Pterocanium prismatium Riedel, 1957, p.87, pl.3, figs.4-5; emend. Riedel and Sanfilippo, 1970, p.529
Cephalis subspherical, pitted, bearing a sharply pointed, cylindro-conical apical horn with a length of approximately 1.5 times cephalic diameter, and sometimes smaller accessory spines. Thorax having overall shape of a triangular prism surmounted by an obtuse triangular pyramid, terminating at pronounced collar stricture: this general shape often modified by shallow concavity of the prismatic faces, and slight swellings between the proximal parts of the 3 thoracic ribs. In relation to the general thorax surface, the thoracic ribs are usually depressed in furrows proximally, and raised on ridges distally. Thoracic pores subpolygonal to almost circular, separated by thin bars, arranged approximately hexagonally in apparent longitudinal rows. Upper part of thorax usually bears small spines: these spines often concentrated on the proximal swellings, and thorns often occur on the ridged part of the thoracic ribs. Opening at base of thorax somewhat constricted. Feet three-bladed, proximally fenestrated, straight or slightly curved, usually almost parallel or somewhat divergent, usually shorter than thorax. Abdomen, when present, short, delicate, with irregular meshes smaller than those of thorax, usually entirely separate from and surrounded by the feet (Riedel, 1957).
The original description of this species admitted specimens without thorns on the three thoracic ribs, but such forms are now excluded (Riedel and Sanfilippo, 1970).
Usual length of apical spine 28-40 µm, of cephalis 20-26 µm, of thorax 110-132 µm, of feet 60-120 µm, of abdomen 5-50 µm. Breadth of thorax usually 110-150 µm (Riedel, 1957).
Major part of the long thorax triangular prismatic, with a distinct "shoulder"-like change in contour in the upper part. At this "shoulder", the ribs extending from the collar region to the feet are distinctly thorny. Feet short, straight, approximately parallel, proximally latticed (Riedel and Sanfilippo, 1978a).
P. prismatium is distinguished from co-occurring P. praetextum (Ehrenberg) (Riedel, 1957, p.86, pl.3, figs.13) by its larger, prismatic thorax and shorter feet, and from P. trilobum (Haeckel) (Nigrini, 1967, p.71, pl.7, figs.3a-3b) by its pronounced shoulders with thorns (Sanfilippo et al., 1985).
P. prismatium is a two-segmented theoperid with three feet and a thorax of characteristic prismatic shape. The variable features are the length of the thorax, which tends to be shorter in older specimens (77-110 µm in Spongaster pentas Zone), and longer at the end of the range (90-120 µm in P. prismatium Zone); and the robustness of the skeleton accompanied by a varying degree of thorniness. The test may be very delicate with narrow bars between pores and short thorns on the shoulders, or heavier with thick thorns along the entire length of the ribs as well as on the shoulders (Sanfilippo et al., 1985).
P. prismatium is found in middle to late Pliocene assemblages at latitudes lower than 15°. Its extinction, which a sharp, reliable and easily recognizable event, marks the top of Pterocanium prismatium Zone. The event is globally synchronous and corresponds approximately to the top of the Olduvai and the Pliocene-Pleistocene boundary (Hays et al., 1969; Berggren et al., 1980). However, it is often rare (1-2 specimens in tens of thousands) in the Indian Ocean. Its morphotypic first appearance is approximately synchronous with the lower limit of the Spongaster pentas Zone.
Additional illustrations can be found in Nigrini, 1971, pl.34.1, fig.4; Johnson and Knoll, 1975, pl.1, fig.9.