Shortly before the Cambrian period, animals capable of movement and predation began to diversify. The Cambrian explosion saw a dramatic increase in complexity and ecological variety. Most major animal body plans emerged during this time. Hard shells evolved, leaving abundant fossils - wikipedia 
Cambrian Expansion: 541 Million Years Ago
HTML5 mp4 https://david.hitchhikers.earth/assets/history-of-the-earth/History%20of%20the%20Earth%20%5BQ1OreyX0-fw%5D.mp4#t=9:50 Cambrian Expansion: 541 Million Years Ago
Earth’s continents continued to drift, collide and break apart under plate tectonics. Changing ocean chemistry, sea levels and climates all contributed to bursts of evolutionary innovation and extinction.