Acritarchs appear in quantity in organic-rich shales and siltstone strata 1.4 to 1.6 billion years old. They include a wide range of forms, so it is not clear whether or not they are monophyletic. At about 1 billion years ago they started to increase in abundance, diversity, and especially size and number of spines. Their populations crashed during the Cryogenian period 860 million years ago (the Snowball Earth episodes). They proliferated in the Cambrian explosion and reached their highest diversity in the Palaeozoic. They survived at least until the Ordovician period.