Tiny taxa at the placoderm > spiny shark transition and the genesis of teeth

Recent housekeeping
in the fish subset of the large reptile tree (LRT, 2173 taxa, Fig 5) sheds new light on this newly revised DGS tracing of the crushed skull of the Early Devonian spiny shark, Ischnacanthus (Figs 1, 2). The palate is visible here. That’s rare and attests to the relatively wide skull, wider than in most spiny sharks. That enabled comparison with the palate of the Unnnamed ‘arthrodire placoderm’ with the museum number ANU V244 (Fig 1). Turns out the two are quite similar.

Tiny Early Silurian
Shenacanthus (Fig 3) is ancestral to both the Early Devonian ANU V244 specimen and coeval Ischnacanthus.

The origin of teeth
In these DGS tracings (Figs 1–3) DGS colors are applied with tetrapod homologs. The green patches with bumpy ventral surfaces are maxillae with maxillary teeth. That’s how teeth first appear in vertebraes. In these taxa maxillary teeth are not yet on the margin, but hidden by postorbital cheeks. The yellow patch below the nasal is the premaxilla and its carpet of tiny bumpy teeth. The large tan arches are lacrimals, anchoring the maxillae. Lacrimals are associated with respiration now and so are restricted to the narial area, but in fish respiration occurs at the gills. That’s why lacrimals first appeared below the braincase and in back of the skull. The blue dentaries lie loosely on the mandible. The lavender patches in the palate are vomers, also provided with tiny tooth carpets.

A related tiny taxon,
Shenacanthus vermiformis (Zhu et al 2022, Early Silurian, est 3cm long, Fig 3) is the earliest known of the jawed placoderms and the basalmost member of the spiny sharks. Claspers are present. So are lacrimals and maxillae.

Unnnamed specimen (ANU V244, Young, Lelièvre and Goujet 2001; Hu, Lu and Young 2017; Early Devonian (Fig 1) is a tiny relative to Coccosteus and even tinier Shenacanthus).

Ischnacanthus gracilis (Egerton 1861, orginally Diplacanthus gracilis, Burrow et al 2018; Miles 1973; Early Devonian, 410 mya; up to 2m in length, Figs 1, 2) is an early acanthodian with placoderm-like maxillary pads and a wide skull based on the wide sphenoid (braincase floor), as in the ANU V244 specimen.

The Late Silurian placoderm,
Entelognathus (Zhu et al. 2013), now nests at the base of the Chondrichyes (sharks, rays, ratfish) in the LRT.

The arthrodire placoderm,
Coccosteus. is the last common ancestor of sharks and spiny sharks + bony fish in the LRT (Fig 5). This pushes the origin of placoderms into the Ordovician, prior to the appearance of Early Silurian Shenacanthus, a phylogenetically miniaturized taxon (Fig 3).

Somewhat larger Doliodus also joins Ischnacanthus
(Fig 4) at the base of the spiny sharks in the LRT (Fig 5). Several pectoral spines = rays bind together here to form a robust spine at the anterior margin of the broad pectoral fins, convergent with Iniopterygia and catfish. The robust teeth have twin cusps, convergent with the basal chondrichthyan, Xenacanthus, but all teeth lay flat and point medially. The maxillae are longer than in Ischnacanthus and the lacrimals are flatter. As in Climatius and Brachygnathus (Fig 3), several small post-pectoral spines are present along the ventral abdomen.

This appears to be a novel hypothesis of interrelationships.
If not, please provide a citation so I can promote it here.

References
Burrow CJ et al. 2018. The Early Devonian ischnacanthiform acanthodian Ischnacanthus gracilis (Egerton, 1861) from the Midland Valley of Scotland. Acta Geologica Polonica 68(3): 335–362.
Egerton P de MG 1861. British fossils. (Descriptions of Tristichopterus, Acanthodes, Climatius, Diplacanthus, Cheiracanthus). Memoirs of the Geological Survey of the United Kingdom (British Organic Remains), Decade 10: 51–75.
Hu Y, Lu J and Young 2017. New findings in a 400 million-year-old Devonian placoderm shed light on jaw structure and function in basal gnathostomes. Nature Scientific Reports 7: 7813 DOI:10.1038/s41598-017-07674-y
Miles R 1973. Articulated acanthodian fishes from the Old Red Sandstone of England, with a review of the structure and evolution of the acanthodian shoulder-girdle. Bulletin of the British Museum (Natural History), 24, 111–213.
Young G C, Lelièvre H and Goujet D 2001. Primitive jaw structure in an articulated brachythoracid arthrodire (placoderm fish; Early Devonian) from southeastern Australia. J. Vertebr. Paleontol. 21, 670–678.
Zhu Y-A et al (10 co-authors) 2022. The oldest complete jawed vertebrates from the early Silurian of China. Nature 609:954–958. online

wiki/Ischnacanthus
wiki/Shenacanthus – not yet posted
wiki/ANU v244 not yet posted, not yet named