The petalichthyid placoderm Lunaspis showing some of the general characters of the Petalichthyida. Modified from Benton (2000).
The petalichthyids are chacterised by their dorsally placed orbits (except in the quasipetalichthyids which retain lateral orbits), an X-shaped pattern of sensory lines on their elongate nuchal plate, enclosed sensory lines that communicate with the exterior via large pores, a cranio-thoracic articulation and elongated cornual plates. The body is unknown, except in Lunaspis where it is covered in small scales with a few larger median scutes. There is no evidence for gnathal plates in the petalichthyids, suggesting their jaws were armed with minute separate denticles.
The dorsoventrally flattened shape of the armoured dermal skeleton suggest that this order persued a bentic lifestyle, with the possible exception of the quasipetalichthyids that may have been nektonic.
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