Cirsotrema

“ Cirsotrema ” magellanicum (Philippi, 1845)

(Figures 8, 9)

Scalaria magellanica Philippi, 1845: 65

Scalaria magellanica var. latecostata Strebel, 1905: 658, pl. 23, fig. 43a–d Scala aff. orbignyi: Carcelles, 1944: 248 (in part)

Epitonium (Boreoscala) magellanicum: Clench & Turner, 1952: 324 –325, pl. 156; Rios, 1994: 98, pl. 32, fig. 398 Epitonium magellanicum: Castellanos, 1970: 63 –64 (in part), pl. 4, fig. 15; Cárdenas et al., 2008: 213 –214, fig. 3.34 Epitonium magellanicum latecostatum: Castellanos, 1970: 64 –65 (in part) Boreoscala magellanica: Weil et al., 1999: 12, fig. 9

Type localities. Fretum Magellanicum [= Magellan Strait] (Scalaria magellanica). Magalhen-Str. [= Magellan Strait]; Lennox Island [Beagle Channel] (Scalaria magellanica var. latecostata).

Type material. Two syntypes of Scalaria magellanica var. latecostata (one of them ZMB 2574, here designated as neotype of Scalaria magellanica magellanica; the other SMNH Type-6045).

Additional material examined. Uruguay: 35°37’10’’S 54°55’03’’W, 210.3 m (MACN-In 25696: 5 spm., 2 sh.); 35°42’S 52°52’ W, 184 m (MACN-In 24186: 1 sh.); 36°30’S 54°44’W, 26.5 m (MACN-In 23693-1: 1 sh.); 36°42’S 53°50’ W, 382 m (MACN-In 15648-10: 41 sh.). Argentina: Buenos Aires Province: 38°31’S 55°42’W, 109 m (MACN-In 23356: 1 sh.); [38°34’S 58°42’W], Quequén, 18 m (MACN-In 18676-1: 1 spm.); 38°40’S 56°00’W, 90 m (MACN-In: 8635-7: 1 sh.); 39°28’S 57°02’W, 90.5 m (MACN-In 25130: 2 sh.). Río Negro Province: 41°51’25.2’’S 58°09’12’’W, 106 m (MACN-In 40399: 1 spm., 1 sh.); 42°30’S 59°15’W, 96 m (MACN-In 37901: 1 spm., 18 sh.). Chubut Province: 42°31’24.6’’S 59°20’37.8’’W, 91 m (MACN-In 40398: 1 spm.); 44°30’ to 44°00’S 59°30’ to 60°30’W (MACN-In 40397: 1 spm.). Santa Cruz Province: 46°32.13’S 64°41.74’W, 107 m (MACN-In 40400: 3 spm., 1 sh.); [47°45’S 65°54’W], Puerto Deseado (MLP-Ma 13531: 6 sh.). Tierra del Fuego Province: 54°26’30’’S 64°53’ W, 111.6 m (MACN-In 25028: 4 sh.); 55°06’S 66°29’ W, Cabo San Pío, 65-80 m (MLP-Ma 14175: 1 sh.). Burdwood Bank: 54°53’13.08’’S 59°48’54’’W, 785 m (MACN-In 40781: 1 sh). Chile: 41°50’56.4’’S 73°23’52.8’’W, Golfo de Ancud, 214 m (MZUC 32579: 1 spm., 1 sh.); 42°43’40.8’’S 73°23’31.2’’W, Golfo Corcovado, 169 m (MNHN-Cl 7250: 1 spm.).

Known distribution. Rio Grande do Sul (30°S), Brazil (Rios 1994), south to Tierra del Fuego Province (55°41’S), and in the Pacific Ocean north to Golfo de Ancud (41°50’S), Chile. Living specimens: 18 to 214 m; shells up to 785 m.

Weil et al. (1999) also mentioned Antarctica in the distribution of “ Cirsotrema ” magellanicum (under Epitonium), although the source of this record is unknown and no other finding from this area is currently available to confirm its presence therein.

Description. Shell large (maximum L observed = 28.2 mm, apex missing; to 30 mm fide Clench & Turner (1952)), narrowly conical, thick, white, chalky (Fig. 8 A–C, E, F). Protoconch glossy, elongate, of about 1,000 µm in length and 500 µm in maximum diameter; composed of 2 bulbous whorls (Fig. 8 D); usually lost in larger specimens. Protoconch sculpture unknown (eroded in the available material). Limit between protoconch and teleoconch clearly evidenced by a change in sculpture. Teleoconch with up to 8 whorls, markedly convex in outline, attached (Fig. 8 A–C, E, F); suture crossed by axial sculpture (Fig. 8 K). Last whorl markedly flattened at the base. Aperture subcircular; outer margin thick, auriculate at base; inner margin either completely attached to last whorl or partially or completely detached; in the last two cases, originating a narrow and small to wide and large umbilicus. Fasciole small or absent (Fig. 8 A–C, E, F).

