Trilobites (pronounced "TRY-luh-BYTS" or "TRIH-luh-BYTS") are extinct marine arthropods that belong to the class Trilobita. They are among the earliest arthropods found in the fossil record and were some of the most successful early animals. Trilobites lived in oceans for nearly 270 million years, and more than 22,000 species have been identified. Their hard outer covering, made of calcite, made them easy to fossilize, leaving behind a large number of fossils. These fossils have helped scientists study Earth's history, including how life evolved, how rock layers are dated, and how continents moved over time. Trilobites are part of a group called Artiopoda, which includes many organisms that look similar to trilobites but lack hard parts. Scientists are still unsure how Artiopoda relates to other arthropods.
Trilobites filled many different roles in ancient ecosystems. Some crawled along the seafloor as predators, scavengers, or filter feeders, while others swam and ate plankton. A few even moved onto land. Trilobites had most of the lifestyles seen in modern marine arthropods, except for parasitism, which is still debated by scientists. Some trilobites, like those in the Olenidae family, may have lived with sulfur-eating bacteria, using them for food. The largest trilobites grew over 70 centimeters (28 inches) long and could weigh up to 4.5 kilograms (9.9 pounds).
The first trilobites appeared in the fossil record around 521 million years ago, marking the start of the Atdabanian/Cambrian Stage 3 in the Early Cambrian period. Soon after their appearance, trilobites were widespread and diverse. Their greatest variety occurred during the late Cambrian and Ordovician periods. Trilobites remained diverse through the Silurian and early Devonian periods but declined sharply during the mid- to late Devonian due to several extinction events, including the Taghanic extinction, the Late Devonian mass extinction, and the Hangenberg extinction. These events nearly wiped out all trilobites, leaving only the Proetida order. Trilobites slightly recovered during the Early Carboniferous period but then declined again during the late Carboniferous and Permian periods. They remained common until the end-Permian mass extinction about 251.9 million years ago, when only a few species were left.
Evolution
Trilobites are part of a group called Artiopoda, which includes extinct arthropods that look similar to trilobites. Only trilobites had hard, mineralized exoskeletons. Other artiopodans are usually found only in rare, well-preserved fossils from the Cambrian period.
Scientists are not sure how artiopods are related to other arthropods. Some think they are closely related to chelicerates, like horseshoe crabs and spiders, as part of a group called Arachnomorpha. Others believe they are more closely related to Mandibulata, which includes insects and crustaceans, as part of a group called Antennulata.
The earliest trilobites in the fossil record are redlichiids and ptychopariid bigotinids, dating back about 520 million years. Some possible early trilobites include species like Profallotaspis jakutensis (Siberia), Fritzaspis (western US), Hupetina antiqua (Morocco), and Serrania gordaensis (Spain). These trilobites appeared around the same time in Laurentia, Siberia, and West Gondwana.
Olenellina trilobites do not have facial sutures, which scientists think was their original feature. The earliest trilobite with facial sutures, Lemdadella, appeared nearly at the same time as Olenellina, suggesting trilobites may have existed before the Atdabanian period but left no fossils. Other groups, like Agnostina and some Phacopina, later lost their facial sutures. Olenellina also lack early stages called protaspid, which may mean these stages were not calcified, a feature scientists think was original.
Three trilobites from Morocco, Megistaspis hammondi, dated 478 million years old, have preserved soft parts. In 2024, researchers found soft tissues and structures like the labrum in well-preserved trilobites from the Cambrian Stage 4 in Morocco. These discoveries provide new information about trilobite anatomy and suggest their preservation was caused by rapid death after an underwater volcanic event.
Trilobites diversified over time. As a long-lived group, their history includes many extinction events that wiped out some groups while allowing others to adapt and fill new roles. Trilobites remained diverse during the Cambrian and Ordovician periods but declined gradually in the Devonian, ending with their extinction at the end of the Permian.
Key evolutionary changes from early trilobites, like Eoredlichia, include new eye types, better enrollment and articulation mechanisms, larger pygidium (from micropygy to isopygy), and extreme spines in some groups. Other changes involved narrower thoraxes, varying numbers of thoracic segments, and changes to the cephalon, such as glabella shape, eye position, and hypostome specialization. Some features, like eye reduction or miniaturization, appeared independently in different groups.
