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In Images: 'Field Guide' Showcases Bizarre and Magnificent Prehistoric Mammals

Xenocranium

prehistoric mammals

(Image credit: Kenneth D. Rose)

Xenocranium lived during the Eocene epoch, 55 million to 34 million years ago. It had a skull tipped with a broad scoop, which it likely used for digging. In fact, its entire skeleton, with its short, strong limbs, was specialized for burrowing.

Livyatan melvillei

prehistoric mammals

(Image credit: Mary Persis Williams)

The gigantic extinct sperm whale Livyatan melvillei measured a whopping 60 feet (18 meters) in length. It had huge curved teeth lining its upper and lower jaws — unlike modern sperm whales, which grow conical teeth only on their lower jaws.