Geologic Time Scale 4.6 Billion years ago to the present

1. Geologic Time Scale 4.6 Billion years ago to the present Precambrian Time From 4.6 billion years ago until 3.2 billion years ago there is no fossil record of life on Earth. The environment at this time is low in oxygen. Fossils of cyanobacteria appear in the fossil record At 3.2 billion years ago. They are photosynthetic marine prokaryotes, living in an oxygen poor environment.

2. At 1.8 billion years ago, first evidence of eukaryotic cells. Oxygen levels are increasing in the atmosphere. 1.2 billion years ago first fossil of jellyfish Around 600 million years ago, multicellular animals are abundant; jelly fish, worms, sponges. All are soft bodied invertebrates, all are marine

3. At 543 million years ago, the Precambrian ends and the Paleozoic Era begins. There is a much more complete fossil record because of the presence of organisms with hard body part. Cambrian period; 1st shell fish appear in the fossil record. Coral are in the fossil record Trilobites appear in the fossil record These can be used as index fossils.

4. Ordovician; The first record of vertebrate animals 1st fishes appear (jawless) Cephalopods appear (ammonites) There is an enormous variety to the types of trilobites that are in the fossil record.

5. Silurian; Climate is warming up, glaciers are melting at the poles , ocean levels are rising. Large coral reefs form 1st fish with jaws, sharks appear in the fossil record 1st fresh water fishes 1st land plants (Cooksonia) 1st land animals (similar to scorpions and centipedes

6. Devonian; 1st trees appear, and forests. Appalachian mountains begin to form Temperatures are warming, glaciers are beginning to melt and sea levels are rising. Much of North America is covered by a shallow sea. North America and Europe together near the equator. Bony fishes appear in the fossil record 1st amphibians in the fossil record

7. Carboniferous; Temperatures become moderate, but large ice sheets form at the south pole. Ocean levels drop, causing large swampy areas to form. Most of the coal deposits that we have in the United States are the result of these swamps. Land snails appear in the fossil record. Winged insects appear. These are the first flying animals.

8. Reptiles appear in the fossil record for the 1st time. This is the first appearance of an amniotic egg. Trilobites are becoming more scarce. By the end of the Carboniferous period, North America and Europe are colliding with one another.

9. Permian; Pangaea is nearly formed by the beginning of this period, and will be complete by the time this period ends. Global climate fluctuates, but there are many long dry stretches of time. Conifers appear. They are virtually identical to the ones we have today. Many large reptiles appear. At 251 million years ago, there is a mass extinction that kills about 90% of the known species in the fossil record.

11. Mesozoic Era: Began after the Paleozoic extinction event. Triassic Period; There are still reptiles, amphibians, and fishes. There are still insects and plants. Many of the reptiles are still fairly large at this time. Dinosaurs first appear in the fossil record. Most of them are not very large, but by the end of the Triassic they are increasing in size. Toward the end of the Triassic, small mammals appear in the fossil record. Pangaea is slowly starting to separate.

12. The Triassic period ends with a mass extinction. Jurassic period; New dinosaur species appear in the fossil record. Many species are very large. Apatosaurus, brachiosaurus and diplodocus are some of the large sauropods that appear. Allosaurus and stegosaurus appears in the fossil record. There are still many large reptiles. There are a lot of marine reptiles such as plesiosaurs and ichthyosaurs.

13. Flowering plants appear in the fossil record about mid-way through the Jurassic. Mammals are still fairly small and insignificant at this time. Birds appear in the fossil record during the last third of the Jurassic. Many different types of large turtles appear in the fossil record. Many pterosaurs appear in the fossil record.

14. Pangaea has separated into two large landmasses; Laurasia and Gondwana. The end of the Jurassic is signaled by a mass extinction. Cretaceous period: Great variety to the different types of pterosaurs. Largest dinosaurs that ever existed. Extremely large marine reptiles such as the 43 foot long Kronosaurus. Many well known dinosaurs such as Triceratopsand Tyrannosaurus rexappear in the fossil record.

