Invertebrates
Trilobite
Trilobita · also called trilobites
Trilobites ruled the ancient seas for a quarter of a billion years, longer than almost any other animal group in history, before vanishing entirely in the most severe mass extinction Earth has ever recorded. They left behind one of the richest fossil records of any extinct animal — segmented shells so well preserved that scientists can still study the fine structure of eyes that saw their last light 252 million years ago.
Overview
Trilobites, subphylum Trilobita, were hard-shelled marine arthropods that thrived from roughly 521 to 252 million years ago, diversifying into an estimated 20,000 or more species. Named for a body divided into three lengthwise lobes, many had well-developed compound eyes made of calcite, a mineral that fossilises exceptionally well.
Built to fossilise, built to last
Trilobites moulted their hard exoskeleton repeatedly to grow, leaving behind multiple fossils per individual, and some species could roll into a defensive ball much like a modern pill bug. This combination has left an unusually complete fossil record spanning a quarter of a billion years.
Status
Extinct — wiped out at the end of the Permian period roughly 252 million years ago in the most severe mass extinction in the history of complex life, after already surviving several earlier extinction events across an extraordinarily long run.
Trilobite: common questions
Why did trilobites go extinct?
Trilobites died out at the end of the Permian period, approximately 252 million years ago, during what is often called the Great Dying, the most catastrophic mass extinction event documented in the history of complex life, thought to have been driven primarily by massive volcanic activity that disrupted global climate and ocean chemistry over a relatively short geological period. This event eliminated the vast majority of marine species alive at the time, and trilobites, despite having already survived for over 250 million years through several earlier extinction events, did not survive this final and most severe crisis.
Why are trilobite fossils so common and well preserved?
Trilobites had a hard, mineralised exoskeleton that fossilised readily under the right conditions, and because they moulted this shell periodically throughout their lives to grow, a single individual trilobite could leave behind numerous separate shell fossils in addition to its final remains after death. Combined with the sheer length of time trilobites existed and the enormous number of individuals and species involved, these factors have resulted in an unusually rich and geographically widespread fossil record compared with most other extinct animal groups.
How do scientists know trilobites had good eyesight?
Many trilobite species had compound eyes built from calcite, a hard mineral that fossilises extremely well and preserves fine structural detail far better than the soft eye tissues typical of most other ancient animals, which almost never survive fossilisation in any recognisable form. This unusual preservation has allowed researchers to study trilobite eye structure in remarkable detail, revealing sophisticated compound eyes in some species and providing rare, direct fossil evidence for how vision evolved and diversified during a critical early period of animal evolution.
Were all trilobites the same size and shape?
No, trilobites varied enormously across their roughly 20,000 known species, ranging from tiny forms only a few millimetres long to giant species that grew to over 70 centimetres, and displaying an equally wide range of body shapes, spine arrangements, and eye structures suited to very different marine lifestyles and habitats. This diversity reflects the sheer length of time, well over 250 million years, that trilobites had to diversify and adapt to different ecological niches across the world's ancient oceans before the group's eventual extinction.