Sphenosphaera sp.

Sphenosphaera sp.

Fig. 2A, B View Fig .

2020 Sphenosphaera View in CoL ? sp.; Bertero 2020: 37, pl. 1: 1–4.

Material.— CEGH-UNC 24795–24800 , six recrystallized teleoconchs; from the Sandbian, Upper Ordovician , La Pola Formation, Sierra de Villicum, San Juan Province, Argentina .

Description.—Anomphalous, rounded but not globose, small-sized shell with a height and length of 8–15 mm. Flanks rapidly expanding; median-dorsal shell slightly elevated with a distinct selenizone. The aperture is 1.98 mm wider than long, and the margins flared laterally and posteriorly forming flat to slightly excavated posterolateral shelves. The spire is sub-rhomboidal in apical view and growth lines weakly visibly on the shell surface. As wide and deep and a V-shaped sinus ( Fig. 2A View Fig 3 View Fig ).

Remarks.—The type species of Sphenosphaera was originally ascribed to the genus Bellerophon by Ulrich in Ulrich and Scofield (1897). Wahlman (1992) suggested that Sphenosphaera differs from Bellerophon in being less globose, in having more flared apertural margins which form prominent posterolateral shelves and in having a distinct rounded median ridge on the whorl floor. The Argentinean species is comparable to Sphenosphaera clausus (Ulrich in Ulrich and Scofield, 1897) (in Wahlman 1992: 167, pl. 30: 1–23, pl. 31: 1–13, pl. 32: 1–15), from the Katian, Upper Ordovician of North America. The Laurentian species, however, has a selenizone on an elevated flat-topped median carina bordered by fine revolving threads and distinct lunulae, and closely spaced and regular growth lines which are broadly curved over the dorsal slopes in maturity. Sphenosphaera troosti ( Férussac and d’Orbigny, 1840) (in Wahlman 1992: 168, pl. 29: 1–22), from the Katian, Upper Ordovician of North America, differs from Sphenosphaera sp. in having markedly broader and inflated whorls, a more prominent selenizone located on a rounded dorsomedian ridge, and closely spaced and fine growth lines. Sphenosphaera burginensis (Ulrich in Ulrich and Scofield, 1897) (in Wahlman 1992: 170, pl. 28: 10–23), also from the Katian, Upper Ordovician of North America, has narrower whorls that are expanding rather slowly, a selenizone elevated on a thin dorsomedian carina, and closely spaced and thin growth lines. Sphenosphaera recurvus (Ulrich in Ulrich & Scofield, 1897) (in Wahlman 1992: 171, pl. 33: 1–6), from the Katian, Upper Ordovician of North America, is much narrower in dorsal view than Sphenosphaera sp. , has the aperture more subtriangularly elongated and not expanding laterally, has strongly curved growth lines, a deep sinus, and the selenizone is markedly elevated and truncated. Finally, Sphenosphaera capax (Ulrich in Ulrich & Scofield, 1897) (in Wahlman 1992: 172, pl. 34: 1–13), also from the Katian, Upper Ordovician of North America, is very similar to the Argentinean species in shell shape, although the Laurentian form is more globose than Sphenosphaera sp. and much larger with a length and height of 38– 26 mm, respectively.

Family Bucaniidae Ulrich & Scofield, 1897 View in CoL

Subfamily Bucaniinae Ulrich & Scofield, 1897

[= Grandostomatinae Horný, 1962 ]

Genus Bucania Hall, 1847 View in CoL

Type species: Bellerophon sulcatinus Emmons, 1842 , by subsequent designation of Waagen (1880); from the Chazyan Stage of the Middle Ordovician, North America.

Remarks.—According to Wahlman (1992: 127), representatives of Bucania are characterized by having a small to largesized shell, with rounded or depressed whorls gradually expanding, and with a medium or large umbilicus. The aperture has a thin lip with a broad V-shaped sinus and a central slit; the slit is shallow or quite deep. The selenizone is generally narrow, being slightly elevated, flat or channel-like. The shell surface is ornamented by transverse growth lines or lamellae and spiral threads; spiral threads run normal and are generally interrupted by growth lines. Growth lines may become lamellose in mature growth stages and accompanied by a laterally thickening. Considering these characters, the specimens analyzed here are assigned to Bucania .

This is the first occurrence of Bucania in the lower Palaeozoic (Upper Ordovician) of Argentina and South America.

Stratigraphic and geographic range.—Lower–Middle Ordovician to middle Silurian; North America, Europe, Asia and South America.