Placoderms are the only entirely extinct class of vertebrates. The class is distinguished by a craniovertebral joint, a ball-and-socket joint on either side of the cranium that articulates with the vertebral column. Flexion of the cranium backward increases the gape of the mouth. The five orders make up the first adaptive radiation in vertebrate evolution, and include diverse forms such as Arthrodira (pelagic marine predators, such as Dinicthys, shown above) and Antiarchs (benthic freshwater herbivores, such as Bothriolepis, which show evidence of well-developed paired lungs). The first jawed vertebrates, *Acanthodii, are sometimes included in this group.