A member of the extinct class Heterostraca; an early fish with bony armor made of different plate types.
From 'heterostraca' + '-an' (individual member suffix). Singular form referring to a single member of this extinct fish class.
Each heterostracan was a living patchwork quilt—their armor was made of bone plates of different shapes and thicknesses, giving them a weird, asymmetrical look!
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