Effacement, the loss of surface details on the cephalon, pygidium, or thorax, is a common trend. Orders like Agnostida and Asaphida, and the suborder Illaenina, show this. Effacement may indicate burrowing or pelagic lifestyles, but it makes it harder for scientists to classify trilobites.
Trilobites did not originate in the Precambrian, as once thought. They likely appeared shortly before their first fossils in the lower Cambrian. Soon after, they diversified into major orders like Redlichiida, Ptychopariida, Agnostida, and Corynexochida. A major crisis in the Middle Cambrian led surviving orders to develop thicker cuticles and isopygius or macropygius bodies for better predator defense. The Late Cambrian marked the peak of trilobite diversity, but the end-Cambrian extinction event wiped out most Redlichiida and Late Cambrian trilobites.
Notable Cambrian trilobite genera include:
• Abadiella (Lower Cambrian)
• Buenellus (Lower Cambrian)
• Judomia (Lower Cambrian)
• Olenellus (Lower Cambrian)
• Ellipsocephalus (Middle Cambrian)
• Elrathia (Middle Cambrian)
• Paradoxides (Middle Cambrian)
• Peronopsis (Middle Cambrian)
• Xiuqiella (Middle Cambrian)
• Yiliangella (Middle Cambrian)
• Yiliangellina (Middle Cambrian)
• Olenus (Late Cambrian)
The Early Ordovician saw the rise of articulate brachiopods, bryozoans, bivalves, echinoderms, and graptolites, many of which first appeared in the fossil record. While Cambrian trilobite diversity peaked, they still played a role in the Ordovician radiation event. Groups like Phacopida and Trinucleioidea became highly diverse but had uncertain ancestors. The Ordovician mass extinction affected some trilobite groups, like Telephinidae and Agnostida, but many survived into the Silurian. The Ordovician marked the last major diversification of trilobites, with few new patterns emerging afterward.
Notable Ordovician trilobite genera include:
• Cyclopyge (Early to Late Ordovician)
• Selenopeltis (Early to Late Ordovician)
• Parabolina (Early Ordovician)
• Cheirurus (Middle Ordovician)
• Eodalmanitina (Middle Ordovician)
• Trinucleus (Middle Ordovician)
• Triarthrus (Late Ordovician)
Most Early Silurian trilobite families are part of the Late Ordovician fauna. Few Early Ordovician groups survived the Ordovician extinction, but 74% of Late Ordovician trilobites did. These survivors formed all post-Ordovician trilobite groups except Harpetida. Silurian and Devonian trilobites continued to evolve but showed fewer major changes compared to earlier periods.
Fossil distribution
Trilobites were mostly marine animals, as their fossils are always found in rocks that also contain fossils of other salt-water creatures like brachiopods, crinoids, and corals. Some evidence suggests trilobites occasionally moved onto land. In the ocean, trilobites lived in many environments, from very shallow water to the deep sea. Trilobites, like brachiopods, crinoids, and corals, are found on all modern continents and were present in every ancient ocean where Paleozoic fossils have been found. Fossil remains of trilobites can include their entire bodies or parts of their exoskeletons, which they shed during a process called ecdysis. Also, the tracks trilobites left on the sea floor are often preserved as trace fossils.
Three main types of trace fossils are linked to trilobites: Rusophycus, Cruziana, and Diplichnites. These fossils show the activities of trilobites on the sea floor. Rusophycus are traces made when trilobites rested, hid, or hunted, with little movement. Cruziana are furrows in sediment, likely made by trilobites as they fed on the seafloor. Diplichnites are often tracks left by trilobites walking on the sediment. Care is needed because similar fossils in freshwater or later rock layers may not be from trilobites.
Trilobite fossils are found worldwide, with thousands of species identified. They appeared quickly in Earth’s history and molted like other arthropods, making them useful index fossils that help geologists determine the age of rocks. Trilobites were among the first fossils to draw attention, and new species are still being discovered.
In the United States, a famous public trilobite collection is in Hamburg, New York, at a site called Penn Dixie. The quarry stopped mining in the 1960s, and trilobites were found there in the 1970s by Dan Cooper. He helped raise interest in the site. The fossils are from the Givetian period (about 387 to 382 million years ago), when western New York was near the equator and covered by water. The town of Hamburg bought the land to protect it from development, and in 1994, it became Penn Dixie Fossil Park & Nature Reserve. The two most common trilobite species found there are Eldredgeops rana and Greenops.
