14 DAY TRIAL // In the 19th century, scientists discovered that the rocks with fossils are deposited in a defined stratigraphical order, an this is how Paleontology emerged. Already in 1761, Giovanni Arduino saw that the rocks could be assigned to ages: Primary, Secondary, and Tertiary. In 1829, J. Desnoyers added the Quaternary.
Today, this is the classification of the geological epochs:
A. Precambrian is the oldest eon, stretching from the Earth's emergence (4.5 billion years ago) till Cambrian (542 million of years ago). This is the epoch when life appeared, and the oldest traces of organic carbon seem to be 3.8 billion years old.
During the Proterozoic eon (1 billion years ago-542 million years ago) the oxygen starts to accumulate in the atmosphere at the levels we know today. When the first fossils of complex animals appeared, the Cambrian started.
B. Phanerozoic. Everything that followed the Precambrian till today is called the Phanerozoic, when complex life exploded on Earth.
I. The Paleozoic Era (The Primary Era) ("ancient life") lasted from 541 to 251 million years ago.
1. Cambrian (542-488 million years ago) takes its name from old mountains found in Wales, which formed during this period. Most phyla of current animals appeared in this period, but the fauna of the Cambrian was a heavy shelled and also very static one, dominated by trilobites, worms, shelled mollusks and echinoderms.
2. Ordovician (490-440 million years ago) - named after the Welsh tribe of Ordovices. The climate was warm and there was a huge southern continent, Gondwana. The first coral reefs appeared in this period. The fauna was dominated by trilobites, brachiopods, sponges, sea lilies, shelled cephalopods. A mass extinction occurred at the end of this age, when 60 % of the species disappeared.
3. Silurian (440-416 million years ago) - named after the Silures, an ancient Welsh tribe. The climate was warm. The fist jawed fish appeared, and seas were dominated by giant sea scorpions, now extinct. Leeches, too, appeared in this period, but also the first terrestrial animals ever, primitive spiders and millipedes. The first vascular plants started the conquest of the ground. Fungi grew as big as trees.
4. Devonian (416-359 million years ago) takes its name from Devon, England. This is when fish developed legs and evolved to land tetrapods around 365 Ma; the first insects also appeared and real spiders colonized terrestrial habitats.
The first seed-bearing plants formed huge forests. Primitive sharks started to dominate the ocean. The first ammonites, shelled mollusks that later evolved in squids and octopuses, appeared in this period.
The land was made by Gondwana to the south, the continent of Siberia to the north, and the Euramerica in the middle. A mass extinction occurred at the end of this era.
5. Carboniferous (359-300 Ma) means "carrying carbon" as huge deposits of high quality carbon formed in this period, revealing a wet, hot climate that favored the forests' expansion. Oxygen in the air was at a higher level than it is now, 36 %, and there were huge insects, such as 3 ft (1m) wingspan dragonflies. Most primitive insect groups still living emerged in that period. Snails too went to the ground. The amphibians' diversity boomed and the first reptiles appeared.
6. Permian (300 to 250 Ma) takes its name from Perm, a Russian region close to the Ural mountains. The Earth experienced an Ice Age. Ferns were highly diversified, and gymnosperm trees, like ginkgos and cycads appeared in this period. Beetles and flies appeared. The land was dominated by mammal-like reptiles. A huge mass extinction occurred at the end of the Permian, when up to 95 % of the species went extinct.
II. Mezozoic ("middle life", Secondary era) era spanned from 250 to 65 Ma.
1. Triassic (250-200 Ma) is named so because there are three specific stratigraphical layers coming from this era. Triassic was hot and dry, with no Ice Ages.
Many types of huge marine reptiles evolved now. The flora was dominated by gymnosperms and seed-ferns. Mammals, turtles, crocodiles and the first dinosaurs appeared now. The first flying reptiles, pterosaurs, emerged in Triassic. Triassic ended with a mass extinction.
2. Jurassic (200-145 Ma) takes its name from Jura mountains (Germany). This is when Teleostei, the modern bony fish appeared (they form now most of the fish species, from trout to tuna).
The land fauna was dominated by the huge herbivorous dinosaurs named sauropods, the largest beasts that ever roamed the Earth (the typical long-necked, long-tailed, small-headed, elephant-footed dinosaurs). Huge carnivorous dinosaurs preyed on them. There are also stegosaurs (plated dinosaurs). Birds appeared in this period from carnivorous dinosaurs.
3. Cretaceous (145-65 Ma), the "chalk era", named so because of the huge chalk deposits formed in this period. It was wetter but cooler than the Jurassic period. The continents started to split towards their current configuration.
Angiosperms appeared and spread rapidly, co-evolving with the bees. Insects and fish were mainly the same as they are today. First marsupials and placentals were found. Birds were toothed. The main herbivorous dinosaurs were the horned, duck-billed and armored ones. The carnivorous ones were highly diversified and evolved. At the end of the Cretaceous a mass event wiped out the dinosaurs and most reptiles. 50% of the species and 25 % of the known families disappeared.
III. Cenozoic started with the demise of the dinosaurs and it still continues today.
1. Paleogene (65-23 Ma) when small mammals evolved to the high diversity we know today. Many groups died without descendants. Birds and snakes, too, diversified greatly. The clime was somehow cooler. The Paleogene period was made of three epochs: Paleocene, Eocene, Oligocene.
2. Neogene (23-2 Ma) was dominated by megafauna. Most families, genera and even species we know today appeared in this period. The dawn of the human evolution can be traced now. There are two epochs: Miocene and Pliocene.
3. Quaternary started about 1.8 Ma. It's the era of the last Ice Ages. There were alternations between cold Ice Ages and warm inter-glacials.
It has two epochs: Pleistocene and Holocene. At the beginning of the Holocene (the last interglacial in which we still are today), 10-8,000 years ago, many species of the megafauna disappeared worldwide (giant sloths, mastodons, mammoths, saber-toothed cats, glyptodonts, woolly rhinos) while other locally (like horses and camels in North America).