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Ammonites were similar in shape and form to the modern Nautilus. They were very common in the oceans of the Paleozoic and Mesozoic eras, 400 to 65 million years ago, evolving in the Devonian Period. Fossil shells as small as 3 mm and as large as 3 m have been found. This fossil is about 16 cm in diameter.
This fossil ammonite (Mortoniceras inflatum) is from the Upper Albian (Lower Cretaceous), and was collected from Devizes, Wiltshire, in 1932.