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.
In the 1980s and 1990s, ptyctodont skulls were compared to those of other orders. This led to the abandonment of the notion of a gradient from least armored to most armored in placoderms.