|Scavengers and filter-feeders
|0.1–72 cm (0.039–28 in.)
|Paleozoic – Cambrian to Permian
|up to 4.5 kg (10 lbs.)
|Arthropoda, Artiopoda, & Trilobitomorpha
Trilobites are arguably the most recognizable prehistoric animals of all time.
They are marine arthropods, distantly related to crustaceans, chelicerates, insects, and myriapods.
Trilobites first appeared in the Cambrian about 521 million years ago.
This makes them one of the earliest arthropods in the fossil record.
The name trilobites translates as “three lobes,” referring to the three-fold longitudinal division of this arthropod’s body.
Trilobites were around in Earth’s prehistoric seas for most of the Paleozoic Era, a duration of almost 270 million years.
Fossils are found on virtually all continents.
Trilobites are considered one of the most successful groups of prehistoric animals, with more than 22,000 species identified so far.
They were quite diverse in terms of their geographical distribution, morphology, and ecological niche.
Some of them were predators, while others were scavengers and filter feeders.
The diversity of trilobites makes them very important to paleontological studies.
Trilobites are among the most well-studied groups of organisms, and we have learned quite a lot about them.
In this article, we’ll discuss some of the most fascinating facts about trilobites, including their appearance, unique characteristics, and evolutionary and general scientific significance.
Like other arthropods, the body of the trilobites was covered by a hard exoskeleton, which provided protection and support to the soft body part.
The exoskeleton, which covered the arthropod’s dorsal part, was made up of calcite materials.
Following the general body plan of the arthropods, the trilobite’s body is divided into three regions: a head (cephalon), a thorax which consists of several articulated segments, and a tail plate (pygidium) made up of fused segments.
But the most distinctive physical attribute of this group of marine arthropods was their three (tri-) lobed body structure.
This consists of a raised central lobe (axis) and two side lobes (both referred to as pleura).
They’re called “trilobites” because of this tripartite body plan.
Trilobites varied in size considerably.
While some species only measured a few millimeters in length, others grew to an impressive size, several inches long.
The smallest trilobite species found so far is Acanthopleurella stipulate, with a length of about one millimeter.
The largest species is Isotelus rex, which grew to a maximum length of up to 72 centimeters (28 inches). That’s more than two feet!
The shape and general morphology of trilobites also varied from one species to the other.
The head or cephalon was typically the widest part of the body.
It featured a pair of large compound eyes often well-preserved in fossils.
The cephalon was also equipped with various appendages, such as antennae and distinctive mouthparts.
They had three pairs of head legs.
The thorax, located behind the cephalon, consisted of a series of segments, each bearing a pair of legs.
These segments were flexible, allowing the trilobite to adapt to a wide range of environments.
The legs were attached to the thorax just beneath the middle lobe splayed outward underneath the two side lobes.
They had about 11 pairs of legs on their thorax.
Some trilobite species also had spines and other elaborate ornamentation, such as ridges and granules on their thoracic segments.
These features were likely used for defense, camouflage, or species recognition.
The number of thoracic segments varied from 2 to 61, with the number of segments being constant in any given species.
The rear of the trilobite’s body typically had fewer segments compared to the thorax.
The fused segment at the rear is known as the pygidium.
It often had a rounded or tapered shape and served to protect the underside of the trilobite.
Trilobites can be classified based on the size of their tail segments in comparison to their head.
Micropygous trilobites have a pygidium that’s smaller than cephalon.
Isopygous species had a pygidium equal in size to the cephalon, while macropygous trilobites had a pygidium larger than their cephalon.
Habitat and Distribution
Trilobites were highly adaptable marine arthropods, and they lived in a wide range of environments throughout their long existence.
They were around from the Early Cambrian Period (about 521 million years ago) to the end of the Permian Period (about 252 million years ago).
The geographic range of this group of marine arthropods was quite extensive.
Trilobites lived in ancient oceans all over the world.
Fossils have been discovered on every continent, including Antarctica.
The global distribution of this arthropod class highlights their success as a group.
Most trilobite species inhabited shallow marine environments, especially continental shelves and nearshore areas.
However, a few species did venture into deeper waters.
These deep-sea trilobites often had distinctive adaptations, such as reduced eyes and thinner exoskeletons.
Trilobites were primarily benthic organisms.
This means they lived on the ocean floor.
They moved by crawling on soft sediment.
They could also hide among undersea crevices or reefs, and some species burrowed into the substrate.
Trilobites first appeared during a time commonly referred to as the “Cambrian Explosion.”
This event was characterized by the rapid diversification of life forms, including the development of many marine animals with hard exoskeletons like trilobites.
