Describing a vertebra that is concave on both its front and back surfaces.
From Greek 'amphi-' (both) + 'koilos' (hollow), meaning having hollows on both ends. Used in anatomy to describe the shape of certain prehistoric fish and amphibian vertebrae.
Early vertebrates had these double-hollowed vertebrae, which were gradually replaced by more specialized designs—it's like nature was experimenting with different engineering solutions for backbones before settling on the designs we see today.
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