Australian arthrodires

Some of the major classifications of placoderms are briefly reviewed, in their relation to phylogenetic ideas about the group, and the more recent hypotheses of placoderm interrelationships are critically evaluated. Using reinterpreted characters (body armour composition, skull-roof bone pattern, tesserae, number of paranuchal plates, endolymphatic duct, position of nasal capsules, cervical joint, claspers, ornamentation and histology of the dermal bone), a new cladogram, based on 50 synapomorphies, is proposed for all placoderm orders except stensioellids and pseudopetalichthyids, which remain too poorly known. Two main divisions are recognized. In the first branch arthrodires, phyllolepids, petalichthyids and ptyctodontids are characterized by their ventral body armour with an anterior median ventral plate. The second branch groups acanthothoracids, antiarchs and rhenanids on two synapomorphies: a premedian plate, and dorsal nasal capsules. Other synapomorphies (tesserae, structure of the shoulder girdle, and of the cervical joint) require the Acanthothoraci to be dismembered into at least two groups linked respectively to the rhenanids (tesserate forms) or to the antiarchs. The position of Radotina prima is also discussed. As a main conclusion, arthrodires and antiarchs are no longer considered as sister-groups. Their elongated body armour is seen as a result of convergent adaptation. The various acanthothoracids share synapomorphies with one or other of the two main divisions, and the Acanthothoraci therefore forms a stem· group of imprecise phylogenetic status.