Mammals (Mammalia) are synapsid animals that are characterised by their hair, three middle ear bones, mammary glands in females, they are also warm-blooded. Mammals evolved from earlier cynodont therapsids during the Triassic/Jurassic period.
Mammals and other animals
Unlike their earlier ancestors that ruled the world before the time of the dinosaurs mammals were not the rulers of the time they first appeared. During that time at first the crocodile relatives ruled the world of Triassic, but during the Late Triassic and the later Jurassic and Cretaceous periods the dinosaurs ruled the land and sea reptiles ruled the oceans.
During the dinosaur times most mammals were numerous very small. Most of them were not larger than modern small mammals like shrews. But some of them like Didelphodon and Repenomamus were somewhat larger (Repenomamus was about 1 m long). Repenomamus is even known to eat baby and small dinosaurs but the larger meat eating dinosaurs were something that could of course eat Repenomamus.
The time of mammals
After the extinction of all the non-bird dinosaurs, mammals quite soon became the rulers of the planet that they remain to this day. During these times mammals became very diverse and many different forms of plant eaters (like Chalicotherium) and predators (like Smilodon) appeared.
Most large land animals today are mammals and in fact some of them like whales are gigantic in size. But no land mammal has ever grown as large as largest sauropod dinosaurs. But some land mammals like Paraceratherium were the size of medium sauropods.
- Ausktribosphenidae (poorly known extinct Cretaceous Australia mammals)
- Monotremata (monotremes, includes modern egg-laying mammals)
- Multituberculata (multituberculates, extinct mammals resembling rodents)
- Gobiconodonta (Repenomamus etc.)
- Eutriconodonta (group of Jurassic and Cretaceous mammals)
- Metatheria (marsupial mammals and relatives)
- Eutheria (placental mammals and relatives)