Plural form referring to ray-finned fishes collectively, or a taxonomic grouping of fishes characterized by radiating fin structures.
From actino- (ray) + pteri (plural of pteris, wing/fin). A taxonomic plural form used in older ichthyological classifications.
When early fish taxonomists studied actinopteri, they could count the exact number of rays in each fin and use these patterns like fingerprints to identify species—a system that still works perfectly today.
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