270 - 264
264 - 258
258 - 252
252 - 246
246 - 240
270 - 240 M
2nd Galactic year 500 - 250
1st Galactic year 250 to present
Phanerozoic Era 543 - Present
Permian Period 290 - 248
Triassic Period 248 - 206
Mesozoic Era (Middle Life) 248 - 65

Triassic Period 248 - 206 Geologic - Pangaea covers nearly a quarter of the Earth's surface. The Triassic Period, unlike the previous periods, is marked by few significant geologic events. Toward the end of the Triassic Period, continental rifting begins to break apart the supercontinent.
The general climate is warm, becoming semiarid to arid.
Biologic - The first dinosaurs such as Coelophosis and Euskelosaurus, and mammals, turtles, crocodiles and frogs appeared. Life began to diversify after the end-Permian extinction. Early dinosaurs evolve. Many are bipedal, fast, and relatively small. The largest Triassic dinosaurs are only 20 feet (6 meters) in length—small when compared to later Mesozoic forms.
Marine reptiles evolve, such as ichthyosaurs and plesiosaurs.
Ferns, cycads, ginkgoes, and conifers flourish.
Mass extinctions occur at the end of the Triassic Period, reducing some marine and terrestrial groups, such as the ammonites, therapsids, early reptiles, and primitive amphibians, by as much as 75 percent.

Permian Period 290 - 248
Geologic - A single supercontinent, Pangaea, forms as Earth's landmasses collide and merge. Pangaea extends across all climatic zones and nearly from one pole to the other. This supercontinent is surrounded by an immense world ocean.
Extensive glaciation persists in what is now India, Australia, and Antarctica. Hot, dry conditions prevail elsewhere on Pangaea, and deserts become widespread.
Biologic - The first sailback reptiles such as Dimetrodon appeared. Invertebrate marine life is rich and diverse at the beginning of the Permian period. Toward the end of this period, mass extinctions occur among large groups of corals, bryozoans, arthropods, and other invertebrates. 99% of all life perishes.
On land, insects evolve into their modern forms; dragonflies and beetles appear.
Amphibians decline in number, but reptiles undergo a spectacular evolutionary development of carnivorous and herbivorous, terrestrial and aquatic forms.
Ferns and conifers persist in the cooler air.

- 256 Phtinosuchus, an early Therapsid. Shortly after the appearance of the first reptiles, two branches split off. One is Synapsida: they had a pair of holes in their skulls behind the eyes, which were used to increase the space for jaw muscles. The other branch is Diapsida.
From synapsids came the Therapsida, the direct ancestor of mammals. They are often called mammal-like reptiles.
The earliest mammal-like reptilian are the pelycosaurs. The pelycosaurs was the first animals to have temporal fenestra.Pelycosaurs are not Therapsida but soon they gave rise to them. The therapsids have temporal fenestrae larger and more mammal-like than pelycosaurs, their teeth show more serial differentiation; and later forms had evolved a secondary palate. A secondary palate enables the animal to eat and breathe at the same time and is a sign of a more active, perhaps warm-blooded, way of life.
- 250 One Galactic year ago
- 248 - 65 Mesozoic Era (Middle Life)