LIFE × EARTH

Animals Learn to Dig

~521 to 509 million years ago

Phanerozoic · Paleozoic · Cambrian

Animals stopped grazing across the sediment and started working their way down into it. The Precambrian seabed had been firm, sealed under a skin of microbial mat; it gave way to a soupy layer, stirred and re-stirred by burrowers, which has covered the ocean floor ever since.

Why it matters

Digging moves oxygen down and buried chemistry up, and it turns a firm floor into one that will not hold an animal's weight. Every bottom-dweller had to adapt to a surface that had changed under it. Nothing has reversed since: the mixed layer is still there, and still being stirred.

Dating & uncertainty

The trace fossil record puts the change in Cambrian Age 2 rather than at the period boundary, so the seafloor did not turn over when the Cambrian began. Burrowing had already started in the latest Ediacaran, and mat-covered bottoms survived locally for millions of years after.

Sources

1. Mángano & Buatois (2017), Earth-Science Reviews · The Cambrian revolutions: trace-fossil record, timing, links and geobiological impact
2. Dornbos et al. (2005), Palaeogeography, Palaeoclimatology, Palaeoecology · Paleoecology of benthic metazoans in the Early Cambrian Maotianshan Shale biota and the Middle Cambrian Burgess Shale biota: evidence for the Cambrian substrate revolution
3. Cribb et al. (2019), Royal Society Open Science · Increase in metazoan ecosystem engineering prior to the Ediacaran-Cambrian boundary in the Nama Group, Namibia

Earth history as one day

21:14:442 h 45 min before midnight

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