In the United Kingdom, a famous trilobite site is Wren’s Nest in Dudley, West Midlands, where Calymene blumenbachii is found in the Silurian Wenlock Group. This trilobite is on the town’s coat of arms and was called the Dudley Bug or Dudley Locust by workers in old quarries. Another site is Llandrindod Wells in Powys, Wales. In Utah, the Elrathia kingi trilobite is found in the Cambrian Wheeler Shale.
Well-preserved trilobite fossils, including soft body parts like legs and gills, have been found in places like the Cambrian Burgess Shale in British Columbia, Canada; the Ordovician Walcott–Rust quarry and Beecher’s Trilobite Bed in New York, U.S.; the Lower Cambrian Maotianshan Shales in China; the Devonian Hunsrück Slates in Germany; and the Cambrian Wheeler Shale in Utah.
In Morocco, trilobites are often found perfectly preserved in mudslides. Their recovery has led to debates about restoration practices. These fossils show a wide variety of eye shapes and body features, similar to how bodies were preserved in Pompeii.
The French paleontologist Joachim Barrande studied trilobites in the Cambrian, Ordovician, and Silurian periods of Bohemia. He published the first volume of Système silurien du centre de la Bohême in 1852.
Importance
Niles Eldredge's study of Paleozoic trilobites along the Welsh-English borders was important in helping to develop and test punctuated equilibrium, a theory about how species change over time.
Finding differences between 'Atlantic' and 'Pacific' trilobite groups in North America and Europe suggested that the Iapetus Ocean had closed, forming the Iapetus suture. This discovery supported the idea that continents have moved over Earth's history.
Trilobites are useful for understanding how quickly new species appeared during the Cambrian explosion because they are the most varied group of multicellular animals found in early Cambrian fossils.
Trilobites help scientists determine the age of rock layers from the Cambrian period. For example, finding trilobites with specific features, such as alimentary prosopon and micropygium, indicates that the rocks are from the Early Cambrian. Much of what scientists know about the Cambrian period comes from studying these trilobite fossils.
Trilobites are the official state fossils of Ohio (Isotelus), Wisconsin (Calymene celebra), and Pennsylvania (Phacops rana).
Taxonomy
The 10 most commonly recognized trilobite orders are Agnostida, Redlichiida, Corynexochida, Lichida, Odontopleurida, Phacopida, Proetida, Asaphida, Harpetida, and Ptychopariida. In 2020, a new order called Trinucleida was proposed. It was moved to a higher classification level from the asaphid superfamily Trinucleioidea. Sometimes, Nektaspida are considered trilobites, but these lack a hard outer shell and eyes. Some scientists believe the order Agnostida may have multiple origins, with the suborder Agnostina representing non-trilobite arthropods not related to the suborder Eodiscina. Under this idea, Eodiscina would be raised to a new order called Eodiscida.
Over 22,000 trilobite species have been described.
Although trilobites have a rich fossil record with thousands of genera found worldwide, their classification and evolutionary relationships remain uncertain. Except for the orders Phacopida and Lichida (which first appear in the early Ordovician), nine of the eleven trilobite orders existed before the end of the Cambrian period. Most scientists think the order Redlichiida, specifically its suborder Redlichiina, contains the common ancestor of all other orders, except possibly Agnostina. Many studies suggest that Redlichiina gave rise to the orders Corynexochida and Ptychopariida during the Lower Cambrian, and Lichida descended from either Redlichiida or Corynexochida in the Middle Cambrian. The order Ptychopariida is the most difficult to classify. In the 1959 Treatise on Invertebrate Paleontology, what are now separate orders (Ptychopariida, Asaphida, Proetida, and Harpetida) were grouped together as Ptychopariida. In 1990, a subclass called Librostoma was created to include these orders, based on their shared feature of a natant (unattached) hypostome. The most recently recognized of the nine trilobite orders, Harpetida, was established in 2002. The origin of the order Phacopida is still unclear.
