Some comments on the Placoderm parasphenoid

Parasphenoids have or are being described in thirty-nine genera of the Placodermi. All but three of the genera are arthrodires. The actual number of parasphenoids preserved is very small. A review of the literature reveals that many questions regarding this bone remain unanswered e.g., its origin, what is the primitive and advanced condition, what changes occur during growth. Though used by some authors as a character that is diagnostic of particular groups of arthrodire, it seems on existing evidence that the parasphenoid is species specific. There are few indications, at the present time, that the bone is useful in working out placoderm relationships.