The remains or imprint of an organism from a previous geologic time. A fossil can consist of the preserved tissues of an organism, as when encased in amber, ice, or pitch, or more commonly of the hardened relic of such tissues, as when organic matter is replaced by dissolved minerals. Hardened fossils are often found in layers of sedimentary rock and along the beds of rivers that flow through them.The word fossil comes from the latin word fossilis,which means "DUG UP".
Most fossils are excavated  from sediment rock layers . Sedimentary rock is rock that has formed from sediment, like sand, mud, small pieces of rocks. Over long periods of time, these small pieces of debris are compressed (squeezed) as they are buried under more and more layers of sediment that piles up on top of it. Eventually, they are compressed into sedimentary rock. The layers that are farther down in the Earth are older than the top layers.

The fossil of a bone doesn't have any bone in it! A fossilized object has the same shape as the original object, but is chemically more like a rock.



570 Million Years Ago .
The period between the formation of the Earth and the Paleozoic era is called the Precambrian Era by geologists and paleontologists. The Precambrian era ends 570 million years ago.
Most Precambrian fossils are very small, almost microscopic. The larger species of Precambrian life that lived in later Precambrian time had soft bodies and lacked the shells or bones ordinarily necessary for the formation of readily identifiable fossils.
The first abundant fossils of larger animals date from about 600 million years ago. The first mollusk fossils appear in early Cambrian rocks, about 600 million years old.
570 to 225 Million Years Ago (The Paleozoic Era)
The Paleozoic Era lasted from 570 to 225 Million Years Ago. Complex forms of life appear. During this 345 million year period, plants and animals underwent rapid evolution.
The Paleozoic era lasted about 345 million years. It includes the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian periods. Index fossils of the first half of the Paleozoic era are those of invertebrates, such as trilobites, graptolites, and crinoids. Remains of plants and such vertebrates as fish and reptiles make up the index fossils of the second half of this era.
570 Million to 500 Million Years Ago.
The Cambrian Period began about 570 million years ago and ended 70 million years later.
At the beginning of this 70 million year period, the only animal life we know about lived in the oceans. However, by the end of the period, all
the phyla of the animal kingdom existed, excepting only animals with backbones, the vertebrates.
Animals common to this period include trilobites (a primitive arthropod, related to the insects, lobsters and ticks), snails, cephalopod mollusks, brachiopods, bryozoans, and foraminifers. Plants of the Cambrian period included seaweeds in the oceans and lichens on land.
500 to 430 Million Years Ago
The Ordovician Period began about 500 million years ago and ended 70 million years later. Small hemichordate worms (graptolites), with an anatomical structure that appear to be a precursor of a spinal cord, appear at the beginning of this period. The end of this period saw the emergence of the first vertebrates, animals with backbones, in the form of primitive fish.
430 to 395 Million Years Ago
The Silurian Period began about 430 million years ago and ended 35 million years later. About 430 million years ago, the first air breathing animal appeared, a scorpion. Simple land plants lacking separate stems and leaves first appeared.
395 to 345 Million Years Ago
The Devonian Period began about 395 million years ago and ended 50 million years later. During this period, fish were dominant, including sharks and lungfish.
The ancestors of the amphibians appeared in the form of a primitive form of hard scaled ganoid fish. Early proto-amphibians may have existed during this period. The earliest fossil insect is found in the Devonian period.
This is the first period during which significant numbers of fossilized plants have been found. This was the period during which the first woody plants developed. By the end
of the period ferns, ferns, scouring rushes and scale trees had developed. There were forests for the first time.
345 to 280 Million Years Ago
The Carboniferous or Mississippian Period began about 345 million years ago and ended 65 million
years later. During this period stegocephalia —amphibian land animals developed from the lungfish— first appeared. Plant got larger and more diverse in form.
In the second part of the Carboniferous period, known as the Pennsylvanian, the first reptiles appeared, having developed from amphibians. The reptiles lived entirely on land. Insects, spiders, snails, scorpions and over 800 species of cockroaches flourished. The first true conifers developed from gymnosperms during the Pennsylvanian.
280 to 225 Million Years Ago
The Permian Period began about 280 million years ago and ended 45 million years later. During this period
many forms of marine animals disappeared, but the reptiles (mostly lizard like) rapidly evolved and spread.
Importantly for us, there was one small group of reptiles called the Theriodontia that evolved during this period. This group included the ancestor of all mammals. The forests were largely made up of ferns and conifers.
225 to 65 Million Years Ago (The Mesozoic Era)
The Mesozoic Era lasts from 225 to 65 Million Years Ago, a 165 million year period.
