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Palaeocene epochthe first epoch in the TERTIARY PERIOD lasting from 65 to 54 million years ago. see GEOLOGICAL TIME. This was the time of the explosive evolution of placental mammals; marine reptiles such as ichthyosaurs, mesosaurs and plesiosaurs and the last of the large land dinosaurs became extinct at the end of the CRETACEOUS PERIOD.
Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005