The Fossil Collector is found at the Museum Camp on Fossil Island. She gives numulites in exchange for fossils (identified or unidentified) found around the island:
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Trivia
The Fossil Collector's and her dog's names, Mary and Tray, are a reference to English palaeontologist Mary Anning, who lived in Dorset from 1799-1847.
She owns a dog named Tray, but upon her release the dog was not visible. It was added in a later update.