Petalichthyida is an extinct order of small, flattened placoderm fish.

They are typified by their splayed pectoral fins, exaggerated lateral spines, flattened bodies, and numerous tubercles that decorated all of the plates and scales of their armor.

Because they had compressed body forms, it is speculated they were bottom-dwellers that chased after or ambushed smaller fish.

According to Zhu's 1991 redescription of Diandongpetalichthys,[1] that genus represents the most basal petalichthyid known, as it has a comparatively elongated head, and shares certain anatomical features with acanthothoracids and primitive arthrodires such as the actinolepids.

In Zhu's redescription, "Quasipetalichthyidae" (comprising Quasipetalichthys, Eurycaraspis, and possibly Neopetalichthys) and "Macropetalichthyidae" (comprising all the other "advanced" petalichthyid genera not including Neopetalichthys, or Diandongpetalichthys) form a dichotomy, with Diandongpetalichthys as a sister taxon.