Our story today is about trilobites, and the curious reader will learn much about their habits and history.

Trilobites are a class of marine arthropods that were widespread in the Paleozoic seas and became extinct at the end of the Permian period. Among living creatures, their closest relatives are horseshoe crabs, although they are only distantly related to these ancient beings.

Trilobites attract the attention of many—paleontologists, evolutionary biologists, collectors, and filmmakers alike. They represent an incredibly successful evolutionary group. Despite maintaining a consistent body plan, trilobites managed to occupy numerous ecological niches and thrived for over 300 million years.

Today, we will set aside the usual study of fossils and computer reconstructions. To get to know the protagonists of our story, we will travel back in time to the Silurian period.

So, 430 million years ago, in the Southern Hemisphere, on the coast of the supercontinent Gondwana. The vast ocean stretches to the horizon, and somewhere to the north, its waves crash against the shores of Laurentia, Baltica, and Angarida—other continents of this young world.

Inland, rocky hills give way to mountains, beyond which lie vast wastelands. There is no familiar greenery, but lichens paint the rocks with color, and damp areas are covered with a soft carpet of moss, above which rise delicate shrubs of some kind. The day is warm, even hot, but we must wear special suits and breathe through oxygen masks—the ozone layer has not yet formed, and there is too little oxygen in the atmosphere (about 10%), with three times more carbon dioxide than today.

The tide begins to recede, and not all sea creatures can keep up with the retreating waters. Here, on the exposed seabed, among the brown-green clumps of algae, brachiopod shells, sea stars, and puddles of water, strange creatures crawl slowly. Covered with segmented armor, they resemble giant pill bugs. On the powerful head shield of their armor, compound eyes stand out, and long antennae probe the surface ahead. At our approach, trilobites try to hide in the puddles, and some curl up into a ball. This behavior can save them from predators, but we easily capture a few specimens, and now it is time to examine them more closely.

Trilobites are arthropods, members of the same phylum as spiders, centipedes, crabs, beetles, and butterflies.

The general body plan of trilobites is consistent despite differences in size and lifestyle. The body is divided into three parts both lengthwise and crosswise. In the transverse plane, these are: the head (cephalon), the body (thorax), and the tail (pygidium). In the longitudinal plane, there is the central axial lobe (rhachis) and two pleural lobes/plates on the right and left sides. The head shield conceals the most important organs—the brain and stomach. The other body segments have a uniform structure. Each segment contains a nerve cord with ganglia, intestines, and a long, multi-chambered vessel—the heart.

Like all arthropods, the body of a trilobite is covered with a chitinous carapace that acts as an exoskeleton. From the inside, connective tissue, muscles, and internal organs are attached to the carapace. The carapace is not a solid, monolithic structure; it consists of many segments, providing the animal with good mobility. Its thickness ranges from 1 mm or more in large species. High strength is achieved in two ways: by mineralizing the chitin with calcium salts and through structural features. Various ridges, spines, and other outgrowths create additional stiffening ribs that reinforce the trilobite’s armor.

The trilobites we caught have a dark, uniform gray-green color, but some individuals have a reddish-brown carapace with spots resembling camouflage.

One of the interesting features of trilobites is their ability to curl up into a ball in times of danger. Our specimens now look like oversized pill bugs. In this state, they can remain for a long time, but if left undisturbed, they slowly uncoil.

If you turn a trilobite over, you will see many articulated legs moving. Since limbs rarely fossilize, we now have a unique opportunity to study them alive. At first glance, they seem identical, but this is not the case. The very first pair of limbs has been modified into antennae—with them, the trilobite feels and smells objects. Beneath the head shield are four pairs of strong legs for grasping and shredding food. The limbs on the abdomen and tail are divided into two branches. The main one is the walking leg. And from its base extends a special bristle-covered plate. These are the gills, part of the trilobite’s respiratory system. In the rear, under the pygidium, these bristles are much larger and more massive, serving as swimming paddles.

A sturdy external skeleton is good protection against enemies, but it has a significant drawback—it cannot stretch as the animal grows. Therefore, the carapace must be periodically shed—molted. On the trilobite’s head shield, special sutures are located along which the old shell would split. The molting process began with the release of the eyes, and their absence in a fossil is one sign that we are dealing with a shed exoskeleton. Here, on the shore of the Silurian Sea, these exoskeletons are found in large heaps, covered with a layer of sand and silt. It is evident that trilobites gathered in one place for molting, just as modern crabs do, to protect each other from predators.

Trilobites were among the first animals to develop complex eyes. Look into these facets—their stony gaze is mesmerizing even after millions of years. And “stony” is not just a metaphor. The eye of a trilobite consists of many facets (from 70 to 10-15 thousand). Each facet contains two lenses. The lower one is made of chitin, while the upper one is a calcite crystal with magnesium impurities. This is very unusual. Similar “mineral” lenses in eyes are found only in two other groups of living creatures—chitons, armored mollusks, and ophiuroids, a group of echinoderms. But only in trilobites are they highly developed and provide excellent vision. Their origin is the result of parallel evolution and the unique metabolism of these animals.

