A small calcareous plate or pair of plates found as a fossil, thought to have served as a closure mechanism for the shells of certain extinct cephalopods.
Greek ana- (back) + ptyx- (fold); coined by 19th-century paleontologists describing structures preserved in ammonite and nautiloid fossils.
Paleontologists spent years finding these mysterious stones in fossils before realizing they were ancient 'trap doors'—evidence that evolution experimented with different ways to seal and protect soft bodies inside hard shells.
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