This era is often called the Age of Reptiles because dinosaurs and other reptiles were the dominant land animals throughout this era. During this period the seven continents are formed out of one large continent called Pangaea. This period includes the Triassic, Jurassic &Cenozoic.
225 to 195 Million Years Ago
The Triassic Period began about 225 million years ago and ended 30 million years later. The dinosaur first evolved during this period. During this period the dinosaurs were smaller than later, seldom exceeding 15 feet. Sea going reptiles, the ichthyosaurs, and flying reptiles, the pterosaurs also evolved at this time.
Of particular importance to us, this was the period in which the first mammals appeared. These were small reptile like creatures, though probably warm blooded. The first bony fish appeared in the ocean.
Evergreens, including ginkgoes, conifers and palms dominated the forests. Ferns and small scouring rushes were still present, as they are today, but in smaller numbers, and the larger members of these groups had become extinct.
195 to 136 Million Years Ago
The Jurassic Period began about 196 million years ago and ended 41 million years later. This was the beginning of the age of the large dinosaurs, such as Brontosaurus and Tyrannosaurus. Four orders of mammals now existed by this time, all smaller than the modern dog. Many new insect orders appeared for the first time, such as beetles, grasshoppers and moths. Forests of thick
stemmed palm like plants known as cycads were uniformly common, including in the polar regions,
indicating the presence of a mild climate over most all of the earth.
136 to 65 Million Years Ago
The Cretaceous Period began about 136 million years ago and ended 71 million years later. Fossils of pterodactyl (flying dinosaurs) were discovered in Texas with wingspreads of up to 50 feet. The first snakes appeared at this time as did several types of birds. During this period the first marsupials and placental mammals appeared, belonging to the group of insectivores.
Deciduous plants (Angiosperms) first appeared during the Cretaceous. Most (over 90%) of the modern varieties of trees and shrubs were present by the close of the period. Some paleontologists believe that deciduous woody plants were present during the Jurassic period, but in conditions unfavorable to fossil preservation.
For reasons not yet entirely clear, by the end of the Cretaceous period the dinosaurs had become extinct.
65 Million Years Ago to the Present (The Cenozoic Era)
The Cenozoic Era began about 65 million years ago and has not yet ended. The beginning of the Cenozoic marks an abrupt transition from the age of the reptiles to the age the mammals, marked by a dramatic die off of the former. It includes the Paleocene, Eocene, Oligocene, Miocene, Pliocene, Pleistocene, & the Postglacial, epoch (the Holocene).
65 to 54 Million Years Ago
The Paleocene Epoch began about 65 million years ago and ended 11 million years later.
There are seven groups of mammals known in this period, each of which apparently developed in Northern Asia
before migrating. All had similar features
. The largest species was the size of a small bear. They all had four feet and five toes on each foot, and most had slim heads with narrow muzzles and small brains. The three predominate groups are now extinct. One, the creodonts, evolved into the modern carnivores. The other four that survived are the marsupials, the insectivores, the primates, and the rodents.
54 to 38 Million Years Ago
The Eocene Epoch began about 54 million years ago and ended 16 million years later. During this period the ancestors of the horse, rhinoceros, camel, rodent, many modern birds, and monkey all appeared. They were all small in size. The first aquatic mammals, such as the ancestor of the whale appeared.
38 to 26 Million Years Ago
The Oligocene Epoch began about 38 million years ago and ended 12 million years later. The first dog and cat like carnivores appeared, evolving from the creodonts, which became extinct. Several mammal groups that are now extinct flourished, including the titanotheres, which are related to the rhinoceros and the horse, and the oreodonts, which were small, doglike, grazing animals.
26 to 12 Million Years Ago
The Miocene Epoch began about 26 million years ago and ended 14 million years later. The grass family of plants made their appearance for the first time in the Miocene Epoch. This in turn
encouraged the further devilment of the grazing animals. The mastodon evolved at this time.
12 Million to 2 Million Years Ago
The Pliocene Epoch began about 12 million years ago and ended 10 million years later. This epoch was similar to the Miocene, but is regarded by some as the climax of the age of mammals.
2 Million Years Ago to the Present (The Quaternary Period)
The Quaternary Period is made up of the Pleistocene epoch and the Postglacial, epoch
, the Holocene. It begins 2 million years ago and continues to this day.
2 Million Years Ago to Ten Thousand Years Ago
The Pleistocene epoch began about 2 million years ago and ended ten thousand years ago.. During this period of time as much as 25% of the world was covered with ice. The period is marked by the presence of large mammals, many of which are now extinct, very possibly hunted to extinction by humans.