Trilobites constitute a large and diverse class of extinct marine arthropods with more than 10,000 species in about 2,000 genera. They occurred, proliferated, and became extinct during the Paleozoic Era. At times, notably in the early Paleozoic, the trilobite proliferation was so remarkable that they surpassed in number and diversity of species most groups of multicellular animals living then. Just as the Mesozoic may be considered the dinosaur era, so the Paleozoic is believed to be the era of trilobites. The trilobite body was shelled with a chitinous carapace, hard and resistant to chemical reactions.
The carapace protected the animal from the outside and also was used for attaching inner organs (primarily the motor muscles); therefore, it is thought to be a peculiar kind of armored exoskeleton (exposed, outer skeleton). The trilobite carapace was saturated with mineral salts, mainly calcium carbonate, which made it particularly strong. Owing to this calcification, the trilobite carapace has been well preserved in the ground over a few hundred million years.
The trilobite carapace can be divided both lengthwise and across into three parts. The lateral division includes:
Cephalon (head) with well-developed eyes.
Thorax, comprising a varied number of segments flexibly attached to one another.
Pigidium (tail piece) which is distinguished from the thorax because its segments are securely attached and could not be moved independent of one another.
Two longitudinal, almost parallel spinal valleculae (grooves), divide the shell into three lobes, an axial lobe and two pleural ones. These three lobes, which run from the cephalon to the pygidium, are what give trilobites their name; “trilobite” means “three-lobed”.
Only the dorsal part of the carapace was calcified; the ventral, or under, side—comprising limbs and other organs of motion, feeding, breathing, and sense—was very soft and tender. Most trilobites were able to enroll (curl tightly, like the fingers in a fist) to protect this soft abdomen In the event of danger. But they didn’t develop this ability overnight. During the Cambrian period, when trilobites first emerged and reproduced, few species had the ability to enroll. As early as the Ordovician, however, hardly any unenrolled species remained. It is assumed that this was due to the scarcity of cephalopods (the major predators of marine arthopods) in the Cambrian, whereas cephalopods reproduced vigorously and sometimes reached gigantic sizes during the Ordovician period.
The majority of trilobites had a remarkably developed vision. Their eyes have an compound structure (composed of many tiny facets, or lenses), similar to that of modern insect eyes. The visual surfaces of the huge, faceted eyes of trilobites were arranged in such a way that many of these animals had a 360 degree visual field, but only along the horizon line and ten degrees above it. A single eye in the occipital part of the trilobite head shield enabled the animal to observe the “blind space” in the apex. The eyes of some trilobite species, which searched for food by digging into the upper layer of mud, were elevated on long pedicles—hard, upright growths of carapace. Because the chitinous carapace of arthropods cannot stretch, their growth is accompanied by several moltings. Once the carapace becomes too tight, it breaks and the animal drops it. During this interim period, after the old carapace has been shed but the new one is not yet hardened, the animal’s size increases dramatically. The posture during the shedding of the old carapace is very peculiar and varies in different trilobite species.
The trilobite carapace is very diverse in shape: smooth, bumpy, or spiny, with huge or reduced eyes that may be situated low or elevated on long pedicles, with long branching growths or none, with a thorax of just two segments or of dozens, and so on.
A wide, flat carapace was characteristic of those species that crawled slowly on the bottom surface. The ones that moved actively along the bottom and dug into the bottom sediments feature a convex carapace with deep grooves. The species that were constantly digging in the ground had a thick, arched carapace with a smooth surface. In some trilobite species, the carapace suggests a planktonic way of life. They had small bodies and huge eyes and, when they were enrolled, there remained two big unprotected holes—the place where long amphibian limbs were located.
Trilobite legs are amazing, enabling the animals to dig into the mud, swim near the bottom surface, and even to breathe, since gills were attached to them. Dozens of trilobite species with different types of carapace could inhabit the same place. This suggests that their feeding and life styles differed greatly, minimizing competition.
Previously, it was believed that trilobites (with the exception of planktonic species) fed by taking in their mouths the upper layer of the benthic mud, rich in organics, because their soft limbs seemed inadaptable to capture prey. Recently, however, new evidence from Sweden suggests that some trilobite species were definitely predators. There, some trilobite tracks covered those of some ground-dwelling animals. In these cases, the tracks of the ground-dwelling animals disappear. Those trilobites obviously hunted and ate animals that inhabited the bottom sediments. In Yakutia, trilobites have been found with well-preserved bowel contents, including particles of bottom-dwelling animals—sponges and brachiopods.
Experts believe that trilobites finally went extinct about 230 million years ago. What caused the trilobite extinction? Findings of trilobite fossils with traces of predator bites have been reported. Apparently, even their hard carapace and ability to enroll into a ball failed to save them from some predators. However, the abundance of predators—ancient cephalopods, fishes, gigantic merostomes—is unlikely the major reason for the overall extinction of trilobites. What caused the disappearance of the trilobites remains unclear to science.
Among modern fauna, there is a class of arthropods (Isopoda) that is remarkably similar in appearance to the late trilobites. It may well be that some of them have filled a niche in the ocean ecology opened up by the extinction of the ancient trilobites.
For a more detailed discussion of trilobite morphology and ecology, see the rich site mounted by S. M. Gon III.