Cambrian substrate revolution

The "Cambrian substrate revolution" or "Agronomic revolution", evidenced in trace fossils, is a sudden diversification of animal burrowing during the early Cambrian period. Before this "widening of the behavioural repertoire", bottom-dwelling animals mainly grazed on the microbial mats that lined the surface of the substrate, crawling above (like how freshwater snails still do) or burrowing just below them.

Source: Wikipedia — Cambrian substrate revolution (CC BY-SA 4.0)