Life › The Cambrian explosion › Cambrian explosion
Shells & skeletons
from c. 539 million years ago· Evolutionary innovation· Animals
Well establishedStrong fossil, genetic or geological evidence.
Animals began building hard parts from minerals, such as shells, spines and plates, which spread widely in the early Cambrian.
See it on the Life map →What happened
Biomineralization means growing hard parts from minerals in seawater: calcium carbonate shells, calcium phosphate plates or silica spicules. An early well-known example is Cloudina, a stacked-tube fossil about 550 million years old, some with bore holes left by attackers. From about 539 million years ago, millimeter-scale 'small shelly fossils' appear, followed by trilobite armor and reefs built by sponge-like archaeocyaths. Skeletons evolved independently in many groups.
Why it matters
Hard parts gave protection, support and anchorage, and they fossilize far better, so they shape what we can see of early animal history.
The evidence (fossils, genes, rocks)
Fossils of Cloudina and small shelly faunas in Namibia, China and Siberia, plus repair scars and bore holes that record attacks; seawater chemistry inferred from carbonate rocks.
Still debated
Why so many groups started making skeletons at nearly the same time: predation pressure, rising oxygen, changing seawater calcium, or a mix.
Connections (2)
▸Cambrian explosion saw the spread of Shells & skeletonsWell established
The connection
Mineral skeletons existed just before the Cambrian, but during the explosion they spread to many groups at once: molluscs, brachiopods, trilobites, sponges and echinoderms. Hard parts became normal for seafloor animals, and because they fossilize well, they make the explosion vivid in the rock record.
When and how
Oceans worldwide, from about 539 million years ago; first prominent in the 'small shelly fossils' of Siberia and China.
The evidence
Fossil counts show skeleton-building groups multiplying sharply in early Cambrian rocks, independently in different lineages using different minerals.
Debates and open questions
Whether skeletons spread mainly as armor against predators or because seawater chemistry made them cheaper to build.
▸Shells & skeletons were a defence in Eyes & the predator–prey arms raceStrong evidence
The connection
Once predators could see their prey, armor paid off. Shells, spines and hard plates protected animals from visual hunters such as Anomalocaris, while predators evolved stronger mouthparts in return. Bite marks and repaired injuries on Cambrian shells record this escalation.
When and how
Cambrian seas, about 530–505 million years ago; bore holes in Cloudina, about 550 million years ago, show attacks even earlier.
How sure are we?
Healed wounds on trilobites, drill holes in shells, and the near-simultaneous appearance of armor and eyes support the link.
Debates and open questions
Skeletons also give support and store minerals, and seawater chemistry changed too; predation may be only part of the story.