I looked the word origin and found Placodermi is PLATE SKINNED while Ostracodermi is Shell Skinned. Can someone please explain the difference between being Plate skinned and Shell skinned?
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If in reference to the prehistoric groups of fish of the respective classifications, ostracoderms would have had a bony head much like a shell, and placoderms had articulated plates covering the head/thoracic area. The differences are relatively straightforward, see the wikipedia pages for each:
