Geologic Time Scale 4.6 Billion years ago to the present

Most of the intervals of geologic time are recognized with the appearance and/ or disappearance of organisms in the fossil record. Precambrian Time- from 4.6 billion, to 542 million years ago.

From 4.6 billion years ago until 3.2 billion years ago there is no fossil record of life on Earth.

The environment the early Precambrian is low in oxygen, but high in methane and ammonia.

At 3.2 billion years ago, the first fossil forming organisms appear. Cyanobacteria are photosynthetic marine prokaryotes. They are capable of living in an oxygen poor environment.

At 2.1 billion years ago, first evidence of eukaryotic cells. These are thought to be related to some of today’s photosynthetic protists.

Around 600 million years ago, multicellular organisms become abundant; jellyfish, worms, sponges and algae are common.

1.2 billion years ago there is evidence of the first multicellular organisms with the fossils of jellyfish.

At this time, oxygen levels are about 15% higher than they are today.

All are soft bodied invertebrates/ all are marine.

At 542 million years ago, the Precambrian ends and the Paleozoic Era begins. The Paleozoic is subdivided into 6 periods.

Cambrian period (542-488 mya); 1st shell fish appear in the fossil record.

Coral are appear in the fossil recordTrilobites appear in the fossil record

Trilobites can be used as index fossils to help determine the age of a rock layer.

There is a much more complete fossil record because of the presence of organisms with hard body part.

Ordovician (488-444 mya); The first record of vertebrate animals. At the beginning of the Ordovician, temperatures are very hot (ave. temps of about 70◦ F. Today’s ave is 57◦ F), but temperatures start to cool as the period progresses.

Cephalopods appear (ammonites)

drawing actual fossil

1st fishes appear (jawless)

There is an enormous variety to the types of trilobites that are in the fossil record.

Massive glaciers begin to forming toward the end of the Ordovician, but average temperatures are still hotter than today.

The end of the Ordovician is marked by the 2nd largest mass extinction on record.

The glacier formations cause sea levels drop and a mass extinction occurs.

Sea levels begin to slowly rise, and large coral reefs begin to form.

Silurian ( 444-416 mya); Climate begins to slowly warm up.

1st jawed fishes appear.

1st fresh water fishes appear.

1st primitive sharks appear.

1st land plants (Cooksonia) appears.

1st land animals (similar to scorpions, spiders and centipedes)

Devonian (416-359 mya)- Temperatures continue to warm, glaciers are melting and sea levels are rising.

1st trees appear, and then forests appear.

1st Bony fishes appear in the fossil record.1st amphibians appear in the fossil record

North America and Europe are together near the equator.

Much of North America is covered by a shallow sea.

Appalachian mountains are just beginning to form.

Sharks become abundant, in many different forms.

Carboniferous (359-299 mya); Temperatures become moderate, but cooling occurs throughout the period. By the end of the carboniferous, large ice sheets form at the south pole and the climate becomes much drier.

1st Land snails appear in the fossil record.

1st Winged insects appear. These are the first flying animals.

Most of the coal deposits that we have in the United States are the result of these swamps.

Ocean levels drop, causing large swampy areas to form.

1st Reptiles appear in the fossil record. They are very small.

Trilobites are becoming more scarce.

By the end of the Carboniferous period, North America and Europe are colliding with one another.

With reptiles, we have the first appearance of an amniotic egg.

Likely, because of the cooler, dryer climate, there is a mass extinction.

Permian (299-251mya); Pangaea is nearly formed and will be complete by the time this period ends.

1st Conifers appear. They are virtually identical to the ones we have today.

Many large reptiles appear.

Global climate fluctuates, but there are many long, dry stretches of time.

As the Permian Period ends, this also ends the Paleozoic Era.

The Permian ends with the largest mass extinction on record. It is estimated that 70% of all terrestrial organisms become extinct.

Estimates for marine organisms are between 90 and 95%.

This is the only known mass extinction of insects.

All trilobites become extinct.

Mesozoic Era: Begins after the Paleozoic extinction event. There are 3 periods during this era.

Triassic Period (251-200 mya) Pangaea is slowly starting to separate.

Many of the reptiles are still fairly large. Some of them have characteristics similar to early mammals.

