A type of ancient marine organism (brachiopod) with a specialized shell structure, extinct for millions of years.
From Greek 'a-' (without) + 'thyra' (door), referring to the absence of a particular opening found in related brachiopods; '-id' makes it a taxonomic name.
Athyrids were successful ocean creatures for 300 million years, filtering food through openings in their shells—then they vanished completely while more flexible brachiopods survived, suggesting flexibility sometimes beats specialization in evolution.
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