Trilobite

A trilobite is an extinct classification of animal that lived in the ancient seas of the Paleozoic era from 521 to 250 million years ago. The name trilobite means "three lobes," a reference to the organism's body type, which was characterized by three segmented parts. At the time they first evolved, trilobites were one of the most advanced life forms on the planet. Scientists estimate that more than twenty thousand species of trilobite existed, with some growing larger than 2 feet (0.6 meters) long. The creatures were an early form of arthropod, an invertebrate group that includes insects, spiders, and crustaceans.

Background

The first life-forms on Earth are believed to have developed about 3.8 billion years ago. These single-celled organisms took about another 3 billion years to evolve into more complex multicellular organisms—tiny sponges that would eventually lead to the development of worms, anemones, and jellyfish. About 540 million years ago, creatures began developing the first primitive backbones and hard-shelled exoskeletons.

It was about this time that Earth experienced a relatively sudden growth of biological diversity known as the Cambrian explosion. In the span of about twenty to thirty million years—a very short period in evolutionary terms—the fossil record shows that the planet's oceans were filled with millions of complex species. The biological ancestors of almost every modern-day species is believed to have originated during the Cambrian explosion. The event was named after the Cambrian period, a geological time frame that lasted from about 542 to 490 million years ago. Scientists are unsure of its exact cause but speculate that it may have been the result of sudden evolutionary or environmental changes.

Overview

Trilobite fossils first appeared in the geological record about 521 million years ago. These early fossils showed creatures that were already biologically advanced, leading scientists to believe that they had evolved millions of years earlier. The ancestors of the first trilobites may have originated about 550 million years ago and resembled segmented worms with primitive spines. The first trilobites were among the most advanced life forms on Earth at the time. They had complex digestive systems and a protective outer shell made of calcite, a carbon-based mineral. When threatened, trilobites could curl into a ball for added protection. Some evidence suggests that trilobites may have migrated in "herds" in search of food.

Most trilobites had highly developed compound eyes that gave them a wider sense of vision. This allowed them to hunt prey and see approaching predators. Trilobites occupied three spaces in the Paleozoic food chain; they were predators, scavengers, and prey. Some trilobites may have fed on marine worms while others are believed to have eaten plankton or algae. All trilobites had a head, a segmented abdomen, and a tail. Their bodies were divided into three sections, or lobes. The central axial lobe resembled a primitive spine. It was flanked on either side by the right and left pleural lobes. Beneath their shell, trilobites had several pairs of jointed legs. They outgrew their shells over time, shedding their old ones and growing new, larger versions in a process called molting. Some scientists believe trilobites may have been the first organisms to molt. Many trilobite fossils are nothing more than the remains of their discarded exoskeletons.

These evolutionary advantages allowed trilobites to proliferate throughout the Paleozoic oceans. During the nearly 300 million years trilobites existed on Earth, they numbered almost twenty-five thousand species. They reached their peak population about 500 to 488 million years ago, with an estimated sixty-three biological families. A family is a category of biological classification ranked between order and genus.

The average trilobite was about 1 inch (2.54 centimeters) long, while the smallest were nearly microscopic at less than a millimeter. The largest trilobite ever discovered was the 28-inch (71-centimeter) long Isotelus rex, "king of trilobites," found along Hudson Bay in Manitoba, Canada. Isotelus maximus, a type of trilobite found in the American Midwest, could grow up to 16 inches (41 centimeters) long and was named the state fossil of Ohio.

Some trilobites had 2-inch-long (5-centimeter-long) eye stalks, curved ram-like horns, or spines that resembled wings. A species called Cyclopyge bohemica had a large, bulbous eye at the front of its head. Actinopeltis globosus had an almost perfectly round ball growing between its eyes, and Walliserops trifurcatus had a three-pronged, trident-like fork protruding from its head.

The trilobite populations began to decline in the mid-Ordovician period about 472 million years ago. In the Silurian period about 440 million years ago, the number of biological trilobite families had dropped below twenty. Near the end of the late Permian period 260 million years ago, only two trilobite families remained. The last species died out in a mass extinction event about 250 million years ago. The event, sometimes called the "Great Dying," wiped out about 90 percent of plant and animal life on Earth. The exact cause of the extinction is unknown, though scientists speculate that it may have been a massive volcanic eruption or catastrophic climate change.

Trilobites were the most prolific and diverse type of extinct organism in Earth's history. Trilobite fossils are some of the most commonly discovered remnants of ancient life on the planet. Some Native Americans used them as charms and called them "little water bug living in a house of stone." Their shape confused some seventeenth- and eighteenth-century scientists who thought they resembled a flat fish or a petrified insect. Trilobites are part of the same biological category as insects—the arthropods. An arthropod is a classification of animal with a segmented body, an exoskeleton, and paired, jointed appendages. Spiders, crabs, lobsters, scorpions, and centipedes are also considered arthropods. The closest living relative to the trilobite is the modern horseshoe crab.

Bibliography

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"What Is a Trilobite?" FossilEra, www.fossilera.com/pages/about-trilobites. Accessed 19 Sept. 2017.