Trilobite

Trilobites are extinct sea creatures called marine arthropods, a group related to today's insects, spiders, and crabs. They belong to the class Trilobita, and their name means "three-lobed entities." They were among the earliest arthropods to appear in the fossil record and were one of the most successful early animals. They lived in oceans for almost 270 million years, and over 22,000 species have been described.

Trilobites had a hard outer shell called an exoskeleton, made of calcite, a mineral. This shell turned into a fossil easily, so many trilobite fossils have survived. Scientists have used these fossils to learn about biostratigraphy, paleontology, evolutionary biology, and plate tectonics. Trilobites are placed in a group called Artiopoda, which includes many animals that look a bit like trilobites but mostly did not have hard shells.

Trilobites lived in many different ways. Some crawled across the seafloor as predators, scavengers, or filter feeders. Others swam and ate plankton, and some even crawled onto land. Most of the ways modern sea arthropods live can also be seen in trilobites. One exception may be parasitism, though scientists still debate this. Some trilobites, especially the family Olenidae, may have had a symbiotic relationship with sulfur-eating bacteria, which gave them food. The largest trilobites were more than 70 centimetres (28 in) long and may have weighed as much as 4.5 kilograms (9.9 lb).

Trilobites first appear in the fossil record at the start of the Atdabanian/Cambrian Stage 3 time period, about 521 million years ago. Soon after they appeared, they were already varied and had spread around the world. They were most varied during the late Cambrian and Ordovician periods, and they stayed varied during the Silurian and early Devonian. In the middle and late Devonian, their numbers dropped sharply after several mass extinctions, including the Taghanic event, the Late Devonian mass extinction/Kellwasser event, and the Hangenberg/end-Devonian mass extinction. These events wiped out most trilobite species, and only the Proetida group survived. Trilobites recovered somewhat in the early Carboniferous period, but stayed few in number through the late Carboniferous and Permian periods. They remained spread across the world until the end. The last trilobites disappeared in the end-Permian mass extinction event, about 251.9 million years ago, when only a few species were left.

Evolution

Trilobites belong to the Artiopoda, a group of extinct arthropods that look similar to trilobites. Only trilobites had heavily mineralised shells. Scientists are not sure how artiopods are related to other arthropods. Some think they are close relatives of chelicerates, which include horseshoe crabs, sea spiders, and arachnids. Others think they are more closely related to Mandibulata, which includes insects, crustaceans, and myriapods. The earliest known trilobites in the fossil record date to around 520 million years ago. Soon after they appeared, they spread into many major groups.

Over time, trilobites changed in many ways. They developed new types of eyes and new body shapes. Their history includes several extinction events, when some groups died out. The groups that survived spread to new ways of living. Trilobites stayed very diverse through the Cambrian and Ordovician periods. Then they slowly declined during the Devonian. The last few survivors died out at the end of the Permian period. Scientists are not certain exactly why trilobites became extinct, but a combination of causes is likely.

Fossil distribution

Trilobites were mostly sea animals. Their fossils are usually found in rocks that also hold fossils of other sea animals, such as brachiopods, crinoids, and corals. Some trackways suggest that trilobites sometimes walked onto land for short trips. They lived in water from very shallow to very deep, and their fossils are found on every modern continent. We find either whole bodies or pieces of the shell they shed as they grew. This shedding is called ecdysis. The tracks they left on the sea floor are often kept as trace fossils. There are three main kinds: Rusophycus, which shows resting, Cruziana, which shows feeding, and Diplichnites, which shows walking. Similar trace fossils are also found in freshwater and later rocks, so experts must be careful to tell them apart. Trilobite fossils are found all over the world, and thousands of kinds are known. They appeared quickly in Earth's history and shed their shells like other arthropods. This makes them useful index fossils, which help geologists work out the age of rocks. They were among the first fossils to attract wide attention, and new kinds are still found every year.

The best public collection of trilobites in the United States is in Hamburg, New York. The shale quarry there, known as Penn Dixie, stopped mining in the 1960s. In the 1970s, rock collector Dan Cooper found large amounts of trilobites, which sparked interest from scientists and the public. The fossils date to the Givetian (387.2–382.7 million years ago), when the Western New York Region was 30 degrees south of the equator and fully underwater. The Town of Hamburg bought the site from Vincent C. Bonerb to protect it from building, working with the Hamburg Natural History Society. In 1994, the quarry became Penn Dixie Fossil Park & Nature Reserve, where visitors can come and collect trilobites. The two most common finds are Eldredgeops rana and Greenops. The Trilobite Wilderness in California is named for the many fossils found there. A famous trilobite site in the United Kingdom is Wren's Nest in Dudley, in the West Midlands. There, Calymene blumenbachii is found in the Silurian Wenlock Group. This trilobite appears on the town's coat of arms. Quarry workers who worked the now abandoned limestone quarries called it the Dudley Bug or Dudley Locust. Llandrindod Wells, in Powys, Wales, is another famous site. The well-known Elrathia kingi is found in large numbers in the Cambrian Wheeler Shale of Utah. Very well-preserved trilobites, often showing soft body parts like legs, gills, and antennae, have been found in British Columbia, Canada, in the Cambrian Burgess Shale. They have also been found in New York, US, at the Ordovician Walcott–Rust quarry near Russia, New York, and at Beecher's Trilobite Bed near Rome, New York. In China, they are found at the Lower Cambrian Maotianshan Shales near Chengjiang. In Germany, they are found at the Devonian Hunsrück Slates near Bundenbach. Such finds are much rarer in Utah's other rock layers, in Ontario, and at Manuels River in Newfoundland and Labrador. Sites in Morocco also have very well-preserved trilobites. Collecting them has become a large industry, which has led to disagreements about how they are repaired. The French palaeontologist Joachim Barrande (1799–1883) carried out his major study of trilobites in the Cambrian, Ordovician, and Silurian of Bohemia. He published the first volume of Système silurien du centre de la Bohême in 1852.

