Acritarch

Acritarchs are early microfossils, the remains, probably, of eukaryote cells. It is the resistant single-layered cell walls, or perhaps a cover secreted by cell walls, that we see in early rocks of the Proterozoic era. These structures are organic in their chemical composition, not calcium carbonate. Their precise nature is not known, but they look like the resting stages of modern dinoflagellates (the 'red tide' organisms).

"These microfossils represent the resting stage in the reproductive cycle of eukaryote algae".^[1]p258

Acritarchs appear in quantity in organic-rich shales and siltstone strata 1.4 to 1.6 billion years old.^[2]p57 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.^[1]p256

The increased spininess possibly resulted from the need for defence against predators large enough to swallow them or tear them apart. Other groups of small organisms from the Neoproterozoic era also show signs of anti-predator defences.^[3][4]

1. ↑ ^{1.0 1.1} Levin, Harold. 2006. The Earth through time. Wiley, Hoboken N.J.
2. ↑ Clarkson E.N.K. 1998. Invertbrate palaeontology and evolution. Blackwell, Oxford.
3. ↑ Bengtson S. (2002), "Origins and early evolution of predation", in Kowalewski M. and Kelley P.H. (ed.), The fossil record of predation. The Paleontological Society Papers 8 (PDF), The Paleontological Society, pp. 289–317, archived from the original (Free full text) on 2008-09-10, retrieved 2010-05-20
4. ↑ Stanley S.M. 2008. Predation defeats competition on the seafloor. Paleobiology 34: 1–0. doi:10.1666/07026.1.