Tropidodiscus sp.

Tropidodiscus sp.

Fig. 3A, B View Fig .

2020 Tropidodiscus sp. Bertero 2020: 47, pl. 1: 9, 10.

Material.— CEGH-UNC 24677–24693; seventeen incomplete specimens preserved as inner and external moulds; from the Sandbian, Upper Ordovician, La Pola Formation,

Sierra de Villicum and Quebrada de Don Braulio, San Juan Province, Argentina.

Description.—Lenticular, planispiral and strongly compressed shell, with a wide and open umbilicus. The typical acute keel is not clearly visible. The teleoconch consists of about 3 whorls. The profile is triangular in transversal section ( Fig. 3A View Fig ). The length of the shell 20−70 mm and the umbilical angle 80−90º. The median selenizone is slightly acute, and growth lines are not clearly visible in the available specimens.

Remarks.—The specimens analyzed show the typical character of Tropidodiscus . Tropidodiscus subacutus ( Ulrich & Scofield, 1897) (in Wahlman 1992: 118, pl. 41: 9–13), from the Katian, Upper Ordovician of North America, differs from the Argentinean form in being slightly smaller, has the dorsum acutely carinate with a narrow truncation at the crest of the selenizone, a wider umbilicus, a very narrow and V-shaped sinus, and fine growth lines. Another similar species to Tropidodiscus sp. is Tropidodiscus cristatus ( Safford, 1869) (in Wahlman 1992: 119, pl. 41: 6–8), from the Katian, Upper Ordovician of North America; although the former has a strongly acute whorl floor keel. Tropidodiscus magnus

Miller, 1878) (in Wahlman 1992: 119, pl. 42: 1, 2), from the Katian, Upper Ordovician of North America, has a slightly more compressed shell, a narrower umbilicus and less expanded last teleoconch whorl. Tropidodicus ( Peruniscus ) sp. (in Rohr 1980: 150, pl. 1: 15–16), from the Middle/Upper Ordovician of North America, has a slightly more acute and subangular keel. Tropidodiscus sp. (in Ebbestad et al. 2008: 610, fig. 3.18–20), from the Upper Ordovician of Iran, is much smaller than the Argentinean species. Tropidodiscus laticonchus Ebbestad & Yochelson, 2000 (194, fig. 8A–G), from the Upper Ordovician of Norway, has a more inflated and lenticular shell, with more whorls of uniform expansion rate, a distinct raised median selenizone, a wider umbilicus and ornament with clearly visible prosocyrt growth lines. Topidodiscus maximus Ebbestad & Yochelson, 2000

196, fig. 9A–H), also from the Katian, Upper Ordovician of Norway, is much larger that the Argentinean species with a length of 380 mm, the teleoconch consists of 5 to 6 whorls, and it has a distinct raised and narrow selenizone and weak prosocyrt growth lines. Tropidodiscus pusillus (Barrande in Perner, 1903) (in Horný 1997: 52, pl. 9: 10, 11, pl. 10: 1–5), from the Darriwilian, Middle Ordovician, of Morocco, has a more laterally compressed shell, a more acute and conspicuous median selenizone, and the collabral lines are clearly visible in the shell surface.

Order uncertain (Palaeozoic Basal Taxa that are Certainly Gastropoda sensu Bouchet et al. 2017)

Superfamily Trochonematoidea Zittel, 1895

Family Trochonematidae Zittel, 1895 View in CoL

Genus Trochonemella Okulitch, 1935 View in CoL

Type species: Trochonemella notablis ( Ulrich & Scofield, 1897) , by original designation; from the Black River Group of lower Silurian , Kentucky, North America .

Remarks.—Members of Trochonemella are characterized by the presence of a narrowly phaneromphalous shell; sinus relatively shallow culminating in a wide notch that gives rise to a selenizone ( Knight et al. 1960).

Stratigraphic and geographic range.—Ordovician of North America and South America.

Trochonemella ? sp.

Fig. 3C View Fig .

2020 Trochonemella sp. ; Bertero 2020: 81, pl. 7: 5, 7, 10.

Material.— CEGH-UNC 24752 ; one recrystallized teleoconch; from the Sandbian, Upper Ordovician , La Pola Formation, Sierra de Villicum and Quebrada de Don Braulio, San Juan Province, Argentina .

Description.—Trochiform, gradate, step-like, low-spired and medium-sized shell, with a height of 70 mm and a width of 90 mm; pleural angle of 110º. Sutures impressed. The teleoconch consists of about five whorls, last whorl more expanded than the spire; juvenile whorls are convex ( Fig. 3C View Fig ). On mature whorls the ramp is slightly concave and becomes convex to the lower portion of whorls ( Fig. 3C View Fig 2 View Fig ). A deep spiral furrow is located above mid-whorl and delimiting the sutural ramp and the lower portion of the whorl. The spiral furrow is bordered by two conspicuous spiral ribs. Collabral elements are not visible. The base is flat to slightly angular. Umbilical characters are not preserved. The aperture is subangular.

Remarks.—The present material does not show some of the diagnostic features of Trochonemella , such as the shallow sinus and the wide notch that give rise to the selenizone; thus, it is tentatively assigned to the genus. Trochonemella reusingi Rohr, 1988 (in Blodgett and Rohr 2012: 3), from the Upper Ordovician of North America was interpreted as one of the most common and conspicuous fossils from this region. It differs from Trochonemella ? sp. in being larger and having strongly spinose peripheral spiral cords. Trochonemella churkini Rohr & Blodgett, 1985 (670, figs. 2.7–2.10), from the Katian, Upper Ordovician of west-central Alaska, differs from the Argentinean species in having a more turbinate and gradate outline shell, a selenizone at mid-whorl with clearly visible lunulae, an upper-outer angulation of whorls with scoop-like protrusions and collabral threads above sutures. Another species comparable to Trochonemella ? sp. is Trochonemella? mikulici Rohr, 1980 (176, pl. 8: 21–24), from the Katian, Late Ordovician of North America, although T.? milkulici has strong orthocline growth lines, a strong raised selenizone above mid-whorl, lunulae at selenizone clearly visible in mature whorls, and orthocline growth lines on the shell surface. Trochonemella antelopensis Rohr, 1996 (59, figs. 3.14–3.17), from the Middle and Upper Ordovician of North America, differs from the Argentinean species in having a raised selenizone located above mid-whorl, a slightly concave whorl surface below selenizone, labral edge with strongly prosocline to opisthocyrt growth lines, lower– upper angulation with short frill, and a circumumbilical spiral cord. Trochonemella sp. ( Ebbestad and Stott 2008: 235), from the Upper Ordovician of Canada, is smaller than the Argentinean species with a maximum height of 27 mm.