Some fusulinids are so similar in shape that scientists have to use a cross section of the fossil to identify them.
Fusulinids probably lived in clear water and may have lived on reefs.
Fusulinids are very large and complex for single-celled life, which is usually microscopic. Fusulinids are marker fossils, which means by looking at the fusulinids in a rock formation, scientists can tell how old the rock is.
Note: Edited 8/31/11 to remove the word "animal" and replace it with "organism." Fusulinida was a protist, not an animal.