Liopleurodon
- When: Middle to Late Jurassic
(166–155 mya) - Where: Europe
- What: Pliosaur
- Weight: 25–35 metric tons
- Length: 10 meters
- Diet: Carnivorous
- Discovered: 1873
Livyatan
- When: Miocene
(9–8 mya) - Where: South America
- What: Cetacean Mammal
- Weight: 50 metric tons
- Length: 13–18 meters
- Diet: Carnivorous
- Discovered: 2010
Magyarosaurus
- When: Late Cretaceous
(70–66 mya) - Where: Europe
- What: Sauropod
- Weight: 1 metric ton
- Length: 6 meters
- Diet: Herbivorous
- Discovered: 1932
Maiasaura
- When: Late Cretaceous
(76–65 mya) - Where: North America
- What: Hadrosaur
- Weight: 2.5 metric tons
- Length: 9 meters
- Diet: Herbivorous
- Discovered: 1979
Mamenchisaurus
- When: Late Jurassic
(160–145 mya) - Where: Asia
- What: Sauropod
- Weight: 20–30 tons
- Length: 21–23 meters
- Diet: Herbivorous
- Discovered: 1954
Maraapunisaurus
- When: Late Jurassic
(150 mya) - Where: North America
- What: Sauropod
- Weight: Highly speculative, 100 metric tons
- Length: 30 meters
- Diet: Herbivorous
- Discovered: 1878
Mastodon
- When: Miocene to Pleistocene
(11 million–12,000 years ago) - Where: North America
- What: Mammal
- Weight: 5 metric tons
- Length: 3 meters
- Diet: Herbivorous
- Discovered: 1799
Mastodonsaurus
- When: Middle Triassic
(247–237 mya) - Where: Europe
- What: Amphibian
- Weight: 1 metric ton
- Length: 6 meters
- Diet: Carnivorous
- Discovered: 1828
Megaloceros
- When: Pleistocene
(400,000–8,000 years ago) - Where: Europe, Asia
- What: Mammal
- Weight: 700 kg
- Length: 3.5 meters
- Diet: Herbivorous
- Discovered: 1799