Origin and evolution of gnathostome dentitions

Johanson and Smith 2005
looked at the questions of teeth and pharyngeal denticles in placoderms.

Unfortunately
the large reptile tree (LRT, 1597+ taxa; subset Fig. 1) does not confirm the first sentence of the authors’ abstract: “The fossil group Placodermi is the most phylogenetically basal of the clade of jawed vertebrates but lacks a marginal dentition comparable to that of the dentate Chondrichthyes, Acanthodii and Osteichthyes (crown group Gnathostomata).”

The LRT nests placoderms along with catfish
between Hybodus and spiny sharks, deep into the Gnathostomata. Catfish are not mentioned in the Johanson and Smith text. They do mention, “the rounded or pointed denticles described for the Arthrodira may only be present in a limited number of taxa (Gemuendina (Fig. 2) Traquair, 1903).” Regrettably the authors did not know that some members attributed to this generic wastebasket of Gemuendina are catfish (Fig. 1), a clade closely related to traditional placoderms. So, taxon exclusion, once again, becomes a major issue.

Johanson and Smith also err
when they state, “The Arthrodira is a derived taxon within the Placodermi, hence origin of teeth in placoderms occurs late in the phylogeny and teeth are convergently derived, relative to those of other jawed vertebrates.” The LRT notes that Coccosteus is a basal placoderm, one that is closer to the outgroup taxon, Gregorius than are other less predatory taxa. This exemplifies a problem with this, and many other papers in that without a proper and validated cladogram, it is nearly impossible to determine whether the absence of teeth, or any other trait, represents a vestigial loss or a vestigial genesis situation.

Johanson and Smith report, 
“Tooth sets and tooth whorls in crown-group gnathostomes are suggested to derive from the pharyngeal denticle whorls, at least in sharks, with the patterning mechanisms co-opted to the oral cavity. A comparable co-option is suggested for the Placodermi.”

The authors do not mention
the tooth carpets of Rhincodon (Fig. 2) and Manta. The LRT indicates that these taxa represent the origin of teeth within the jaws, not on the margins, which remain toothless, but on the palate, reusembling shark skin.

The authors likewise do not mention
the angel shark Squatina. The LRT indicates this taxon represents the origin of teeth along the margins of the jaws.

The LRT indicates
placoderms lose teeth and sometimes develop sharp, turtle-like gnathal plates, some of which retain vestigial tooth-like bumps. Their sister clade, the Siluriformes (catfish) lose the maxilla and retain tooth carpets only in the mandible (Fig. 1). This begins with the basalmost catfish, traditionally considered a basal placoderm, Entelognathus.

References
Johanson Z and Smith MM 2005. Origin and evolution of gnathostome dentitions: a question of teeth and pharyngeal denticles in placoderms. Biology Review 80:1–43.