Paleozoic Era: Ancient Life

• The geologic time scale is a record of life forms and geological events in Earth's history, with the epoch being its smallest division.
• Fossils are remnants, impressions, or traces of animals or plants from a past geologic age preserved in the Earth's crust
• An eon represents the largest division of the geologic time scale, spanning from half a billion to nearly two billion years.
• The Precambrian eon represents approximately 4.5 million years, accounting for about 88% of Earth's history.
• Eras are divisions of geologic time, spanning tens to hundreds of millions of years, including the Paleozoic, Mesozoic, and Cenozoic Eras.
• Periods are divisions within geologic eras, generally lasting no more than 100 million years.

Paleozoic Era: Ancient or Old Life

• The Paleozoic Era started >540 million years ago
• Many organisms emerged and invertebrates were abundant.
• The Cambrian Period, from 544-505 million years ago, marked an explosion of new kinds of organisms after the Precambrian mass extinction.
• Sponges were among the primitive animals that evolved during the Cambrian period.
• Trilobites, small ocean invertebrates, were abundant during this period.
• The oceans teemed with diverse invertebrates during the Ordovician Period (505-440 million years ago).
• The first fish evolved during the Ordovician Period.
• Plants colonized the land for the first time during the Ordovician Period, though animals remained primarily in the water.
• The Silurian Period lasted from 440-410 million years ago.
• The Devonian Period spanned from 410-360 million years ago
• Vascular plants appeared on land during the Devonian Period, enabling them to grow larger due to specialized tissues for water and material circulation.
• The first seed plants evolved during the Devonian period, with seeds having protective coats and stored food.
• Fish with lobe fins evolved and could breathe air above water during the Devonian Period, which was necessary for animals to colonize the land.
• The Carboniferous Period spanned from 360-290 million years ago.
• Extensive forests during the Carboniferous Period left carbon deposits that eventually turned into coal.
• The first amphibians evolved to colonize land, but they still needed to return to the water to reproduce.
• The first reptiles evolved soon after amphibians, marking the first animals able to reproduce on dry land.
• Corals appeared and fish continued to evolve in the oceans during the Carboniferous Period.
• The permian ended with a mass extinction.
• During the Permian Period (290-245 million years ago) all major land masses collided to form Pangaea.
• The Permian Period had extreme temperatures and a dry climate.
• Plants and animals evolved adaptations to dryness like waxy leaves or leathery skin.

Mesozoic Era: Middle Life

• The Mesozoic Era started more than 280 million years ago and ended with the biggest mass extinction the world had ever seen, the Permian extinction.
• The Triassic Period saw the first dinosaurs branched off from the reptiles and colonized land, air and water.
• The Jurassic Period was the "Golden Age of Dinosaurs," and the earliest birds evolved from reptile ancestors.
• During the Cretaceous Period, dinosaurs reached their peak in size and distribution.

Cenozoic Era: Recent Life

• The Cenozoic Era began 65 million years ago and continues to the present.
• During the Tertiary Period (65-1.8 million years ago), Earth's climate was generally warm and humid.
• Mammals evolved to fill niches vacated by dinosaurs during the Tertiary Period.
• Mammals, including primates and human ancestors, evolved, and many mammals increased in size.
• Modern rain forests and grasslands appeared, and flowering plants and insects were numerous and widespread during the Tertiary Period.
• During the Quaternary Period (1.8 million years ago-present), Earth's climate cooled, leading to a series of ice ages.
• Sea levels fell during the Quaternary Period because much water froze in glaciers, creating land bridges between continents.
• Some mammals, like wooly mammoths, adapted to the cold by evolving large size and thick fur, while others moved closer to the equator or went extinct, along with many plants.