Wortheniella pygmaea (Stoppani, 1860-65)

Pieroni, Vittorio, 2019, The Norian Worthenia-like gastropods (Main Dolomite, Upper Triassic): reclassification of the specimens housed at the “ Antonio Stoppani ” Natural Museum, Italy, Natural History Sciences 6 (1), pp. 37-40 : 38-39

Wortheniella pygmaea (Stoppani, 1860-65) Comb. nov.

( Fig. 1 A-F)

1860-65 Delphinula pygmaea Stoppani , p. 256-57, pl. 59, fig. 16-17

1860-65 Delphinula regazzoni Stoppani , p. 257, pl. 59, fig. 18

1903 Worthenia pygmaea (Stoppani) – Tommasi, p. 114 [20], pl. 18 [3], fig. 14a-e

Holotype: The holotypes of Delphinula pygmaea and Delphinula regazzoni , like the rest of the specimens from Main Dolomite published by Stoppani in Series III of “Paléontologie Lombarde”, are lost (see above).

Locus typicus: Caino (near Brescia, BS), Italy.

Stratum typicum: Main Dolomite (Norian).

Material: Two specimens [catalogue numbers AS 41/43 ( Fig. 1 A-D) and AS 41/43a ( Fig. 1E,F )], almost totally preserved as pseudoshells recrystallised in dolomite, from Songavazzo (BG), Italy. The specimens are part of the historic Stoppani Collection housed at the Museo Scientifico Naturalistico “Antonio Stoppani” in the Seminario Arcivescovile Pio XI, Diocese of Milan , Venegono Inferiore, Italy.Although not the typical site for the species, Songavazzo is one of the localities where Stoppani collected the Norian gastropods he described in “Paléontologie Lombarde” .

Dimensions: AS 41/43: H= 6.80 mm; W= 7.45 mm; PA =62°. AS 41/43a: H= 10.60 mm; W= 11.80 mm; PA=75°.

Description: Specimen AS 41/43 has a gradate trochiform shell consisting of just over 5 rapidly grown whorls. Adult whorls have an adapical and an abapical spiral carina. The former is at the angle between the ramp and the flank; the latter is at the angle between the flank and the base. The flank is flattened and almost perpendicular to the coil axis. The two early teleoconch whorls are rounded and lack evident ornamentation. The next three whorls have a distinct adapical carina. The abapical carina is exposed only on the last whorl, whereas it corresponds to the suture on the preceding two whorls. The first preserved whorl is almost planispiral. The apical part is strongly recrystallized and thus the characters of the protoconch are not identifiable. A selenizone is present in correspondence with the robust adapical spiral carina separating the sutural ramp, which is inclined and slightly swollen, from the flank of the whorl at an obtuse angle. This carina is sculptured with well-spaced, longitudinally elongate nodules. The abapical carina, thinner than the former, does not have evident nodules. The whole surface of the shell is sculptured with more or less fine threads crossed by growth lines. Three threads are visible on the sutural ramp, of which the two adapical ones are significantly more pronounced and closer together, whereas the abapical one is thinner and located further away from the two adapical threads and from the adapical carina. Three threads are present also on the flank. They are more or less equidistant between the two carinae and well developed. The base, which for the most part is embedded in matrix, is weakly rounded, convex and sculptured with more than five evident threads. The growth lines are prosocline and prosocyrt on the ramp, prosocyrt on the flank below the selenizone, prosocline on the base. The aperture, distinctly wider than high, is polygonal in shape being angulated at the two spiral carinae.The specimen AS 41/43a – which has been completely extracted from the matrix, but has an eroded pseudoshell – shows a quite well developed columellar lip that covers almost completely the rather small umbilicus. Evident threads are visible on the base of this specimen, too.

Discussion: This species is described by Stoppani (1860- 65) from Norian Main Dolomite of Caino (BS). In the fauna from some Prealpine Lombard localities in the provinces of Bergamo and Brescia, Stoppani described Turbo songavatii Stoppani, 1860 -65 (= Turbo solitarius Benecke, 1866 , which according to De Stefani 1880 was a junior synonym of Turbo songavatii ,whichHaas1953consideredasynonymof Trochus contabulatus Costa, 1864, p. 232, pl. 5, fig. 4), Turbo pusillus Stoppani, 1860 -65, Pleurotomaria inzini Stoppani, 1860 -65, Delphinula escheri Stoppani, 1860 -65, Delphinula diadema Stoppani, 1860 -65, Delphinula regazzoni Stoppani, 1860 -65, Delphinula meriani Stoppani, 1860 -65, and Delphinula inzini Stoppani, 1860 -65. In the same publication, he described a species from the Rhaetian of Lombardy – Neritopsis ? oldae Stoppani, 1860-65 – which was closely related to the former.

