Characteristics of Trilobites

About Trilobites

• The trilobites are part of the Arthropoda phylum. The trilobite class is divided into ten orders; over 20,000 species are believed to have existed. The class is hugely diverse. Sizes of trilobites range from one millimeter to nearly 70 centimeters. They also vary widely in shape and texture; however, a few characteristics are common to trilobites.

Hard Shell

• Trilobites have a hard exterior, or exoskeleton, which is believed to have helped protect them against predators. This shell, made up of three layers, was very strong, but varied in thickness and shape among the trilobite species. Trilobite exoskeletons have distinguishing details called prosopon, which include nodes, pits, ridges, spines and tubercles.

Body Sections

• All trilobites have three distinct body sections. These are the cephalon, the thorax and the pygidium. The cephalon, or head section, contains the eyes, mouthparts and antennae. The thorax, or middle section, is comprised of many separate segments providing flexibility for the trilobite to bend or roll up into a ball (referred to as enrollment). The pygidium, or tail section, is comprised of several fused segments and varies in size among species.

Body Lobes

• The "tri" in trilobite refers to the three lobes of the body. Trilobites have a central axial lobe with pleural lobes on either side; this too is common to all trilobites. The central axial lobe runs the length of the body and contains the glabella, or middle portion of the cephalon.

Eyes and Vision

• Trilobites were among the first creatures to develop sophisticated eyes. Most trilobites had eyes made up of many units, or compound eyes, similar to those found in insects today. Their vision was highly sensitive to motion and had good depth of field with minimal distortion. Although there are huge variations in vision systems among the trilobite species, the types of eyes tend to fall into three main types: holochroal, schizochroal and abathochroal.