Mammals evolved during the Triassic period, about the same time that the first dinosaurs appeared. The first placental mammals appeared at the beginning of the Cretaceous period. The earliest mammals were tiny, shrew-like mammals.

The long line of animals leading to the mammals includes:

• Jawless fish (Cambrian period-mid Ordovician periods)
• Bony fish (mid Ordovician period-late Devonian period)
• Amphibians (late Devonian period-early Carboniferous period)
• Reptiles (late Carboniferous period)
• Pelycosaurs ("mammal-like reptiles") (late Carboniferous period to very early Triassic period)
• Mammals (mid-Triassic period to today).

Earliest-known Mammals:
Some of the earliest-known mammals (the Mesozoic Era) include Eozostrodon, Deltatheridium, Jeholodens, Megazostrodon, Triconodon, and Zalambdalestes.

EOZOSTRONDON Eozostrodon was one of the first true mammals; it lived during the late Triassic period and early Jurassic period, about 210 million years ago. This small, primitive, egg-laying mammal fed the young with mother's milk. Eozostrodon was a quadruped with short legs, a long, pointed snout, five-toed feet with claws, and a long, hairy tail (it looked like a modern-day shrew). Eozostrodon was about 42 inches (107 cm) long. It was a triconodont that belonged to the family Morganucodontidae, which had true mammalian teeth (the cheek teeth were differentiated into simple premolars plus more complex molars, and the teeth were replaced only once, and the molars had triangular cusps).

DELTATHERIDIUM (pronounced DEL-tah-ther-ID-ee-um) Deltatheridium was an early mammal from the late Cretaceous period, about 80 million years ago. This opossum-like quadruped was about 6 inches (15 cm) long and had a long tail. Its had sharp canine teeth and its cheek teeth were wide with triangular crowns. Deltatheridium was an insectivore that may have also eaten small reptiles and perhaps scavenged. Deltatheridium had characteristics of a very early marsupial (pouched mammal), and was a possible kangaroo ancestor. Fossils have been found in Mongolia.

JEHOLODENS Jeholodens was a triconodont, an early mammal. Jeholodens jenkinsi, named by Luo Zhexi and Ji Qiang, dates from the middle Cretaceous period, about 125 million years ago. Its almost complete fossil was recently found in Lianong, China. This long-tailed, quadrupedal mammal was about 5 inches ( cm) long. An insectivore, it had relatively advanced, grasping hands, but a primitive pelvis and hind limbs. It may have had large eyes, and may have been nocturnal (most active at night).

MEGAZOSTRODON Megazostrodon was one of the earliest mammals. It was a tiny quadruped with a long tail, a long body, and a long snout. It lived from the late Triassic period through the early Jurassic period. This primitive mammal was about 4 inches (10 cm) long and weighed only a few ounces. It may have eaten insects. A complete fossilized skeleton was found in Lesotho, South Africa.

ZALAMBDALESTES Zalambdalestes was an early mammal that lived during the late Cretaceous period. Fossils of this shrew-like quadruped have been found in Mongolia. This tiny mammal was about 8 inches (20 cm) long and it's skull was only 2 inches long and. This insectivore (insect eater) had very sharp, interlocking teeth, a long, upturned snout, long foot bones, large eyes, a small brain, and a long tail. Classification: Subclass Pantotheria, Infraclass Eutheria (placental mammals), Genus Zalambdalestes

During the last Ice Age, there were many large, interesting mammals, like the saber-toothed cats, giant ground sloths, mastodons, and mammoths. These animals have long since gone extinct and are known mostly from fossils, from frozen, mummified carcasses, and even from ancient cave drawings.