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Science Olympiad
Phylum Heterokontophyta
Diatoms (Phylum Heterokontophyta formerly Chrysophyta) make their first appearance during the Jurassic. Diatoms are unicellular or colonial eukaryotic phytoplankton. Diatoms are the most successful of the phytoplankton in today’s oceans in terms of biomass and net primary production. Diatoms are encased in a two-part, asymmetrical silica cell wall. The two halves of the silica case fit together like the parts of a petri dish. The diatom’s silica shell is called a frustule.

Diatoms are not only major primary producers in oceans, especially in cold marine environments, they also make up the base of the food chain for many freshwater environments. The deposition of large numbers of diatoms can create the rock diatomite or diatomaceous earth. Diatomaceous earth is mined commercially and used as a filtering material, mineral filler, and fine abrasive. Fossil diatoms can be used for a multitude of purposes (Prothero, 1998, p 206):

  • late Cretaceous and Cenozoic biostratigraphic indicators
  • paleoenvironmental indicators for temperature, salinity, pH, and fertility of ancient lake environments
  • used to determine ancient upwelling conditions in marine environments

Evolutionary Shifts In Oceanic Primary Production

Earth's oceans have experienced two major shifts in the composition of primary producers. Initially, cyanobacteria along with other photosynthetic bacteria were the primary producers during the Proterozoic eon. The first shift occurred during the early Paleozoic era when eukaryotic green algae joined cyanobacteria in being major primary producers. The second shift would occur during the Mesozoic era when dinoflagellates and coccolithofores would be joined by diatoms in the Jurassic. Diatoms, dinoflagellates, and coccolithophores would assume their dominant role as the base of many modern marine ecosystems by Cretaceous times. (Knoll, Summons, Waldbauer, and Zumberge, 2007, p. 155).

Click on the word diatom to access images from the Microfossil Image Recovery and Circulation for Learning and Education website. The website will open as a new window.

Science Olympiad Fossil Event

The 2016 Science Olympiad Fossil List includes diatoms under the Kingdom Chromista and class Bacillariophyceae.


Knoll, Summons, Waldbauer, and Zumberge. (2007). The Geological Succession of Primary Producers in the Oceans. In Falkowski, P.G. Knoll, A.H. [Eds] Evolution of Primary Producers in the Sea. (pp. 133-163). China: Elsevier Academic Press.

Olney, M. (2008). MIRACLE: Microfossil Image Recovery and Circulation for Learning and Education: University College London. See:

Prothero, D.R. (1998). Bringing Fossils to Life: An Introduction to Paleobiology. New York: McGraw-Hill.


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