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.
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.