Importance

Niles Eldredge studied Paleozoic trilobites in the Welsh-English borders. His work helped form and test punctuated equilibrium, an idea about how evolution happens. Trilobite fossils found in North America and Europe showed that the Iapetus Ocean had closed. This gave strong support to the idea of continental drift.

Trilobites help scientists estimate how quickly new species appeared during the Cambrian explosion. They are the most varied group of metazoans known from early Cambrian fossils. They are also very useful for dating Cambrian rock layers. Most of the Cambrian stratigraphy is based on trilobite marker fossils. Trilobites are also the state fossils of Ohio (Isotelus), Wisconsin (Calymene celebra) and Pennsylvania (Phacops rana).

See also: List of trilobite genera

Scientists most commonly recognize 10 trilobite orders: Agnostida, Redlichiida, Corynexochida, Lichida, Odontopleurida, Phacopida, Proetida, Asaphida, Harpetida and Ptychopariida. In 2020, an 11th order, Trinucleida, was proposed to be separated from the Trinucleioidea group of asaphids. Some scientists think the Nektaspida are trilobites, but they do not have a hard outer shell made of calcite or any eyes. Some scholars think that Agnostida is not one single group. In that idea, the Agnostina are not trilobites at all, and the Eodiscina would become a new order, Eodiscida. More than 22,000 trilobite species have been described.

Trilobites have thousands of described genera found all over the world. However, their taxonomy and phylogeny still have many uncertainties. Nine of the eleven orders appear before the end of the Cambrian. The exceptions are Phacopida and Lichida, which first appear in the early Ordovician. Most scientists believe that Redlichiida, especially its suborder Redlichiina, contains a common ancestor of all the other orders, except possibly the Agnostina. Most proposed family trees show Redlichiina giving rise to Corynexochida and Ptychopariida, and Lichida coming from either Redlichiida or Corynexochida. Ptychopariida is the hardest order to classify. In the 1959 Treatise on Invertebrate Paleontology, members of Ptychopariida, Asaphida, Proetida and Harpetida were grouped together as Ptychopariida. In 1990, the subclass Librostoma was created to include all of these orders, based on their shared ancestral feature of an unattached hypostome. The most recently recognized of the nine orders, Harpetida, was set up in 2002. The progenitor of Phacopida is unclear.

Morphology

See also: Cephalon (arthropod head)

Further information: Hypostome (trilobite)

Most trilobite fossils contain only the hard outer shell, called the exoskeleton, and it is often broken. Only a few special places, called Lagerstätten, have kept soft body parts such as legs and gills. Of the 20,000 known species, only 38 have fossils with preserved legs. Trilobites ranged from less than 1 millimetre to over 70 centimetres long, but most were about 3 to 10 centimetres. The largest known one, from Hudson Bay, is 72 centimetres long. The shell is hardened with calcite on its upper side. It is split into three parts: a head, a body made of segments called the thorax, and a tail called the pygidium.

Trilobites got their name from the three lobes running along their bodies. Their head shield, called the cephalon, had a central dome called the glabella. It also had a small plate on the underside called the hypostome. Lines on the head, called facial sutures, helped them shed their old shell when moulting. Many fossils are curled up, a position called volvation, which may have protected them. Some species grew long spines on their bodies or heads. These may have helped protect them from predators.

Soft body parts

Only about 21 species have preserved soft body parts. Because of this, some features, such as the rear feelers of Olenoides serratus, are still hard to study.

Trilobites had one pair of antennae in front of the mouth. Their other limbs were mostly alike. They had biramous limbs on the head, one pair on each body segment, and some on the tail. Each walking leg had six or seven segments. Each leg also had a feather-like branch. This branch helped with breathing and, in some species, swimming. A 2021 study found that the upper branch is a "well-developed gill" that adds oxygen to the hemolymph. This is similar to the book gill of the modern horseshoe crab Limulus. Spines on the inner part of the leg probably helped process food. The last leg segment usually had claws or spines. Many hairs on the legs suggest they helped with feeding or sensing.

The toothless mouth was on the back edge of the hypostome.

