The placoderms are an extinct clade of gnathostomes characterised by their extensive dermal armour which is made up of two parts in most groups that cover the head and the anterior part of the trunk respectively. An articulating joint evolved independently in the arthrodires, antiarchs, petalichthyids and ptyctodonts between the head armour and the thoracic armour, where primatively there would been a gap.
There was a single large branchial opening, situated between the thoracic armour and the cheek plates that formed a mobile operculum in life. The dermal bones also bore a senory-line system that was housed in open grooves and pit lines on their surface.
Another placoderm characteristic is the presence of a type of dermal hard tissue called semidentine in the tubercles that cover the dermal bones of the armour in many species and in the jawbones.
The head armour of the arthrodires Coccosteus (a) and Dunkleosteus (b) showing the extensive dermal armour of the group and the hinges between them allowing articulation. From Benton (2000).
The endoskeleton was cartilaginous with varying degrees of ossification throughout the body and supported paired pectoral and pelvic fins covered in small scales, except in the antiarchs, plus a single dorsal fin and a strongly epicercal tail fin. The anal fin is believed to be absent.
Return to Fossil groups home page