In the revision proposed by Tommasi (1903) who compared Stoppani’s types with new material collected specifically in the type locality (specimens housed at the Museo di Storia Naturale dell’Università di Pavia), all these forms were redescribed with the following generic designations: Worthenia songavatii , Worthenia pusilla , Worthenia inzini , Worthenia pygmaea , Worthenia meriani , Worthenia sp. , and the new species Worthenia stoppanii . Delphinula escheri and Delphinula diadema were tentatively attributed to the genus Schizogonium Koken, 1889 , considering possible attribution also to the genus Guidonia De Stefani, 1880 . This distinction regarding the genera Delphinula escheri and Delphinula diadema is questionable: the specimens described in Tommasi 1903 display great uniformity in their main characters, so should have been attributed to a single genus. In particular, the six specimens classified as Schizogonium (?) escheri (cat. n. MSNP 20814) – the best preserved of which was illustrated by Tommasi in Table 3, Fig. 19 – do not display any substantial differences with the other forms attributed by him to the genus Worthenia . Indeed, the description of the growth lines and upper carina of Schizogonium (?) escheri made by Tommasi implicitly suggests the presence of a selenizone. The author considered Delphinula regazzoni synonym of W. pygmaea and deemed Stoppani’s drawing of D. regazzoni to be based on a single, poorly preserved specimen that, in reality, is very similar to W. pygmaea . Even Worthenia meriani is very similar to W. pygmaea , but Tommasi thought W. meriani to be distinct in that, in contrast with the figure in Stoppani (1860-65), the holotype had a base that lacked ornamentation.

The general appearance of these Norian species is always the same, with ornaments that are more or less accentuated. Haas (1953) extensively discussed these forms (pp. 56–63), but he did not have access to Stoppani’s or Costa’s species type specimens of Trochus contabulatus Costa, 1864, and Worthenia contabulata , which had probably already been lost during WWII (p. 62). Not being able to study the types, Haas compared his Peruvian specimens with the material from Main Dolomite of Esztergár Valley, Hungary, illustrated by Kittl (1900). According to Haas, in that material the growth lines, which in the figures in Kittl (1900) suggest the presence of a selenizone, are in reality deformed and rather axial on the flanks indicating that the shell lacks a true selenizone. Haas attributed those forms to the genus Guidonia De Stefani, 1880 (family Trochonematidae ), amending the diagnosis of Guidonia by identifying a type species ( Trochus rotulus Stoliczka, 1861 ), because De Stefani formally did not indicate it (for a history of the genus see: Gatto & Monari, 2010). In contrast, the specimens described here, like those in Tommasi (1903), display evidently prosocyrt growth lines on the sutural ramp and on the flank, with a marked edge in correspondence with the adapical carina, which is very much like the typical selenizone of Worthenia . The presence of a true selenizone was hypothesised also in Tichy (1975) on specimens of Worthenia contabulata from Main Dolomite from Austria. However, Tichy did not have well-preserved specimens at his disposal, so was not able to observe the growth lines.

Thus, giving as certain the attribution of these specimens to the “ Worthenia group”, these forms must be classified more precisely at the genus level. A possible solution derives from the study of the first whorls that appear fairly well preserved in AS 41/43. Research on the juvenile stages of the “ Worthenia group” has brought to the designation of new genera that are rather similar in relation to their teleoconchs. As mentioned by Nuetzel & Senowbari-Darian (1999), Chronic (1952) erected the genus Platyworthenia for some Permian US forms that differ from Worthenia Koninck, 1883 in having very low early whorls. Subsequently, Knight et al. (1960, p. I209) considered Platyworthenia and Worthenia as synonyms, on account of their great similarity. Later Yoo (1994) illustrated the protoconch and the first teleoconch whorls of the Carboniferous specimens attributed by him to Worthenia demonstrating that the early shell is trochiform. Schwardt (1992) erected the genus Wortheniella for specimens of the Upper Triassic St. Cassian Formation characterised by planispiral juvenile whorls. Nuetzel & Senowbari-Darian (1999, p. 97) assigned the Norian species from the Iranian Nayband Formation to the genus Wortheniella , and discussed the differences between the genera Worthenia De Koninck, 1883 , Wortheniella Schwardt, 1992 and Platyworthenia Chronic, 1952 , which are certainly related to each other. They concluded that this distinction depends upon the importance one wants to bestow upon the morphology of first teleoconch whorls. Conversely, Nuetzel & Erwin (2004) assigned the species of this group from the Norian deposits of Idaho ( U.S.A.) to Worthenia . Finally, Bandel (2009, p. 17) erected the family Wortheniellidae , describing the protoconchs of the Carnian forms from the St. Cassian Formation as sinistral and situated a little under the first whorl of the teleoconch.

Figures

Fig. 1 - A) Wortheniella pygmaea (Stoppani) AS 41/43, lateral view, showing the pattern of the growth lines, which curve backwards above and below the nodose adapical carina (corresponding to the selenizone).Songavazzo (BG), Norian,Main Dolomite. B) Wortheniella pygmaea (Stoppani) AS 41/43, latero-basal view, showing the very conspicuous threads on the base. C) Wortheniella pygmaea (Stoppani) AS 41/43, apical view of the early whorls of the spire. D) Wortheniella pygmaea (Stoppani) AS 41/43, lateral view of the early whorls of the spire. E) Wortheniella pygmaea (Stoppani) AS 41/43a, lateral view. F) Wortheniella pygmaea (Stoppani) AS 41/43a, latero-basal view, showing the region of the umbilicus, which is largely occupied by a columellar lip.