A genus of fossil algae. Girvanella is characterized by flexuous, tubular filaments of uniform diameter, composed of thick, calcareous walls (see illus.). External diameters average between 10 and 30 micrometers. Filaments may occur free (unattached), but usually occur in groups, twisted together to form nodules and encrusting masses on various objects. The genus is intergrown with encrusting foraminifers in some Paleozoic limestones.
Girvanella in a thin section of Cambrian limestone. Tube diameter is about 20 μm.
Girvanella is now generally placed in the bluegreen algae (Cyanophyta). This genus is a very common fossil, with a worldwide distribution. Occurring mainly in marine rocks, it has been reported from the Cambrian to Cretaceous. The apparent absence of Girvanella in rocks younger than Cretaceous age has not been satisfactorily explained. See also Algae.
