SINCLAIR STAMMERS / SCIENCE PHOTO LIBRARY. SINCLAIR STAMMERS / SCIENCE PHOTO LIBRARY.
Fossil ammonites Arnioceras semicostatum, in a sample of rock from the lower Jurassic period (195 to 172 m. y.). The sample was found at Robin Hoods Bay, Yorkshire. Ammonites, shelled cephalopods, lived from 400 to 65 m.y., becoming extinct at the end of the Cretaceous period in the massive upheavals that saw the end of the dinosaurs. Ammonites are known as zone fossils, meaning that certain species appear in specific geological formations and therefore enable accurate dating.
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