15. Climate is very warm at this time and flowering plants become abundant. Bees appear in the fossil record. Early relatives of ants and termites appear. The early relatives of butterflies and moths appear. By the end of the Cretaceous period, small there are a lot of marsupial mammals and placental mammals appear. The end of the Cretaceous (and the Mesozoic Era) is marked by a mass extinction of about 75% of known species.

17. Cenozoic Era: Recent time, has such a complete fossil record that the Periods can be divided into smaller units called Epochs. The Era is only divided into two periods. Tertiary Period lasted from 65 million years ago until 1.8 million years ago. It is divided into 5 Epochs. Paleocene epoch; The climate is cooler than it was during the Cretaceous period, but warmer than it is today.

18. A variety of mammal sizes appear during this epoch. Some are as small as mice, and the largest are about the size of a small bear. Most do not look like any mammals that are on the planet today. The earliest primate ancestors appear in the fossil record, but they do not look like modern primates. Brain cases from all of these early mammals are small, even when compared to the mammal of the Mesozoic. Shark species become abundant. Several varieties of large, flightless birds are present.

19. Eocene epoch; Temperatures become hotter than today’s climate. Average temperatures globally at currently at 57 degrees F. During the Eocene, global averages were about 82 F. Most modern orders of mammals appear at this time. These include the ungulates (animals with hooves), rodents, bats, and aquatic mammals. *Relatives of the modern whale had previously been terrestrial. Many birds related to modern species appear, such as vultures, eagles and pelicans. Most Eocene animals are small in size. Hyracotherium, an early relative to today’s horse, is about the size of a large house cat.

20. Oligocene epoch; Climate slowly begins to cool, but remains warmer than it is today. Grasses become abundant for the 1st time in history. Many grazing animals such as ancestors to modern camels become abundant. Large variety of early horse ancestors appear. 1st elephant relatives that have trunks appear. Hamsters appear in North America.

21. Miocene epoch: Global temperatures start to increase again. Grasslands continue to spread in size. Merychippus and other horse relatives are living in the grasslands. Short legged rhino and rhino with two horns (side by side) Early mastodons appear. Also raccoons, weasels, deer and giraffes appear in the fossil record. Several large predators: relatives to modern dogs, but they are the size of a black bear. They have jaws that can crush large bones. Large cats that are related to the saber-toothed cats.

22. The Cascade and Sierra Nevada mountain ranges are forming in North America. The Andes Mountains are forming in South America. Pliocene epoch: India and Asia are colliding. The Himalaya Mountains are forming. In North America, the Rocky Mountains are forming. The Alps are forming in Europe. Temperatures begin to decrease again. Large polar ice caps begin to form, and Antarctica is starting to become the continental glacier that we are familiar with today.

23. The climate, is still warmer than it is today. In parts of Africa, Australopithecus afarensis appears in the fossil record. This is a primate that walks upright. Quaternary Period; Divided into two epochs. Pleistocene epoch: From 3 million years ago, to about 10,000 years ago. Periods of global cooling produce large glaciers each for about a 100,000 year period of time, and then warming occurs.

24. There were at least 5 major glacial events during this epoch. Many of the major groups of plants and animals from this epoch are virtually unchanged from what we see today. There were some changes however; Mammoths slowly replace mastodons on the planet. There is a long horned bison, giant ground sloth and saber toothed cat in North America. There are also camels, recent relatives of the modern horse and Dire wolves (large ) and cave bears (very large).

25. Early human ancestors have spread to every continent except Antarctica. Homo sapiens appear in the fossil record about 200,000 years ago. Neanderthals, which are considered a subspecies of Homo sapiens, first show up around 130,000 years ago. What we consider to be modern humans appear in the fossil record about 115,000 years ago.

26. Holocene epoch; 12,000 years ago up to the present.