They look like little footballs, at least the American variety of football. Fusulinids (the name indicating the fusiform shape) are about the size and shape of wheat grains. They were marine protists (single-celled eucaryotes) that lived from the late Early Carboniferous to the end of the Permian Period. Fusulinids are foraminiferans of the Superfamily Fusulinoidea named by Valerïan Ivanovich Möller (Imperial School of Mines, St. Petersburg) in 1878. They are critical index fossils for the Late Paleozoic, and I knew them intimately during my dissertation work in southern Nevada.
The shell of a fusulinid is very complex. It is made of a granular calcite wrapped along the axis of the football in a series of chambers with internal walls. Each coil wrapped completely over the earlier coils, making the shells involute. They are most commonly studied in section to reveal the internal complexity.
Cross-section of a fusulinid (Triticites) from the Permian of Iowa.
Fusulinid evolution was dramatic for a single-celled group. The earliest varieties were very small (one or two millimeters in length), and the later ones up to five centimenters long. Their internal features also increased in complexity, making each successive new species very easy to identify. This is why they are such good indications of geological time intervals. It is this biostratigraphic value that proved most useful to me as a young graduate student working in what seemed to me to be virtually featureless Carboniferous limestones.
Hageman, S.A., Kaesler, R.L. and Broadhead, T.W. 2004. Fusulinid taphonomy: encrustation, corrasion, compaction, and dissolution. Palaios 19: 610-617.
Möller, V.I., von. 1878. Die Spiral-gewundenen Foraminiferen des russischen Kohlenkalks. Mémoires de l’académie impériale des sciences de St-Pétersbourg, VII Série, Tome XXV, No. 9 et dernier.
Ross, C.A. 1967. Development of fusulinid (Foraminiferida) faunal realms. Journal of Paleontology 41: 1341-1354.
Stevens, C.H. and Stone, P. 2007. The Pennsylvanian–Early Permian Bird Spring carbonate shelf, southeastern California: Fusulinid biostratigraphy, paleogeographic evolution, and tectonic implications. Geological Society of America Special Paper 429, 82 p.