Trilobites were masters of biochemical work with calcium carbonate. Analysis of fossilized carapaces and samples taken from living trilobites show a high degree of mineralization of their chitinous armor, which is also unmatched among arthropods.

Such an eye structure allowed them to see objects clearly at distances of up to several meters, with numerous facets forming a three-dimensional, focused image of the object. In fossils, trilobite eyes are preserved almost unchanged, including their color—in some, they may be turquoise, emerald green, or yellow.

Like most modern arthropods, trilobites were dioecious and laid eggs.

To distinguish between a male and female trilobite, one must pay attention to body shape and the sculptural elements of the carapace. Males have narrower bodies, but their carapaces may be richly decorated with outgrowths. Females have fewer spines and protrusions, but their bodies are wider and stronger. However, the main external difference is the brood pouch, a special adaptation on the head shield or its underside where eggs are stored.

Trilobite larvae look very different from their parents—they have no eyes, and their bodies are covered by an undivided shield. These larvae swim freely in the water column and are carried over long distances by waves and currents. As the young trilobite grows, it becomes more and more like an adult. Many modern marine arthropods also have a planktonic stage in their reproduction.

Like some crustaceans, trilobites were capable of migrating as adults. It is known that trilobites could form chains, like lobsters, and move in large groups over long distances. What causes them to migrate, whether it’s a seasonal phenomenon or not, remains to be determined.

To study the lifestyle of trilobites in their natural environment, we will have to dive to the bottom of the Silurian Sea in a mobile laboratory and set up observation cameras.

Our shallow lagoon is separated from the ocean by a barrier reef. Its builders are stromatoporoids, colonial organisms related to sponges. Along with single-celled algae and corals (rugose and tabulate), they form a complex community where sea lilies, anemones, brachiopods, and mollusks settle.

Here, among the coral bushes, we encounter Paralejurus trilobites, small creatures with a smooth, massive cephalon and large eyes. They are numerous and slowly crawl from place to place in search of food—mollusks, a dead horseshoe crab, or a fish trapped among the sea lilies will become their prey. Another pair of similar specimens is busily gnawing on a sponge, trying to extract something edible from it.

Overall, trilobites are predators that lead a benthic lifestyle. Their primary prey is a variety of worms and other soft-bodied invertebrates. Some species ambush passing prey—burying themselves in the sand, leaving only their eyes visible, and pouncing on their victim with a swift motion. For example, Asaphus kowalewskii, which has eyes on long stalks, hunted this way, though we do not observe similar species in the Silurian. However, at night, the camera captured a remarkable scene of another trilobite, Cheirurus, stalking its prey. With elongated and sharp horns extending backward from its cephalon, and two extended spines on its pygidium, Cheirurus looks formidable. Swiftly moving along the seabed, it examines the burrows of polychaete worms. One of them catches its attention, and the trilobite lies in wait. About an hour passes before the worm emerges from its burrow, and as soon as it fully exits, a rapid lunge follows, sending a cloud of sand and silt into the water. The worm is seized across its body by the trilobite’s front legs, and after a brief struggle, it is torn into several pieces.

Studying the camera recordings and making dives in scuba gear, we found that the diversity of trilobites is vast even in such a small area as a tiny lagoon. The coastal algae thickets are inhabited by small, fingernail-sized trilobites, nibbling on their juicy stems. In the depressions where silt and viscous mud accumulate, we found spiny creatures resembling Dicranurus. The long outgrowths on their carapace help them avoid sinking into this thick sand-silt mire.

Some of the burrows we initially thought were made by worms turned out to be trilobite burrows. These trilobites have smooth, flattened bodies and tiny eyes. Eating detritus and small bottom-dwellers, they dig their tunnels and occasionally emerge on the surface at night. A major discovery, in every sense, was finding the true king of trilobites—Isotelus. Several large individuals, up to a meter in length, inhabited the base of the reef, tirelessly plowing the lagoon floor in search of prey.

Having gathered the necessary information and samples, we return to our own time. Our journey to the Silurian period is over, but the story of trilobites will continue for another 200 million years.

After the flourishing of the Silurian, there will be a gradual decline in the diversity and numbers of these remarkable creatures. The Devonian period, the next epoch, will bring significant changes. Fast fish with powerful jaws capable of crushing the hard carapaces of arthropods will come onto the scene. Ammonites will appear. Among benthic predators, a special place will be occupied by eurypterids—sea scorpions up to two meters long. Marine ecosystems will be reorganized, becoming increasingly complex. By the Permian period, almost nothing will remain of the former diversity of trilobites. The last of the ancient ones will disappear at the Permian-Triassic boundary, during the Great Dying, and their ecological niche will eventually be taken over by isopods—similar to woodlice.