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If we will base it on the recent fossil which was recently found in Japan-- the Unique Mosasaur fossil last December 8, 2015, ( this was found after the discovery of the Dinosaur relatives and first dinosaurs (Dec. 7, 2015) by the University of Utah), most possibly, these creatures existed during the Mesozoic Era. During this period, Dinosaurs surround the planet for about 165 million years. In the Earth’s history, this era is called the Mesozoic Era.
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