During the 1980s and '90's, ptyctodont skulls were compared with skulls from other orders. From these analysises, this idea of a gradient from least armored to most armored in placoderms was discarded.
During the 1980s and 1990s, researchers analyzed ptyctodont skulls alongside those from other fish groups. This analysis ultimately refuted the idea that placoderms exhibited a progressive increase in armor strength.