Morphology
When trilobites are found in fossils, only their hard outer covering, called the exoskeleton, is usually preserved. In most cases, these fossils are incomplete. However, in a few special locations called Lagerstätten, soft body parts like legs, gills, muscles, and the digestive system may also be preserved. These locations can even show fine details of structures such as eye parts. Out of the 20,000 known trilobite species, only 38 have fossils that include preserved appendages.
Trilobites varied greatly in size. The smallest species measured less than 1 millimeter (0.039 inch), while the largest reached over 70 centimeters (28 inches). The smallest known trilobite is Acanthopleurella stipulae, which was no more than 1.5 millimeters (0.059 inch) long. The largest known trilobite, Isotelus rex, was 72 centimeters (28 inches) long and was discovered in 1998 by scientists in Canada on the shores of Hudson Bay. Another large trilobite, Hungioides bohemicus, was found in Portugal in 2009. When complete, it was estimated to be 86.5 centimeters (34.1 inches) long.
Only the top (dorsal) part of the trilobite’s exoskeleton becomes hardened, made of minerals like calcite and calcium phosphate arranged in a chitin lattice. This part curls around the bottom edge to form a small fringe called the "doublure." The appendages and soft underside were not hardened. Trilobites had three main body sections: the cephalon (head), thorax (body), and pygidium (tail).
Trilobites are a diverse group with about 5,000 genera, so their body shapes and features can be complex. Despite this, they have unique traits that set them apart from other arthropods. These include a hard, chitinous exoskeleton shaped like an oval and divided into three lobes (which gives them their name), a large head shield (cephalon) that connects to the body segments, and a tail shield (pygidium) formed by fused body segments. When comparing trilobite species, scientists often examine the size, shape, and presence of features on the head.
During molting, the exoskeleton typically splits between the head and body, which is why many trilobite fossils are missing either the head or body. Facial sutures on the head helped trilobites shed their old exoskeleton. Like lobsters and crabs, trilobites likely grew between molting and when their new exoskeleton hardened.
The head section (cephalon) of trilobites varies widely in shape. A dome-like structure called the glabella sits beneath the head and holds the "crop" or stomach. The exoskeleton on the underside has few unique features, but the head often shows marks where muscles attached. A small, rigid plate called the hypostome, similar to a ventral plate in other arthropods, is sometimes preserved. The mouth and stomach were toothless and positioned above the hypostome, with the mouth facing backward at the hypostome’s rear edge.
The hypostome’s shape and placement vary. Sometimes it is supported by a soft membrane, sometimes fused to the front edge of the doublure with an outline similar to the glabella, or fused with a different outline. These variations are linked to different lifestyles, diets, and ecological roles. In some trilobites, the head’s front and side edges are greatly expanded, while in others, a bulge suggests a brood pouch. Complex compound eyes are also a notable feature of the head.
Facial or cephalic sutures are natural lines of weakness in the head that helped trilobites shed their old exoskeleton during molting. All species in the suborder Olenellina, which went extinct at the end of the Early Cambrian, lacked facial sutures. These trilobites are considered early ancestors of later species because they predated the evolution of facial sutures. Some later trilobites also lost facial sutures, and the type of sutures found in different species is important for classifying and understanding their evolutionary relationships.
The top surface of the trilobite’s head can be divided into two parts: the cranidium (central area) and the librigena (free cheeks). The cranidium includes the glabella (central lobe) and the fixigena (fixed cheeks). Facial sutures are located along the front edge, where the cranidium and librigena meet.
On the top of the head, facial sutures can be grouped into five main types based on where they end relative to the genal angle (the edges where the side and rear of the head meet).
- Absent: Trilobites in the Olenellina lacked facial sutures. This is considered a primitive trait and is always paired with the presence of eyes.
- Proparian: The suture ends before the genal angle. Examples include Dalmanites (Phacopida) and Ekwipagetia (Agnostida).
- Gonatoparian: The suture ends at the tip of the genal angle. Examples include Calymene and Trimerus (Phacopida).
- Opisthoparian: The suture ends at the rear edge of the head. Examples include Peltura (Ptychopariida) and Bumastus (Corynexochida). This is the most common type.
- Hypoparian or marginal: In some trilobites, the suture may disappear over time. This often occurs as the eye structure on the free cheek shrinks, eventually leading to blindness. This type can develop from proparian or opisthoparian sutures.
