A new 'acanthothoracid' placoderm from the Arctic Canada (Early Devonian) and its bearing on the evolution of jaws and teeth.

The origin of jaws and teeth represents one of the most formative episodes in our own evolutionary history. However, this event is poorly understood because of a lack of detailed knowledge of key lineages, including the 'acanthothoracid' placoderms, which were among the earliest jawed vertebrates. Here, we describe Romundina gagnieri sp. nov., a new species of 'acanthothoracid' from the Early Devonian of Arctic Canada. The new species displays anterior supragnathal plates with teeth that we have characterized using synchrotron tomography. Our study shows that teeth are arranged in a concentric manner and that the pattern of tooth addition is centrifugal, including an anterior addition. Overgrowing odontodes, present on the anterior part of the gnathal plates, are covering teeth that can display an hypermineralized layer (probably reflecting the earliest stage of teeth during the ontogeny) or be partially broken. These overgrowing odontodes develop in successive steps and without obvious organization. The presence of a pair of anterior supragnathal plates on the ethmoid part of the endocranium, as well as the growth process of these plates in R. gagnieri sp. nov. are similar to the conditions seen notably in arthrodires, compatible with an ancestral gnathostome ancestral condition.