Sensory organs

Many trilobites had complex eyes and a pair of antennae. The antennae were very flexible. They could be pulled back when the trilobite curled up. One species (Olenoides serratus) also had antenna-like cerci at the rear. Some trilobites were blind, probably because they lived too deep for light to reach them. In some groups, such as the Agnostida and Trinucleioidea, and in Proetida and Phacopina from western Europe, blindness developed over time. Other trilobites, such as Phacops rana and Erbenochile erbeni, had large eyes. They lived in bright waters full of hunters and prey.

Even the earliest trilobites had complex compound eyes. Their lenses were made of calcite (calcium carbonate, CaCO₃). This suggests that the eyes of arthropods could have developed before the Cambrian explosion. Better eyesight for both hunters and prey may have been one of the evolutionary pressures behind the fast growth of new life forms at that time. Rigid calcite could not accommodate to a change of focus like a human eye. So some trilobites had a doublet structure. This gave good depth of field and little spherical aberration. Ideas about this were discovered by René Descartes and Christiaan Huygens in the 17th century. A living animal with similar lenses is the brittle star Ophiocoma wendtii. Other trilobites may have used a gradient-index lens, whose refractive index changes toward the center. Some phacopid trilobites had sensory structures under their lenses. These are similar to those in the apposition eyes of many modern arthropods, especially Limulus, a genus of horseshoe crabs.

• Holochroal eyes had a great number of small lenses, sometimes over 15,000. The lenses were hexagonally close packed, touching each other, with one corneal membrane covering them all. These were the earliest type of trilobite eye. They were by far the most common, found in all groups except the Agnostida. How well an eye works depends on light intensity, circular motion, and how signals from single rhabdoms are combined. Larger lenses helped in low light, as in Pricyclopyge compared with Carolinites. They also helped fast-moving hunters and prey. Fast-moving trilobites like Carolinites had eyes flattened from the side, with ommatidia pointing toward the front or back. So eye shape can give clues about a trilobite's environment.

• Schizochroal eyes had fewer, larger lenses, around 700. They are found only in Phacopina. Each lens had its own cornea, and the lenses were separated by thick cuticle called sclera. They appear suddenly in the early Ordovician and probably came from a holochroal ancestor. Their wide view and placement suggest they worked more as an "early warning" system than as a tool for hunting food. Scientists did not think modern eyes like these existed, but they are found in the modern insect Xenos peckii.
• Abathochroal eyes are found only in Cambrian Eodiscina. They had around 70 small separate lenses, each with its own cornea. Their sclera was separate from the cornea and was not as thick as in schizochroal eyes. Well-preserved examples are rare. These eyes have been recorded in the lower Cambrian, making them among the oldest known. Environmental conditions seem to have led to the later loss of visual organs in many Eodiscina.

Some other body structures may have sensed light. Small areas of thin cuticle on the underside of the hypostome, called macula, may have worked as simple "ventral eyes." These could tell night from day or help a trilobite find its way while swimming upside down.

Some types of prosopon may have collected chemical or vibration signals. Large pitted fringes on the head of Harpetida and Trinucleioidea may have worked as a "compound ear." These animals had small eyes or no eyes at all.

Development

Trilobites grew by shedding their shells in stages called instars. New body segments first appeared at the back of the body. After that, the animal kept growing and shedding its shell, but it did not add new segments. Trilobites grew mostly when their shell was soft, right after shedding and before the new shell hardened.

Trilobite larvae are known from the Cambrian to the Carboniferous periods. Scientists believe trilobites laid eggs, but no fossils show this. Young trilobites went through early stages called protaspid, meraspid, and holaspid. Some trilobites changed a lot in form at one stage. This helps scientists see how their way of life changed as they grew. Trilobites that spent their whole young lives floating in the water did not survive the Ordovician extinctions. Those that settled on the sea floor earlier did.

History of usage and research

In 1698, Rev. Edward Lhwyd published a page of fossil drawings in Philosophical Transactions of the Royal Society, one of the oldest science journals in English. One drawing showed a trilobite he found near Llandeilo. He described it as "the skeleton of some flat Fish." In 1749, Charles Lyttleton discovered Calymene blumenbachii (the Dudley locust), which is often seen as the start of trilobite research. In 1754, Manuel Mendez da Costa said trilobites were not insects but belonged to "the crustaceous tribe of animals." German naturalist Johann Walch made the first full study of the group. He gave them the name "trilobite," which means they have three lobes along the body.

People may have written about trilobites as early as the third century BC, and almost certainly by the fourth century AD. Some Greek and Latin lapidaries may describe trilobite fossils. In the New World, fossil hunters found many Elrathia kingi in western Utah in the 1860s. Until the early 1900s, the Ute Native Americans wore these trilobites as amulets. The Ute believed trilobite necklaces protected people from bullets and illnesses such as diphtheria. In 1931, Frank Beckwith photographed two petroglyphs that most likely show trilobites. In the 1880s, archaeologists found a trilobite fossil with a hole drilled through it in the Caves of Arcy-sur-Cure in Yonne, France. It may have been worn as a pendant.