One-celled Organisms: Phylum Protozoa

One-celled Organisms: Phylum Protozoa johncallahan Mon, 07/13/2009 - 15:54

Protozoans are one-celled organisms that include the amoeba. One group of protozoans, the Foraminifera ("forams"), are among the most common fossils found in the Cretaceous of Delaware -- but are hard to study without a microscope. Forams build a hard outer covering -- some by secreting calcium carbonate or opaline silica, some by cementing sand grains -- in order to provide support and protection. The resulting many-chambered shells, which are commonly called "tests," are the parts preserved as fossils. Some are very simple, and others are very ornate.

Foraminifera live almost exclusively in marine environments. Some foraminifera live on the sea bottom or within sea bottom sediments -- these are referred to as benthic foraminifera. Others float in the water -- these are called planktonic. Certain species of benthic foraminifera are known to prefer to live in certain marine environments, making them useful in interpreting the marine conditions that existed in the past. Planktonic foraminifera are commonly useful for determining the age of ancient sediments because of the rapid evolution through time of many groups.

Forams are usually collected by special filtration and floatation techniques and studied under the microscope because of their tiny size. However, two genera from the Cretaceous of Delaware, Dentalina and Citharina, are large enough to be seen in a magnifying glass and are fairly common in the Mount Laurel dredge spoils.


Photograph and figures from DGS Special Publication No. 18, by E. M. Lauginiger, 1988.