A group of vertebrates having vertebral centra (the bodies of vertebrae) that are concave on both ends.
From Greek hetero- (different) and koilos (hollow), referring to the differently-shaped cavities in vertebral bones of these ancient fish and reptiles.
When paleontologists first studied ancient fish fossils, they noticed their vertebrae had a completely different shape than modern fish—concave on both ends like double bowls—which led them to create this entire classification category!
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