Radiolarians and foraminifera are both protozoa, specifically belonging to the class Sarcopoda within the phylum Protozoa. Besides Sarcopoda, other protozoa include flagellates, ciliates, and sporozoans.
Protozoa are the lowest animals in the animal kingdom. Their bodies consist of only a single cell, and they are extremely small, generally less than a quarter of a millimeter in size. However, this tiny single cell is a complete living organism capable of independent life, possessing the main life functions required of an animal. The various parts of the cell differentiate to form structures called "organoids," which are used to perform various functions. For example, flagella, cilia, or pseudopodia that protozoa often have are their organs of locomotion.
Most protozoa live in the ocean or freshwater, some live in damp soil, and some are parasitic.
To date, no fossil species have been found in the classes of flagellates and sporozoans. However, a large group of ciliates—the tinids—has had numerous fossils discovered.
Tinid worms typically have a conical or trumpet-shaped body, with cells growing within a rather delicate bottle-shaped shell. The cells are broad at the anterior end and surrounded by feather-like membranes that serve as organs of locomotion. Tinid worms are planktonic and widely distributed in the photic zones of all oceans, with only a few species living in freshwater. Due to their sensitivity to seawater temperature and salinity, modern tinid worm populations have formed diverse combinations in polar, temperate, subtropical, and tropical seas, as well as in estuaries, lakes, and swamps.
The exoskeleton of the tinid beetle is called a tinid shell. The shell wall is usually composed of pseudochitin or organic matter secreted by cells, cemented together with foreign mineral particles and biological hard shells. The shape of the tinid beetle varies from species to species, including bell-shaped, spherical, conical, bottle-shaped, nail-shaped, or bullet-shaped.
Fossils of the tinid worms are mainly preserved in limestone, but some are also found in marl, black silica, shale, glauconite clay, and peat. The fossil record indicates that tinid worms first appeared in the Silurian period, were abundant from the Jurassic to the Cretaceous, and disappeared after the Late Cretaceous. However, they are extremely abundant in modern times, which is truly mysterious.