During this time, the Earth’s continents were arranged differently than they are today, and they changed configuration several times throughout the Paleozoic Era.
Much of the planet’s landmass was concentrated in the Southern Hemisphere, and extensive shallow seas covered some parts of the continents.
Climate conditions also varied during this time.
The changes in Earth’s climate during the Paleozoic have been attributed to variations in greenhouse gas levels and plate tectonics.
Behavior and Diet
Most trilobite species were benthic, which means they lived directly on the seafloor.
They moved by walking on the sea floor with their jointed legs.
Some trilobite species had adaptations for swimming and could use their legs to paddle through the water.
The efficiency of their movement would have varied depending on the species and habitat.
Trilobites had compound eyes, which were often quite sophisticated.
These eyes allowed them to have good vision in their underwater environments.
But some species were completely eyeless and probably relied on chemical cues in the water around them.
They did have a pair of antennae, which may have helped detect movement and other changes in their environment.
There is little evidence to suggest complex social behavior in trilobites.
Like many marine arthropods today, trilobites were primarily solitary organisms.
Their interactions with other trilobites were likely limited to mating and feeding.
Some trilobite species were capable of burrowing into the soft substrate to access buried food sources and escape from predators.
For these species, some of their legs had specialized adaptations that made digging possible.
Trilobites had diverse diets, which varied from one species to the other.
Most of them were filter feeders and scavengers.
They collected small particles, such as plankton or organic detritus, from the water column by sweeping these particles into their mouths.
Many trilobite species were opportunistic scavengers, meaning they fed on dead or decaying organic matter on the ocean floor.
A few trilobite species were also active predators that preyed on smaller organisms within their ecosystem.
These predator trilobites may have had adaptations for capturing and subduing prey, such as sharp spines or appendages.
Trilobites had separate male and female individuals, which means they reproduced sexually.
Initially, scientists suggested that they copulated and eggs were fertilized internally.
This hypothesis was based on the fact that trilobite fossils were often found clustered on top of each other.
However, more recent findings have shown that they probably reproduced by spawning, with males and females releasing gametes into the water.
Trilobite eggs and sperm were released into the water through a genital pore somewhere in the arthropod head, similar to how modern horseshoe crabs do today.
Like their distant relatives, it’s also likely that trilobite males held on to females for long periods to increase the chances of fertilization.
This is evidenced by the discovery of appendages modified to form claspers in some trilobite fossils.
Some trilobite species may have engaged in courtship behaviors, with their unique spines and other body ornamentations helping them to attract mates.
Others simply relied on chemical cues to find mates.
After fertilization, trilobite eggs were likely deposited in a suitable substrate on the ocean floor.
There’s evidence to suggest that some trilobite species stayed close to their eggs to protect the brood.
Trilobite eggs hatched into larvae, which were typically smaller and had fewer body segments than adults.
Like living arthropods, trilobite larvae would have had to go through a series of molts as they grew, which involves shedding their exoskeleton and developing new ones to accommodate a larger body size.
As trilobite larvae molted and grew, they gradually developed into juveniles.
Juveniles were characterized by increased segmentation and the development of more complex appendages.
Trilobites reached adulthood after undergoing multiple molts.
Trilobites had a relatively slow growth rate.
One species (Triarthrus eatoni) was found to have grown by only four centimeters in 10 years.
This growth rate is similar to that of several species of modern marine crustaceans.
Evolution and History
Trilobite had a long evolutionary period that spans nearly 300 million years during the Paleozoic Era.
Although their earliest existence is often dated to the Cambrian Period (541 million years ago), experts think they probably evolved earlier than this.
By the time they appeared in the Cambrian, they were already quite advanced and existed in many diverse forms.
This has prompted speculations that the ancestors of the trilobites were already living in Earth’s prehistoric seas as early as the Ediacaran Period (630 million to 542 million years ago).
One likely trilobite ancestor that has been identified from Precambrian times is the Spriggina.
This trilobite-like arthropod lived in the Precambrian shallow-marine environment of Australia.
Trilobites diversified further during the Cambrian explosion.
During the Early Cambrian, many species with new body plans and ecological roles appeared.
Trilobites were among the most notable groups to emerge during this radiation, filling various ecological niches.
Over the course of the Paleozoic Era, trilobites exhibited remarkable morphological diversity.
Various species developed new adaptations, including different eye types, spines, and ornamentation on their exoskeletons.
These adaptations corresponded to different environments and lifestyles.
Although trilobites are considered arthropods, their relationship with living arthropods isn’t well known.
They’re the most primitive arthropods, and many scientists consider them to be the ancestors of modern crustaceans and chelicerates.