The earliest trilobites had proparian sutures. Opisthoparian sutures evolved multiple times independently. No examples show proparian sutures evolving from opisthoparian ancestors. Some trilobites that have opisthoparian sutures as adults may have proparian sutures as young individuals. Hypoparian sutures also evolved independently in several groups.
The path of facial sutures from the front of the head varies as much as their placement at the rear. However, the lack of a clear reference point similar to the genal angle makes it difficult to classify. In some trilobites, the front of the facial sutures align with the
Soft body parts
Only about 21 species of trilobites have been described with preserved soft body parts, so some features, such as the posterior antenniform cerci found only in Olenoides serratus, are difficult to understand in the broader context of trilobite biology.
Trilobites had one pair of preoral antennae and limbs that were not clearly divided into parts (biramous). These limbs included four pairs on the head, one pair per thoracic segment, and some pairs on the pygidium. Each endopodite (walking leg) had six or seven segments, similar to those of other early arthropods. Endopodites were attached to the coxa, which also had a feather-like exopodite, or gill branch, used for breathing and, in some species, swimming. A 2021 study found that the upper limb branch of trilobites functioned as a "well-developed gill," helping to oxygenate their body fluid, similar to the book gill in modern horseshoe crabs (Limulus). In Olenoides, the connection between the coxa and the body was distinct from the exopods of chelicerates or crustaceans. The inside of the coxa (or gnathobase) had spines, likely used to process prey. The last segment of the exopodite often had claws or spines. Many hairs on the legs suggest adaptations for feeding (like the gnathobases) or sensory functions to aid in walking.
The mouth of trilobites was toothless and located on the rear edge of the hypostome (facing backward), in front of the legs attached to the cephalon. The mouth was connected to the stomach, which was positioned forward of the mouth and below the glabella. The "intestine" extended backward from the stomach to the pygidium. Limbs attached to the cephalon may have helped move food into the mouth, possibly slicing food on the hypostome or gnathobases first. A 2021 study using propagation phase-contrast synchrotron microtomography (PPC-SRμCT), a 3D imaging technique for studying tissue function, found large amounts of undigestible fragments from Conchoprimitia osekensis (a now-extinct, small-shelled ostracod) in the digestive tract of Bohemolichas incola.
These fragments suggest B. incola crushed shells as part of its feeding behavior (durophagous predation). Since the shells were not classified by species but by their physical properties (strength and size), B. incola likely ate opportunistically, similar to scavengers. The presence of shell fragments also indicates that B. incola used enzymes to break down the shells, leaving behind only undigested remains. These findings support the idea that early trilobites may have had glands that secreted enzymes to aid digestion.
Although the mouth, stomach, and digestive tract are directly or indirectly supported by evidence (as described above), the presence of a heart, brain, and liver is only implied (even though many reconstructions show them) with little direct geological proof.
Although rarely preserved, long lateral muscles extended from the cephalon to the middle of the pygidium, attaching to the axial rings to allow enrollment. Separate muscles on the legs retracted them to avoid obstruction.
Sensory organs
Many trilobites had complex eyes and a pair of antennae. Some trilobites were blind, likely because they lived too deep in the ocean where no light could reach them. This blindness developed over time in this group of trilobites. Other trilobites, such as Phacops rana and Erbenochile erbeni, had large eyes that helped them see clearly in bright, dangerous waters filled with predators.
The antennae of most trilobites (and seen in some fossils) were flexible so they could be pulled back when the trilobite curled up for protection. One species, Olenoides serratus, had antenna-like structures called cerci that extended from the back of its body.
Even the earliest trilobites had complex, compound eyes made of calcite (a material found in all trilobite eyes). This shows that eyes in arthropods and possibly other animals may have developed before the Cambrian period. Improved eyesight in both predators and prey is thought to have driven the rapid evolution of new life forms during the Cambrian explosion.
Trilobite eyes were usually compound, with each lens shaped like a long prism. The number of lenses varied: some trilobites had one lens, while others had thousands in a single eye. In compound eyes, lenses were often arranged in a hexagonal pattern. Fossils of trilobite eyes are well-preserved, allowing scientists to study their evolution over time, even though soft body parts rarely survive.
