placoderm
Concise Oxford English Dictionary © 2008 Oxford University Press:
placoderm /ˈplakə(ʊ)dəːm/
▶noun a fossil fish of the Devonian period, having the front part of the body encased in broad flat bony plates.
– origin C19: from Gk plax, plak- ‘flat plate’ + derma ‘skin’.
'placoderm' also found in these Oxford entries:

