LIFE × EVOLUTION

The First Animal Reefs

~525 to 510 million years ago

Phanerozoic · Paleozoic · Cambrian

Archaeocyaths, cup-shaped sponges with perforated double walls, raised the first frameworks that animals ever built above the seafloor. Scanned in three dimensions and compared with living corals, their branching turns out to record the water they grew in: they could stand up to a current and shape it, they probably housed photosynthetic partners, and reef by reef they made the seabed rougher.

Why it matters

A reef is not just a pile of animals. It is a structure that creates places to live, and the rougher it gets the more creatures it can hold. This is the first time animals built one. It is also the first time a reef collapse took a fauna with it, some fifteen million years later.

Dating & uncertainty

Archaeocyaths appear early in Cambrian Series 2 and are gone before its end. Their disappearance is marked by a negative carbon isotope excursion in Nevada, but the correlation between reef sections on different continents is loose enough that the collapse may not have been simultaneous.

Sources

1. Manzuk et al. (2023), Geobiology · Branching archaeocyaths as ecosystem engineers during the Cambrian radiation
2. Pruss et al. (2019), Palaeogeography, Palaeoclimatology, Palaeoecology · Palaeoecology of the archaeocyathan reefs from the lower Cambrian Harkless Formation, southern Nevada, and carbon isotopic evidence for their demise
3. Pruss et al. (2025), Palaeogeography, Palaeoclimatology, Palaeoecology · Rise and fall of archaeocyath reefs shaped early Cambrian skeletal animal abundance

Earth history as one day

21:13:282 h 47 min before midnight

if Earth’s 4.54 billion years were compressed into a single day