February 20. Calcareous algae

We’ve talked about the Cambrian explosion and some of the cool critters that evolved during it, like trilobites and brachiopods. But the less obvious life was still around, if not thriving. That included blue-green algae, or cyanobacteria. Those little guys way back in the Precambrian were the primary builders of our oxygen-rich atmosphere. But during the Cambrian, calcareous algae, that is those that could create fine layers of calcite, calcium carbonate, were still abundant enough to make small reefs in the Cambrian rocks near Saratoga, New York, at Jackson, Wyoming, and elsewhere.

Cambrian stromatolites near Saratoga Springs, NY.
Photo by Michael C. Rygel, via Wikipedia
under Creative Commons Attribution Share Alike unported license

It’s fair to call these things stromatolites, just as we did during the Precambrian. Stromatolites have survived to the present, but they have declined from their peak in the Precambrian. Some scientists think they suffered from the Cambrian proliferation of new grazing animals like trilobites, which might have roamed the surfaces of stromatolites, scraping the living algae off as food. This seems reasonable, and there is also a well-documented example from the Ordovician of stromatolites increasing in abundance during extinction events that killed off marine animals. Conversely, stromatolites decreased as animal life recovered from the extinctions.

Should we care about ancient algae? Well, ancient algae and other plants are the biggest sources of organic matter that becomes oil and natural gas. You decide whether or not to care about them.


Two noteworthy geologists were born on this day. Nathaniel Southgate Shaler was born February 20, 1841, in Newport, Kentucky. He became a fixture in the paleontology and geology departments at Harvard University. Ray C. Moore was also born today, in 1892, in Roslyn, Washington. He worked for the US Geological Survey and the University of Kansas, and he initiated the massive Treatise on Invertebrate Paleontology, 50 volumes, still in progress, and the definitive encyclopedia on invertebrate fossils. And on this day in 1962, John Glenn orbited planet earth.

—Richard I. Gibson

Photo by Michael C. Rygel, via Wikipedia under Creative Commons Attribution Share Alike unported license.