Trilobites were one of the most successful creatures to ever inhabit our planet, with some 15,000 species that survived from the Early Cambrian through the Permian periods (iStock).

      By looking at the genes preserved in and common to modern animals, researchers are trying to determine a possible cause. One study found that an ancient common ancestor—a worm-like animal from which most of the world’s animals were derived—had special genetic machinery that was so successful that it survives to this day. These genes, used to grow appendages (arms, legs, claws, fins, and antennas), were operational at least 600 million years ago. With appendages, animals swam faster, grabbed tighter, and fought with greater efficiency, and, thus, they could eventually dominate the globe.

      What were trilobites and when did they exist?

      Trilobites were some of the most successful creatures to ever live on Earth—and finding one of their fossils is one of the joys of modern fossil collecting. These hard-shelled, segmented animals lived for hundreds of millions of years in the oceans; they are called “three-lobed” because of their head, thorax, and tail. They were one of the first arthropods (animals with jointed legs), comprised over 15,000 different species, and ranged in size from about a half inch (3 millimeters) to 2.3 feet (70 centimeters) long.

      Trilobites first appeared in the Early Cambrian (during what is called the “Cambrian Explosion”); they increased in diversity in the Devonian period; and their numbers decreased in the Silurian (probably due to the appearance of sharks and other predators). They became extinct after the Permian mass extinction. Although they did not exist during the time of the dinosaurs, scientists respect this species for its tenacity and abundance.

      What theories attempt to explain why the Cambrian Explosion occurred?

      In the past, paleontologists have offered many theories as to why the great blossoming of life, known as the Cambrian Explosion, occurred. Some point to the mass extinction of Ediacara organisms (the earliest known complex multi-cellular organisms); another theory states that the development of eyes in animals changed the predator-prey dynamics; and yet another theory suggests that an increase in size of many organisms accelerated diversity.

      There is also one study that places the blame, or credit, for this evolutionary explosion on our planet itself—and although not everyone agrees, it is an interesting theory. More than 500 million years ago, shifting masses in the interior mantle of Earth essentially unbalanced the planet, or “tipped” it, causing the entire surface to reorient itself in an effort to become balanced again. In a process called “true polar wander,” the ancestral North America moved from near the South Pole up to the region of the equator; the large continent of Gondwanaland (made up of South America, Antarctica, Australia, India, and Africa) traveled all the way across the Southern Hemisphere. This movement all happened at more than twice the rate 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.