The lenses of trilobite eyes were made of calcite (calcium carbonate, CaCO₃). Some trilobites used clear calcite crystals to form each lens. However, rigid calcite lenses could not adjust focus like the soft lenses in human eyes. In some trilobites, calcite formed a special structure that improved depth of field and reduced optical errors, based on discoveries by scientists René Descartes and Christiaan Huygens in the 17th century. A modern animal with similar eyes is the brittle star Ophiocoma wendtii.
In other trilobites, a structure called the Huygens interface was missing. Instead, their lenses had a gradient in refractive index, changing toward the center.
Some phacopid trilobites had sensory structures under their lenses. These structures included groups of sensory cells surrounding a rhadomeric structure, similar to those found in the eyes of horseshoe crabs (genus Limulus).
Holochroal eyes had many small lenses (sometimes over 15,000). Each lens was about 30 to 100 micrometers in size and arranged in a hexagonal pattern, touching each other. A single corneal membrane covered all lenses. Holochroal eyes were the earliest type of trilobite eye and are found in most trilobite groups except Agnostida. Little is known about their early history, as many Cambrian trilobites rarely preserved their eye surfaces. The ability of these eyes to see details depends on light intensity, movement, receptor density, and how signals from individual parts of the eye are processed. This suggests that larger lenses were needed in low-light conditions or for fast-moving animals. For example, trilobites like Carolinites had eyes that were flattened on the sides, while others like Pricyclopyge had larger lenses. Eye shape can help scientists infer the environment trilobites lived in. The eye of Erbenochile erbenii shows clear evidence that some trilobites were active during the day.
Schizochroal eyes had fewer (about 700) but larger lenses than holochroal eyes. These eyes were found only in Phacopina. Each lens had its own cornea, and thick layers of cuticle (called sclera) separated the lenses. Schizochroal eyes appeared suddenly in the early Ordovician period and likely evolved from holochroal eyes. These eyes provided wide vision and may have helped trilobites detect threats rather than hunt. Modern insects like Xenos peckii have eyes that function similarly.
Abathochroal eyes were found only in Cambrian Eodiscina and had about 70 small, separate lenses, each with its own cornea. The sclera (thick cuticle) was thinner than in schizochroal eyes. Though few examples are known, abathochroal eyes are among the oldest recorded. Environmental changes may have led to the loss of eyes in some Eodiscina species.
Secondary blindness (losing eyes over time) is common in long-lived groups like Agnostida and Trinucleioidea. In Proetida and Phacopina from western Europe, especially in Tropidocoryphinae from France, well-documented studies show gradual eye reduction in closely related species, eventually leading to blindness.
Other trilobite structures, such as "macula" (thin areas on the underside of the hypostome), are thought to have acted as simple "ventral eyes" to detect light or help with navigation when swimming upside down.
Some prosopon structures on trilobites may have collected chemical or vibrational signals. For example, the large pitted fringes on the heads of Harpetida and Trinucleoidea, paired with small or absent eyes, suggest these fringes might have functioned as a type of "compound ear."
Development
Trilobites grew through a series of stages called instars, during which their existing body parts grew larger, and new sections of their body appeared near the middle of their body during a phase called the anamorphic stage. After this, trilobites entered the epimorphic stage, where they continued to grow and shed their old exoskeletons, but no new body sections formed on their outer shell. The combination of these two growth phases is called hemianamorphic development, a process common in many living arthropods.
Trilobites had a unique way of forming connections between their body segments. These changes in how segments connected led to the three main stages of their life cycle, which are not easily compared to those of other arthropods. Growth and changes in their shape happened when trilobites were soft-bodied, after shedding their old shell and before their new shell hardened.
Trilobite larvae are known from the Cambrian to the Carboniferous periods and are found in all trilobite groups. Instars from closely related trilobite species are more similar to each other than those from distant relatives, making larvae useful for studying how trilobites are related to one another.
Although no fossil evidence of reproduction has been found, trilobites are believed to have reproduced sexually and laid eggs. Some species may have carried their eggs or young in a pouch near their head, especially in environments where it was hard for young trilobites to survive. The size and shape of the first hard shell stage vary among trilobite groups, suggesting some trilobites developed more inside the egg than others. Some early stages before the shell hardened may have existed, but it is also possible that the shell hardened and hatching happened at the same time.
