precambrian marine diversity was dominated by small fossils known as acritarchs. this term describes almost any small organic walled fossilfrom the egg cases of small metazoans to resting cysts of many different kinds of green algae. after appearing around 2000 mya, acritarchs had a boom around 1000 mya, increasing in number, diversity, size, complexity of shape and especially size and number of spines. their increasingly spiny forms in the last billion years may indicate an increased need for defence against predation. other groups of small organisms from the neoproterozoic era also show signs of anti-predator defences. measuring taxon longevity seems to show an increase in predation around this time.