A family of extinct brachiopods (marine animals with shells) that lived from the Paleozoic era and lacked a specific shell opening.
From the Greek root 'a-' (without) + 'thyra' (door), with '-idae' (family suffix in biology), naming this entire family by their lack of a characteristic opening.
Paleontologists used this naming system where 'without the door' (athyra) defined an entire family of creatures—it's like naming a human family 'Nosocks' because that's their most obvious difference, which feels oddly arbitrary until you realize it was the most important feature to early shell-scientists.
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