Sphenosphaera Knight, 1945

Genus Sphenosphaera Knight, 1945 View in CoL

Type species: Bellerophon clausus Ulrich in Ulrich & Scofield, 1897, by original designation; from the Bigby-Cannon Formation of the Upper Ordovician, North America .

Remarks.— Wahlman (1992: 165) proposed a diagnosis for the genus as “shell moderately rapidly expanding, aperture flaring posteriorly and laterally, whorls floor with distinct median ridge, ridge no present on parietal shelf, ornament collabral”. The author also stated that representatives of Sphenosphaera have small to moderate-sized shells, rounded but generally not globose and narrow or closed umbilicus. The aperture is broader than long, and the shell margins flared laterally and posteriorly, producing flat to slightly excavated posterolateral shelves. The slit is shallow to moderately deep, housed in a broad sinus. The selenizone is distinct, either developing a rounded to flat-topped ridge or a concave channel, in many cases with distinct lunulae and a thin revolving thread bordering on each side. The shell surface is marked by fine and closely spaced growth lines ( Wahlman 1992: 165).

The present research provides the first occurrence of Sphenosphaera in the lower Palaeozoic (Upper Ordovician) of Argentina and South America.

Stratigraphic and geographic range.—Ordovician to Silurian; Europe, North and South America ( Wahlman 1992; this paper).