The earliest known stages of trilobite growth after the embryo are called "protaspid" stages, which occur during the anamorphic phase. These stages begin with a head and tail that look the same and end with a line separating the head and tail. New body sections are added near the tail, but all sections remain joined together.
During the "meraspid" stages, which also occur during the anamorphic phase, a joint forms between the head and the fused body sections. Before this stage, the trilobite had a head and a fused body section called the pygidium. During the meraspid stages, new body sections appear near the end of the pygidium, and more joints develop at the front of the pygidium, allowing body sections to move freely in the thorax. One new body section is usually added per moult, though some trilobites added two sections at once or one every other moult. The number of meraspid stages equals the number of thoracic body sections. Growth during this phase was significant, increasing the trilobite’s size by up to 25% to 40%.
The "holaspid" stages, which occur during the epimorphic phase, begin when the trilobite has a stable number of body sections in the thorax. Moulting continued during this phase, but the number of thoracic sections did not change. Some trilobites may have continued to grow and moult throughout their lives, though their growth slowed after reaching maturity.
Some trilobites showed a major change in shape at a specific stage, called "trilobite metamorphosis." This change involved losing or gaining features that indicated a shift in how they lived. Changes in lifestyle during development had evolutionary importance, as trilobites could occupy different environments during their growth, affecting their survival and spread. Trilobites that were planktonic (floating in the water) only during their early protaspid stage and then became benthic (living on the ocean floor) survived the Ordovician extinctions, while those that remained planktonic for longer did not. A life cycle that included a long planktonic stage was poorly suited to the rapid climate changes and loss of tropical habitats during the Ordovician.
There is no evidence that trilobites reabsorbed their old shells during moulting. Some scientists suggest that trilobites’ inability to reabsorb their hard shells made them more vulnerable to predators, as it took longer to rebuild their new shells compared to modern arthropods that can reabsorb their old exoskeletons.
History of usage and research
In 1698, Rev. Edward Lhwyd published a letter titled "Concerning Several Regularly Figured Stones Lately Found by Him" in The Philosophical Transactions of the Royal Society, the oldest scientific journal in the English language. His letter included a page of etchings showing fossils. One etching showed a trilobite he found near Llandeilo, likely on the grounds of Lord Dynefor's castle. He described the trilobite as "the skeleton of some flat Fish."
In 1749, Charles Lyttleton discovered a fossil later called Calymene blumenbachii (the Dudley locust), which is considered the start of trilobite research. In 1750, Lyttleton sent a letter to the Royal Society of London about a "petrified insect" he found in the "limestone pits at Dudley." In 1754, Manuel Mendez da Costa said the Dudley locust was not an insect but belonged to "the crustaceous tribe of animals." He named it Pediculus marinus major trilobos (large trilobed marine louse), a name used until the 19th century. Johann Walch, a German naturalist, first studied this group in detail. He suggested the name "trilobite," based on the three-lobed shape of the fossil's central axis and the two side lobes.
Descriptions of trilobites may date back to the third century BC, but clear written records begin in the fourth century AD. Spanish geologists Eladio Liñán and Rodolfo Gozalo believe some fossils called "scorpion stone," "beetle stone," and "ant stone" in ancient Greek and Latin texts refer to trilobites. More certain references to trilobites appear in Chinese records. Fossils from the Kushan formation in northeastern China were used as inkstones and decorative items.
In the 1860s, American fossil hunters found many Elrathia kingi trilobites in western Utah. Until the early 1900s, the Ute Native Americans of Utah wore these trilobites as amulets, calling them pachavee (little water bug). A hole was drilled in the fossil, and it was worn on a string. The Ute believed the necklaces protected against bullets and diseases like diphtheria. In 1931, Frank Beckwith found evidence of this practice. He photographed petroglyphs that likely show trilobites and examined a burial site with a drilled trilobite fossil in the chest of the deceased. Since then, trilobite amulets have been found across the Great Basin, British Columbia, and Australia.
In the 1880s, archaeologists found a trilobite fossil in the Grotte du Trilobite (Caves of Arcy-sur-Cure, Yonne, France). The fossil had been drilled, possibly to be worn as a pendant. The layer of soil where it was found is dated to 15,000 years ago. Because the pendant was handled so much, the trilobite's species could not be identified. This type of trilobite is not found near Yonne, suggesting it may have been traded from another region and highly valued.