There are still a large variety of fishes, plants, amphibians and reptiles on the earth at this time.

1st Dinosaurs first appear in the fossil record. Most of the early dinosaurs are not very large. By the end of the Triassic, many will have increased in size.

Toward the end of the Triassic, small mammals appear in the fossil record.

Most were only a few inches long, and most were egg layers.

By the end of the Triassic, CO2 concentrations in the atmosphere were 10 times higher than they are today.

The Triassic period ends with a mass extinction, probably related to the climate.

This caused extreme greenhouse effect and very high temperatures.

Jurassic period; New dinosaur species appear in the fossil record. Many species are very large.

There are still many large reptiles too.

Apatosaurus, brachiosaurus and diplodocus are some of the large sauropods that appear.

Allosaurus and stegosaurus appears in the fossil record.

There are a lot of marine reptiles such as plesiosaurs and ichthyosaurs.

Mammals are still fairly small and insignificant at this time.

Flowering plants appear in the fossil record about mid-way through the Jurassic.

Many different types of large turtles appear in the fossil record.

Birds appear in the fossil record during the last third of the Jurassic.

Many pterosaurs appear in the fossil record.

Largest pterosaur- Quetzequatalus had a wing span of 52 feet.

The end of the Jurassic is signaled by a mass extinction.

Pangaea has separated into two large landmasses; Laurasia and Gondwana.

Cretaceous period (145-65 mya): Great variety to the different types of pterosaurs.

Many well known dinosaurs such as Triceratops and Tyrannosaurus rex appear in the fossil record.

Largest dinosaurs that ever existed.

Extremely large marine reptiles such as the 43 foot long Kronosaurus.

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, there are a lot of marsupial mammals and placental mammals appear.

The end of the Cretaceous 65 mya (and the Mesozoic Era) is marked by a mass extinction of about 75% of known species.

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 (65-1.8 mya) 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.

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.

The earliest primate ancestors appear in the fossil record, but they do not look like modern primates.

Most do not look like any mammals that are on the planet today.

Several varieties of large, flightless birds are present.

Shark species become abundant.

Eocene epoch; Temperatures become hotter than today’s climate. During the Eocene, global averages were about 82 F.

Many birds related to modern species appear, such as vultures, eagles and pelicans.

*Relatives of the modern whale had previously been terrestrial.

Most modern orders of mammals appear at this time. These include the ungulates (animals with hooves),

rodents, bats, and aquatic mammals.

Hyracotherium, an early relative to today’s horse, is about the size of a large house cat.

Most Eocene animals are small in size.

Hyracotherium has 4 toes on each front foot and 3 toes on each back foot.

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.

1st elephant relatives to have trunks appear.

1st Hamsters appear in North America.

Large variety of early horse ancestors appear. Most of them are no more than 24 inches tall at the shoulder.

Miocene epoch: Global temperatures start to increase again.

Grasslands continue to spread in size. Merychippus and other horse relatives are living in the grasslands.

Merychippus stands about 3 feet tall, and has 3 toes on each front and rear foot.

Short legged rhino and rhino with two horns (side by side).

Early mastodons appear.

Several large predators are found in the fossil record: relatives to modern dogs, but they are the size of a black bear.

They have jaws that can crush large bones.

The large shark, Megalodon appears in the fossil record for the 1st time.

Large cats that are related to the saber-toothed cats appear in the fossil record.

Also raccoons, weasels, deer and giraffes appear in the fossil record.

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.

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.

The Alps are forming in Europe.

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 12,000 years ago.

Periods of global cooling produce large glaciers each for about a 100,000 year period of time, and then warming occurs.

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 are also camels, recent relatives of the modern horse.

Dire wolves (large ),

and cave bears (very large).

There is a long horned bison

giant ground sloth

and saber-tooth cat in North America

Early human ancestors have spread to every continent except Antarctica.

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

What we consider to be modern humans appear in the fossil record about 115,000 years ago.

Neanderthals, which are considered a subspecies ofHomo sapiens, first show up around 130,000 years ago.

Homo sapiens appear in the fossil record about 200,000 years ago.

A small mass extinction occurs. Most of the largest terrestrial animals on the earth become extinct.