Protozoa The Protozoa embrace a wide and diverse group of organisms. They are regarded by many as a separate phylum, while others view the group as a ‘taxonomic dustbin’. The Protozoa are now classified as a sub-kingdom of the Kingdom Animalia—at least by the protozoologists. This classification, while having many strengths, leaves out many of the more botanical groups that are now separated as the algae. In micropalaeontology many of the protozoan groups, both botanical and animal, are studied as an integral part of the subject and used in both biostratigraphy and palaeoecology. The majority of the organisms included within the sub-kingdom Protozoa have left no fossil record, although many are almost certain to have had a long evolutionary history.
Geologists and micropalaeontologists have a particular interest in those groups that leave behind either organic-walled fossils or calcareous/siliceous skeletons. The important groups are the foraminifera, thecamoebians, radiolaria, silicoflagellates, ebridians, calpionellids, and tintinnids. The other, botanical, microfossil groups that are often included within the Protozoa by geologists include the acritarchs, tasmanitids, chitinozoans, dinoflagellates, haptophytes (calcareous nannofossils), and the diatoms.
Micropalaeontologists often use either the term ‘protozoans’ or ‘protistids’ for these groups of unicellular organisms. Because both groups include organisms that have both animal-like and plant-like characteristics, the classification(s) and phylogentic relationships within the groups are beset with complexities. Protozoologists and phycologists tend to use mutually exclusive classifications. Protists arose from the prokaryotes (unicellular forms that lack a nucleus and any other membrane-bound cell organelles such as plastids or mitochondria) in the Late Archaean or Early Proterozoic (between about 3000 and 2000 million years (Ma) ago). Most protist and protozoan groups are parasitic and non-mineralized. We can infer their presence in the geological record only because they must have been an important part of the food chain. The forms that do produce siliceous or calcareous skeletons or tests are very unrepresentative of the group as a whole. Undisputed protozoans are known from the Early Cambrian (550 Ma ago), but strange flask-like objects are known from rocks dating back to 850 Ma.
Malcolm B. Hart
Bibliography
Hart, M. B., and and Williams, C. L. (1993) Protozoa. In Benton, M. J. (ed.) The fossil record, Vol. 2, pp. 43–70. Chapman and Hall, London.
