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- Diatoms are golden-brown algae that secrete a shell (frustule) of opaline (amorphous) silica.
It was the intricate structure and beauty of the frustule that led amateur microscopists to study diatoms
beginning early in the nineteenth century. They range in size from less than 1 to more than 1000 micrometers,
but most range in size from 10 to 100 micrometers.
One of the characteristics that makes diatom useful in both biological and paleontological studies is that they
are found almost anywhere there is sufficient light and moisture. Habitats include marine, brackish, and
freshwater environments, as well as more unusual environments such as soil and near the mouths of caves.
Diatoms have even been found in rime on antenna wires in the Arctic.
In the modern oceans and lakes, diatoms are important elements in the food chain. A high abundance of diatoms
indicates that the waters are rich in nutrients, particularly nitrogen and phosphorous. Seasonal changes in
light, temperature, and nutrient concentration result in a succession of dominant species in the diatom flora.
In some regions, the high nutrient concentrations lead to the production of diatomaceous oozes. Over time, the
most pure of these oozes lithify to become diatomites.
The susceptibility of opaline silica to dissolution at higher temperatures (>50 degrees C) or pH (>7) limits
the age and geographic distribution of diatomaceous deposits. The earliest record of marine diatoms is from
rocks of Early Jurassic age; freshwater diatoms did not appear until the Paleocene. Diatoms are well preserved
and abundant in rocks of mid-Cretaceous age; the flora is already diverse, including numerous genera with
differing morphologies. This Cretaceous diversity suggests that diatoms have a significantly longer evolutionary
record, but that susceptibility of diatoms to dissolution has removed the earlier evidence of developmental
history. Diatoms did not suffer the same level of extinction at the Cretaceous-Tertiary boundary as the other
microfossil groups.
Although there are several periods of rapid evolutionary turnover during the Cenozoic, there is no evidence for
mass extinction of diatoms. Most of the rapid evolutionary turnover is connected with the increasing development
of a tropical-polar thermal gradient.
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