Placodus gigas (Agassiz 1833), 2.0 meters long, Middle Triassic ~240 mya, was derived from as sister to Paraplacodus and was a sister to Henodus.
Distinct from Paraplacodus, the skull of Placodus was lower and wider. The lateral temporal fenestra had disappeared due to the expansion of the jugal and squamosal. The quadratojugal was incorporated into the squamosal. The teeth were flatter, ideal crushing. The dentary and the coronoid were higher, for greater crushing leverage.
The torso was deeper. The vertebrae were topped by ossificatons. The humerus was shorter and shorter than the ulna. The ischium was smaller than the pubis.
The complete reptile family tree is here. A series on placodont and ichthyosaur palate evolution is here.