Trilobites are fossil animals belonging to the class Trilobita, the phylum Arthropoda. Arthropods include such groups of modern-day animals as those belonging to the classes Insecta (insects), Scorpionamorfa (scorpions), Asamorpha (ticks), Agasinia (spiders) and others. Morphologically speaking, trilobites are most similar to the living horseshoe crabs (class Merostomata) and crustaceans (class Crustacea).
Trilobites appeared on Earth at the beginning of the Cambrian period, about 540 million years ago, and died out without leaving direct descendants in the Permian period, about 250 million years ago.
The Latin term Trilobites was suggested by paleontologists at the beginning of the 19th century. Loosely translated, it means “three-lobed fossil”. Indeed, the dorsal shield of a trilobite consists of three parts: cephalon (head shield), thorax (trunk) and pygidium (tail shield). Each of these parts, in turn, consists of three lobes. There is a prominent glabella in the middle of the cephalon and relatively flat cheeks on both sides of it. The middle part of the thorax is formed by a prominent axis, with pleurae located to the right and left of it. there’s a prominent rachis surrounded by pleurae located in the middle of the tail shield. Thus, the shell (or exoskeleton) of a trilobite is divided into nine parts: three longitudinal and three transverse. There are numerous exceptions to this simple arrangement, however, the cephalon, thorax, and pygidium are always isolated.
The trilobite body was bilaterally symmetrical, i.e. the right parts mirror the left ones. This important sign indicates that trilobites were mobile animals. Each trunk segment had a pair of complex legs or fins, similar to the present-day shrimps, millipedes, or woodlice. Trilobites actively crawled the seabed, could burrow into sediment, and swim but lived only in sea water of normal salinity. Trilobites were deposit and suspension feeders, meaning they scavenged organic detritus. The mouth of trilobites was located below the head shield, had no jaws, and was protected by a special plate called the hypostome in the front. The stomach was located under the glabella, while the intestines were located under the thorax axis. Trilobites were dioecious animals, but the differences in male and female shells are not yet sufficiently understood. Trilobites are believed to pass through an egg stage. The shell of larval specimens consisted of a single tiny plate, up to 1 mm in size, rounded or spiky. The division into sections and segments took place at later stages.
The cephalon was divided into three facial sutures: the cranidium in the middle and the free cheeks (or librigenae) on its sides. The facial sutures are believed to have facilitated the shedding process. The thorax was divided into transverse segments, the number of which varied greatly: from two to more than twenty in different families and genera. These segments allowed the trilobite to bend, which was necessary for crawling or swimming. In addition, many trilobites were able to curl up in case of danger, completely covering the abdominal part of the body under the shell. Some of the present-day crustaceans have the same ability.
In most cases, this structural design caused the shell to fall apart after the animal’s death. Locations where complete trilobites are found are the exception rather than the rule. The shell was composed of chitin and phosphate, but in the fossil state it is often replaced by carbonates or sulfides. The remains of trilobites can be found in different types of sedimentary rocks such as sandstone, siltstone, argillite, marl, or dolomite. However, the best-preserved specimens often occur in limestone.
Like most mobile animals, trilobites had vision. The two compound eyes were located on the head shield, on both sides of the glabella. Like insects, trilobites had schizochroal eyes, consisting of numerous small eyes, each with a separate lens. Many trilobites had large, well-developed eyes. In some species, they are raised on fixed stems. It is assumed that trilobites also had the pineal eye. At the same time, there are known species with reduced eyes, or completely blind. Trilobites, especially the blind ones, had various well-developed sensory organs (tactile or photosensitive) on their glabellae, cheeks, and hypostoma.
The shell of many trilobites may look smooth and shiny. However, microscopic pits, holes, and terrace lines can be seen on it at high magnification. The surface of the shell often features shagreen ornament, as well as various bumps, spikes, or warts. Various spikes can also be present, some longer than the shell itself. Frontal (forward-facing), glabellar (located on the glabella), occipital (on the occipital ring in the back of the head shield), genal (on cheeks), pleural (on the vault or at the ends of trunk segments), marginal and medial pygidial spikes may take place on the cephalon. The actual color of the shell was probably gray, spotty or striped.
Most trilobites weren’t large. The usual size was just a few centimeters. Complete shells with a length of 10–15 cm are rare. At the same time, there are known specimens up to 70 centimeters or more in length. There are many small species not exceeding 10–15 mm.
Trilobites come in a wide range of shapes and sizes. Some hundreds of thousands of species exist. They are found in Paleozoic sediments at all continents around the globe. The majority of species come from the Cambrian and Ordovician periods. That’s also where the largest forms occur. During the Silurian and Devonian periods, the number of trilobites gradually decreased. Carboniferous trilobites are represented by small specimens, which, however, occur as common fossils in some places. Permian trilobites are usually small and rarely found.
