Placoderms and the evolutionary origin of teeth: a comment on Rücklin & Donoghue (2015)

The extinct Devonian placoderms (armoured jawed fishes) [1,2] are central to the question of tooth origins, because some have denticulate ‘toothplates’ within the mouth cavity. A key question is whether these gnathal plates were modified from external dermal bones, or had ‘denticles’ representing true teeth with pulp cavities [3, fig. 2h]. The recent contribution by Rücklin & Donoghue [4] confuses this issue, because their claimed ‘anterior supragnathal’ (ASG) of the placoderm Romundina stellina shows no evidence that it came from the oral cavity, and is more likely an external dermal element. Also, the tissue identified as enameloid is not birefringent and thus not enameloid. Their inferences about growth of toothplates, phylogenetic loss of enameloid, and independent development of teeth and jaws, based on the structure of this plate, are therefore invalid.