Trilobites and Fossil Arthropods

Trilobites are an extinct class of marine arthropods whose calcified exoskeletons make them among the most iconic and biostratigraphically useful Paleozoic fossils.

Definition

Trilobites are dorsally calcified marine arthropods (class Trilobita) divided lengthwise and transversely into three lobes, ranging from the Cambrian to their extinction at the end of the Permian.

Scope

This topic covers trilobite anatomy, molting, compound eyes, enrollment, and classification, together with other fossil arthropods such as eurypterids, ostracods, and the Cambrian arthropods of exceptional faunas. It addresses their evolutionary history through the Paleozoic and their use in zonation.

Core questions

• What is the body plan and exoskeletal structure of a trilobite?
• How did trilobite eyes and enrollment function?
• How are trilobites used to zone Cambrian and Ordovician strata?
• How do trilobites fit within the broader arthropod radiation?

Key concepts

• Cephalon, thorax, and pygidium
• Holochroal and schizochroal eyes
• Enrollment as defense
• Facial sutures and ecdysis

Key theories

Calcite-lens visual systems

Trilobites possessed the earliest well-documented eyes, built of calcite lenses arranged as holochroal or schizochroal compound eyes optimized to correct for spherical aberration.

Molting and the cephalic suture

Growth by ecdysis along facial sutures produced abundant disarticulated sclerites, so many trilobite fossils are molted parts rather than carcasses.

Clinical relevance

Trilobite biozones provide high-resolution dating and correlation of Cambrian through Ordovician marine rocks, and their provinciality helps reconstruct the positions of Paleozoic continents and oceans.

History

Trilobites were among the first fossils described scientifically and were central to nineteenth-century debates over the Cambrian and Silurian systems. Detailed twentieth-century work on their eyes and ontogeny made them a model for functional morphology in the fossil record.

Debates

Causes of trilobite decline and extinction

The long Paleozoic decline of trilobites and their final loss at the end-Permian extinction are attributed variously to competition, predation, and successive mass extinctions.

Key figures

• Harry B. Whittington
• Richard Fortey
• Euan Clarkson

Seminal works

• whittington1992
• clarkson1998

Frequently asked questions

When did trilobites live?

Trilobites appeared in the Early Cambrian, peaked in the Cambrian and Ordovician, and went extinct at the end of the Permian about 252 million years ago.

Why are many trilobite fossils incomplete?

Trilobites grew by molting their exoskeletons, so many specimens are shed parts that broke apart at the facial sutures rather than whole animals.