The oldest fossil foraminifera
“Discussions concerning the appearance of skeletonization near the base of the Cambrian [about 550 Ma (million years ago)] are often restricted to metazoans and take little account of the acquisition of hard parts by protists at the same time. For example, hypotheses relating the evolution of skeletonization to increases in body size and to detoxification of excess calcium in metazoans do not apply to protists and hence are weakened by the appearance of testate protists in the Early Cambrian. However, this appearance is not inconsistent with the hypothesis, applicable to both metazoans and protists, that the initial function of skeletons was to protect the organism, primarily against predation. The presence of agglutinated foraminifera in the Lower Cambrian, probably Atdabanian Stage-equivalent strata, of the Taoudeni Basin, West Africa is reported here. These specimens extend considerably the known geologic range of several genera, they represent the earliest known unequivocal foraminifera, and they further remind us that protists as well as metazoans should be considered in accounting for the origin of skeletalization.” After Culver (1991, p. 689)
Tithanoteka Gaucher & Sprechmann 1999 is probably the oldest fossil foraminifera discovered in the Upper Vendian of Uruguay. Its test is "free, spheroidal to vase shaped, with walls composed of agglutinated rutile grains. The length of the neck increased logarithmically with maximum diameter, the largest resembling long-necked bottles. Wall is single layer, with organic cement. The rutile grains are wellsorted, with grain size fine to medium silt. Grain size is dependent on maximum diameter, with the largest specimens containing the coarsest grains. Grains are arranged with their long axes parallel to the surface of the test, probably acting as spines. Apertural end open." (cited after Gaucher & Sprechmann 1999 from Kaminski 2004)
Gaucher, C. & Sprechmann, P. 1999. Upper Vendian skeletal fauna of the Arroyo de Soldado Group, Uruguay. Beringeria 23: 55-91.