of continental drift found in the normal process of plate tectonics today.
Evidence for this theory comes from Earth itself. During the formation of rocks, the minerals inside naturally align themselves with the existing magnetic field of the planet. By studying the orientation of grains in the minerals, scientists can determine the position of ancient continents relative to the magnetic north pole, which almost always lies close to Earth’s axis of rotation. When the positions of the continents were plotted using this data, scientists found that there was a major movement of the continents within a relatively short period of time around the Cambrian period. The data showed that ancestral North America moved to the equator between 540 and 515 million years ago, while Gondwanaland shifted between 535 and 500 million years ago.
What are the Paleozoic, Mesozoic, and Cenozoic eras?
The divisions between the eras on the geologic time scale represent major changes on Earth. The division between the Pre-Cambrian and Paleozoic, about 543 million years ago, represents an increase of life on Earth. The division between the Paleozoic and Mesozoic represents a major decrease in plant and animal species (called an extinction) about 250 million years ago. It is also called the “Permian Extinction,” or the “Great Dying,” in which up to 90 percent of all species died out. The division between the Mesozoic and Cenozoic, about 65 million years ago, also represents a major extinction of plant and animal species, including the dinosaurs. This extinction was not as extensive as the Great Dying: only about 50 percent of all species died out at this time.
What are the divisions of the Mesozoic era?
The Mesozoic era, often referred to as the “age of the reptiles” or the “age of the dinosaurs” (even though dinosaurs did not evolve until well into the Mesozoic), lasted from approximately 250 to 65 million years ago. It is divided into three periods: the Triassic, Jurassic, and Cretaceous.
What are the more recent time divisions on the geologic time scale?
The Cenozoic era is divided into the Tertiary and Quaternary (or Anthropogene) periods. The Quaternary is further divided into the Pleistocene epoch, a period of advances and retreats of huge ice sheets; and the Holocene epoch, or recent times, which began about 10,000 years ago.
FIRST FOSSILS
What is a fossil?
The remains of plants and animals that have been preserved in the earth, close to their original shape, are called fossils. This word comes from the Latin fossilis, meaning “something dug up.” The different types of fossils depend on the remains and conditions present at the time the organism died. Fossils may be formed from the hard parts of an organism, such as teeth, shells, bones, or wood; they may also be unchanged from their original features, the entire organism having been replaced by minerals such as calcite or pyrite. Animals and plants have also been preserved in other materials besides stone, including ice, tar, peat, and the resin of ancient trees.
Fossils of single-celled organisms have been recovered from rocks as old as 3.8 billion years. Animal fossils first appeared in rocks dating back over one billion years ago. The occurrence of fossils in unusual places, such as dinosaur fossils in Antarctica and fish fossils on the Siberian steppes, is due to the shifting of the continental plates that make up Earth’s crust, and environmental changes over time, such as an ice age. The best explanation of dinosaurs in Antarctica is not that they evolved there, but that Antarctica was once part of a much larger landmass with which it shared many life forms.
How does a fossil form?
There are a number of ways a fossil forms, depending on the type of remains and the environment present. In general, the process for most fossils is much the same: the hard parts of animals, such as bones, teeth, and shells, as well as the seeds or woody parts of plants, are covered by sediment, such as sand or mud. Over millions of years, more and more layers of sediment accumulate, burying these remains deep within the earth. The sediment eventually turns to stone, and often the remains are chemically altered by mineralization, becoming a form of stone themselves (these are the type of fossils often viewed as the recreated dinosaur skeletons seen in many museums). The same process also produces petrified wood, coprolites (petrified excrement), molds, casts, imprints, and trace fossils.
Uncovering a fossil from the surrounding rock is meticulous work that can take hundreds of hours because paleontologists do not wish to damage an artifact that took millions of years to form (iStock).
Most fossils are found in sedimentary rocks—those rocks produced by the accumulation of sediment such as sand or mud. Wind and other weathering conditions wash away sediment on land, depositing it in bodies of water. For this reason, fossils of sea creatures are more common than those of land creatures. Land animals and plants that have been preserved are found mostly in sediment of calm lakes, rivers, and estuaries.
A fossil may also consist of unaltered original material. Bones and teeth are commonly preserved in this way. However, far more often the pores of bone and teeth are filled with minerals in a process called permineralization (what many have called petrifying). Circulating ground water carries silica or calcium carbonate (and sometimes other minerals, such as pyrite) that fill the pores. What remains is, in essence, a duplicate of the original bone or other organic material.
How likely is it that an organism becomes a fossil?
Not all organisms survive to become fossils, and the chance of a living organism becoming a fossil is generally very low. Many organisms completely decay away or are chewed apart by other animals. Because of this, some scientists estimate that although billions of flora and fauna have lived on Earth, very few survived into fossil form. The fossils we do find represent only a fraction of the animals and plants that ever lived.
An organism has the best chance to become a fossil if it is quickly covered by moist sediment after death, protecting the decaying organisms from predators, scavengers, and bacteria. The soft parts of the organisms (such as skin, membranes, tissues, and organs) quickly decay, leaving behind bones and teeth. The majority of found fossils date back no farther than almost 500 million years ago, when organisms first began to develop skeletons and other hard parts.
The following are the steps to fossilization, using a dinosaur as an example. This outline shows how difficult it is for a dinosaur to become a fossil:
Scavenging and decay—When a dinosaur died, it did not take long for scavengers to remove the soft flesh parts of its body. Those parts that were not eaten decayed at a fast or slow rate, depending on the prevailing climate. In any case, within a short amount of time only a skeleton would remain. But even the remaining hard body parts were not impervious to change. They were often weathered by the action of wind, water, sunlight, and chemicals in the surroundings, rounding the bones or reducing them to small pieces.
Location—If the dinosaur’s skeleton was in an area in which rapid burial did not take place, then the chances of fossilization were slim. The bones would break and scatter, often moved by the action of changing river courses or flash floods. But occasionally, this transport increased the chance of fossilization, moving the bones to a better area for preservation, such as a sandbank in a river.
Burial—The most crucial step in becoming a fossil is burial. The sooner the burial of the dinosaur bones, the better
