Trilobites were marine arthropods that lived in ancient oceans for more than 270 million years. They are among the most famous fossils because their hard exoskeletons preserved well in sedimentary rock. Studying trilobites helps paleontologists understand evolution, extinction, and how ocean ecosystems changed through time.
Although they are often shown with dinosaurs in paleontology displays, trilobites went extinct long before dinosaurs appeared.
Understanding Dinosaurs & Paleontology: Trilobites
A trilobite was built for life on or near the sea floor. Beneath its hard outer covering, it had many pairs of jointed legs. Some legs helped it walk across mud or sand.
Other parts near the legs probably moved water toward the gills, allowing the animal to breathe. It had antennae to sense its surroundings and a mouth underneath the front of the body. Its compound eyes were especially remarkable.
In many species, the eye lenses contained calcite, a mineral that gave the lenses a rigid structure. Different eye shapes suggest different lifestyles. Large curved eyes could give a wide view of nearby movement, while reduced eyes may belong to animals that lived in dim water or burrowed through sediment.
Growing created a difficult problem for an animal with a rigid outer skeleton. A trilobite had to split and leave its old covering behind before its body could expand. The fresh new covering was soft for a short time, making the animal more vulnerable to predators and damage.
This is one reason fossil beds can contain many separated pieces, especially head shields. A complete fossil does not always mean an entire animal died in that exact position. Currents could move shells after molting or after death.
Fine mud could then cover them and protect them from scavengers. Over a long time, buried sediment turned into rock, preserving the shell itself or leaving an impression that minerals later filled.
Trilobite species changed quickly enough that geologists can use them to compare rock layers from far apart places. If the same distinctive species occurs in two layers, the layers may have formed during roughly the same part of Earth history. This method helps scientists build a relative timeline even when a rock layer cannot be dated directly.
Trilobites are useful because their fossils occur in many marine sedimentary rocks, including limestone and shale. Their changing forms record natural selection in action. Some developed spines that may have discouraged predators or helped them rest on soft sediment.
Some became smooth and streamlined. Others could curl into a tight ball, placing the softer underside inside a shield of hard plates.
When studying a fossil image, first identify whether you are seeing the upper surface, the underside, or a side view. Fossils are often flattened, broken, worn, or partly hidden in rock, so their original shape may not be obvious. Notice repeated segments and compare the left side with the right side.
Symmetry can reveal where missing parts once belonged. Learn the difference between a body fossil and a trace fossil. A body fossil preserves part of the animal, while tracks, resting marks, and burrows preserve evidence of its activity.
In museums, quarries, and coastal cliffs, fossils provide clues about ancient environments. They should be photographed and recorded carefully, since the rock layer and location can be as informative as the specimen.
Key Facts
- Trilobites lived from about 521 million years ago to 252 million years ago.
- Their bodies had three lengthwise lobes: a central axial lobe and two pleural lobes.
- Their bodies had three main sections: cephalon, thorax, and pygidium.
- Many trilobites molted their exoskeletons, so some fossils are shed shells rather than dead animals.
- Trilobites are index fossils because many species were widespread and lived for short geologic intervals.
- Duration lived = extinction age subtracted from first appearance age = 521 mya - 252 mya = 269 million years.
Vocabulary
- Trilobite
- A trilobite is an extinct marine arthropod with a segmented body and a hard outer skeleton.
- Exoskeleton
- An exoskeleton is a hard external covering that supports and protects an animal's body.
- Cephalon
- The cephalon is the head section of a trilobite, often containing eyes, cheeks, and mouthparts.
- Thorax
- The thorax is the middle section made of movable segments that often allowed a trilobite to curl up.
- Index fossil
- An index fossil is a fossil used to identify the relative age of rock layers because it is widespread and limited to a specific time range.
Common Mistakes to Avoid
- Calling trilobites dinosaurs is wrong because trilobites were marine arthropods and went extinct about 252 million years ago, before dinosaurs evolved.
- Assuming every trilobite fossil is a complete dead animal is wrong because many fossils are molted exoskeletons left behind as the animal grew.
- Thinking all trilobites looked and lived the same way is wrong because different species crawled, burrowed, swam, or fed in different marine habitats.
- Using fossil size alone to identify a trilobite species is wrong because growth stage, preservation, and deformation can change apparent size.
Practice Questions
- 1 A trilobite species first appears in rocks dated to 480 million years ago and disappears in rocks dated to 465 million years ago. How long did the species exist?
- 2 A rock layer contains 36 trilobite fossils. If 2/3 of them are isolated thorax pieces and the rest are complete specimens, how many complete specimens are present?
- 3 A paleontologist finds the same trilobite species in two rock layers on different continents. Explain how this fossil evidence could help compare the relative ages of the two layers.