Most recent classifications consider the chelicerates (especially horseshoe crabs) to be the closest living relatives of trilobites.
Trilobites survived several mass extinction events during their long history.
They were the most dominant marine animals during the Cambrian, but many of the Cambrian trilobites died out at the end of the Ordovician Period.
A new wave of diversification and evolution began after this extinction event.
New powerful predators evolved during the Middle Paleozoic.
To protect themselves against these predators, trilobites evolved spines and other defensive features.
Family-level diversity of the trilobites began to decline in the second part of the Paleozoic Era.
The 270-million-year-long existence of the trilobites finally ended during the Permian mass extinction event, which occurred about 250 million years ago.
Interactions With Other Species
When trilobites emerged during the Cambrian Period, about 520 million years ago, they were among the most complex life forms.
They exhibited a wide range of feeding adaptations.
Most species were specialized filter-feeders, feeding on small microscopic organisms at the bottom of the sea.
Others played a crucial role as scavengers, and they helped to get rid of organic detritus in Earth’s prehistoric seas.
Some of the largest trilobites, such as Redlichia rex, were active predators within their ecosystem.
They were cannibalistic, which means they preyed on other trilobites.
Predatory trilobites also preyed on other hard-shelled and soft-shelled animals within their ecosystem.
Trilobites themselves served as prey for various marine organisms.
Smaller trilobites were consumed by larger trilobites and other large arthropods.
Other predators that were present in the Paleozoic seas that may have preyed on trilobites include primitive jawless fish, early cephalopods (such as nautiloids), and jawed fish species like the Dunkleosteus.
These predators specialized in cracking trilobite exoskeletons to access their soft tissues.
Some trilobite species developed unique adaptations, such as spines, to deter or defend themselves against predators.
Trilobites were officially recognized in the 18th and early 19th centuries, but their discovery dates back hundreds of years before this, even before scientific inquiry into their origin began.
They’re recognized in various myths and legends across various cultures all over the world.
For instance, trilobite fossils identified as “petrified butterflies” are often associated with Legends of the Magician Merlin in South Wales.
In some other ancient cultures, rocks bearing trilobite fossils were worn as necklaces and are believed to have magical properties that could protect their wearer.
Trilobites are one of the most important fossil animals to scientists.
Body fossils (preserved exoskeletons) and trace fossils (evidence of their locomotion and feeding activities) are quite common in rocks worldwide.
Consequently, geologists use trilobite fossils in various ways to understand how the earth developed and changed during the Paleozoic Era.
One of the most significant uses of trilobite fossils is the relative dating and stratigraphical correlation of sedimentary rock formations across various locations.
This group of arthropods is particularly important for dating and correlating Cambrian and early Ordovician rocks.
For instance, one genus of trilobites known as Paradoxides has been found in the Middle Cambrian rocks of England, Wales, Newfoundland, Sweden, Spain, and Siberia.
This shows that these rocks are all of the same age.
Similarly, trilobites in the Merlinia species are used to identify Ordovician rocks, while Calymene trilobites are mainly found in Silurian rocks.
Trilobites are also useful in the field of paleogeography and for the reconstruction of ancient environments.
For instance, some early Ordovician trilobites, such as Petigurus, are found in north-west Scotland and far away North America.
Interestingly, these trilobites are not found anywhere else in Britain.
Similarly, some trilobite species, such as Placoparia, are present in England and Wales but not in North America.
Based on this and other evidence, geologists believe the landmass of present-day Scotland was once connected to North America during the Ordovician but was separated from southern Britain by a prehistoric ocean.
This is just one of numerous practical ways trilobites are useful for biostratigraphic correlation.
Trilobites also provide valuable insights into the evolution of complex life forms during the Paleozoic Era, making them useful to evolutionary biologists.
Beyond their significance to science, trilobite fossils are popular among collectors and are often used as decorative pieces.
Their intricate exoskeletons and distinctive shapes give them a unique aesthetic appeal that makes them prized specimens to collectors.
Many natural history museums also showcase trilobite fossils as part of their exhibit.
Trilobites are a class of primitive arthropods that lived during the Paleozoic Era.
They first emerged during the Cambrian Period and were among the most complex life forms in Earth’s oceans during this period.
Trilobites are one of the earliest known groups of arthropods.
They’re characterized by a three-lobed exoskeleton, which provides support and protection for their body.
Trilobites were quite diverse in terms of their size, body structure, and the ecological niche they occupied.
The diversity of this group of arthropods and its widespread geographical range makes them very important to our understanding of Earth’s prehistoric environment and the changes that took place throughout the Paleozoic.