Histology and affinity of the earliest armoured vertebrate.

Arandaspids are the earliest skeletonizing vertebrates known from articulated remains. Despite a wealth of data, their affinity remains questionable because they exhibit a random mixture of primitive and derived characteristics. We constrain the affinity of arandaspids by providing the first detailed characterization of their dermoskeleton which is revealed to be three-layered, composed of a basal laminated, cancellous middle and tubercular superficial layers. All three layers are composed of acellular bone but the superficial layer also includes dentine and enameloid, comprising the tubercles. As such, the composition of the arandaspid dermoskeleton is common to heterostracans and astraspids, supporting existing hypotheses of early vertebrate phylogeny. This emphasizes the peculiarity of existing interpretations of aranadaspid anatomy and there is need for a complete reappraisal of the existing anatomical data.