An extinct group of jawless fish from the Paleozoic era that had bony external armor plating instead of internal skeletons.
From Greek 'hetero-' (different) and 'straci' relating to armor or shell; refers to the varied protective plating found on these ancient fish compared to other groups.
These fish were like the tanks of the ancient oceans—fully armored on the outside with no jaws to bite—yet they dominated the seas for millions of years before dinosaurs even existed, proving that different body plans can totally work.
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