What on earth is that?! Organic microfossils that cannot be assigned to a specific group are called acritarchs. Acritarchs are defined as small, organic-walled microfossils of unknown biological affinity [1]. They are classified [2,3] according to their morphology, including shape, size, ornamentation and openings that would have allowed encased cell contents to release. Because of their uncertain biological affinity and dominance of simple, long-ranging forms, acritarchs can be difficult to study. Many acritarchs are marine and can include a wide range of organisms, such as metazoan egg cases, copepod eggs, planktonic eukaryotic algae, and cryptospores.
Some likely represent the resting cysts of dinoflagellates and their ancestors [4]. Once the biological affinity is known for a specific taxon, they are no longer classified as an acritarch and placed within the relevant microfossil group. Simple acritarchs have been found since the Archaean [5] and more complex forms since the early Paleozoic (~540 Ma). They are a key organic microfossil group for biostratigraphy and paleoenvironmental studies during the Paleozoic [6-8], as they predate many other organic microfossils.
Additional resources
http://www.ucl.ac.uk/GeolSci/micropal/acritarch.html
References
1. Evitt, W.R. A discussion and proposals concerning fossil dinoflagellates, hystrichospheres and acritarchs. 1963. Proceedings of the National Academy of Sciences, 49, 158-164.
2. Fensome R.A., Williams G.L., Barss M.S., Freeman J.M., Hill J.M. 1990. Acritarchs and fossil prasinophytes: an index to genera, species and infraspecific taxa. American Association of Stratigraphic Palynologists Contributions Series, 25, 771 p.
3. Lister, T.R. 1970. The acritarchs and chitinozoa from the Wenlock and Ludlow Series of the Ludlow and Millichope areas, Shropshire. Part 1. Palaeontographical Society Monographs, 124(1), 100 p.
4. Servais, T., Stricanne, L., Montenari, M., Pross, J. 2004. Population dynamics of galeate acritarchs at the Cambrian-Ordovician transition in the Algerian Sahara. Palaeontology, 47(2), 395-414.
5. Javaux, E., Marshall, C., Bekker, A. 2010. Organic-walled microfossils in 3.2-billion-year-old shallow-marine siliciclastic deposits. Nature, 463, 934–938.
6. Huntley, J.W., Xiao, S., Kowalewski, M. 2006. 1.3 billion years of acritarch history: an empirical morphospace approach. Precambrian Research, 144, 52-68.
7. Munnecke, A., Delabroye, A., Servais, T., Vandenbroucke, T.R.A., Marco Vecoli, M. 2012. Systematic occurrences of malformed (teratological) acritarchs in the run-up of Early Palaeozoic δ13C isotope excursions. Palaeogeography, Palaeoclimatology, Palaeoecology, 367-368, 137-146.
8. Wang, W., Servais, T., Yan, K., Vecoli, M., Li, J. 2014. The Ordovician acritarch Dactylofusa velifera Cocchio 1982: a biostratigraphical and palaeogeographical index species. Palynology, 39, 125-141.