mandible remains immobile and the nasal becomes mobile, scraping bacterial mat perhaps, or ingesting sand, or scraping coral.

PS
Bothriolepis (FIg 5), another placoderm, did not move its mandibles. Instead it moved its nasals, rimmed by scraping premaxillae. The number of mandible elements in Bothriolepis corresponds with those of Rhinopteraspis (Fig 1), including those extra-wide medial elements with a narrow ‘neck’. Based on this example, the premaxilla also evolved several times. And of course it disappeared several times more. That is why phylogenetic analysis can be so vexing to score.

Variation builds upon variation.

References
Zhu et al 2016. A Silurian maxillate placoderm illuminates jaw evolution. Science 354.6310:334-336.
wiki/Gnathostomata
wiki/Qilinyu
wiki/Poraspis

Overlooked interrelationships document yet another origin of jaws