Teleoconch sculptured with slightly prosocline, widely-separated axial ribs and spiral elements. Ribs constant in number throughout the whorls: usually 13 to 15, but exceptionally as few as 10 and up to 18 (Fig. 8 G, H). Ribs on the first whorl are low and rounded, gradually increasing in height in subsequent whorls, where they may either remain erect (Fig. 8 C, F, G) or slightly to markedly recurved (Fig. 8 A, B, E, H); this variation is present even among ribs of a single specimen (Fig. 8 B). Ribs formed by fusion of several layers (Fig. 8 K, L); evenly arcuate along the first two whorls, but showing a flattened, sloping part near the apical suture in subsequent whorls (Fig. 8 C, F). Minute coronations sometimes present on the top, although generally lost by erosion. Ribs of adjoining whorls obliquely aligned and usually fused at their bases. In the last whorl, axial ribs extending to umbilical area, with an abrupt change in curvature at the periphery of base, which is delimited by a strong spiral cord that emerges from the insertion of the outer lip of aperture (Fig. 8 A–C, E, F, K). Interspaces of the axial sculpture showing 5–8 low but wide spiral cords, separated by extremely narrow interspaces, where secondary spiral cords and small, spirally aligned punctae are present (Fig. 8 K, L). Secondary sculpture clearly visible in all whorls, but missing at the base. Spiral sculpture not crossing over axial sculpture.

Operculum: Thick, circular, multispiral, with edge of adjacent coils with raised edges, and subcentrally-located nucleus (Fig. 8 I); outside surface sculptured with 20 to 28 massive, irregular bars per 0.1 mm, obliquely oriented with respect to growth lines (Fig. 8 J). Colour: dark brown.

Jaw (Fig. 9 F): Anterior part uniformly paved with several rows of polygonal, bluntly-pointed, pitted plates that continue to the margin.

Radula: With numerous teeth per row, all of them with a well-developed basal denticle (Fig. 9 B, E). Teeth of the central field short and solid, with an acute apical denticle and two shorter but well-developed, bluntly-pointed secondary denticles (Fig. 9 A). Subsequent teeth gradually increasing the length of the blade, and reducing the number of secondary denticles (Fig. 9 B, E); the outer teeth only having the upturned, acute apical denticle (Fig. 9 C, D). Outermost teeth reduced in size.

Remarks. The original description of “ Cirsotrema ” magellanicum is rather poor and the species was not figured by Philippi. Despite that, two characters clearly stand out from Philippi’s description: 1) the species reaches a large size “Alt. 9½’’’, diam. 4½’’’ [= 24. 3 mm L, 11.4 mm W]; and 2) the shell is sculptured by “circa 15 oblique” axial cords, with spiral threads in the interspaces. No specimen matching exactly that description was found at the MNHN-Cl, ZMB or NHMUK, where other Chilean species studied by Philippi are housed. However, the ZMB houses a topotypic specimen, with an original handwritten label by Philippi, using this name (Fig. 8 B). This is a 20.2 mm L specimen, with 10 axial ribs in the last whorl. Taking into account that this specimen comes from the type locality and was identified by Philippi himself as Scalaria magellanica, and considering the historical confusion surrounding the identity of “ Cirsotrema ” magellanicum (see below), the above-mentioned specimen (ZMB 2574), is here designated as neotype, with the express purpose of clarifying the taxonomic status of this species (ICZN, art. 75).

Strebel (1905) expanded the diagnostic characters of “ Cirsotrema ” magellanicum, particularly focusing on the axial sculpture, which he described as high, recurved ribs that abruptly slope from the suture. Based solely on the number of axial ribs in the last whorl, Strebel (1905) distinguished two varieties: one of them with 14–20 ribs (referred by him as Scalaria magellanica s.s.), and the other, represented by two specimens, one with 10 ribs per whorl, and the other with 12–13 ribs, which he named Scalaria magellanica latecostata. The “subtle” difference in the number of ribs could, at first, suggest that all these specimens correspond to variants of a single entity; however, the excellent figures provided by Strebel (1905: figs. 43a vs. 44a) reveal that he had actually two different taxa in hand. Unfortunately, Strebel (1905) failed to see that these taxa were not particularly different by the number of ribs; the “novelty” actually lay in the lots he referred to as Scalaria magellanica s.s., and not in the lots he assigned his new subspecies. One of these specimens had been originally identified by Philippi as Scalaria magellanica, had come from the type locality and agreed with the original description, except for the lower number of ribs. Under this scenario, Scalaria magellanica Philippi (not Strebel) and Scalaria magellanica latecostata are here regarded as synonyms. However, the specimens misidentified by Strebel (1905) as Scalaria magellanica s.s., correspond to one, or perhaps two new species, described below.

Castellanos (1970) identified specimens housed at the MACN as both Epitonium magellanicum and Epitonium magellanicum latecostatum. The study of this material reveals that some of the lots mentioned by the author (i.e., MACN-In 25130, MACN-In 24186 and MACN-In 25018 in part) actually correspond to “ Cirsotrema ” magellanicum; the identity of the other lots is discussed below. Another specimen of “ Cirsotrema ” magellanicum (currently MACN-In 18676-1) was found among the specimens of “ Scala aff. orbignyi ” reported by Carcelles (1944) from Quequén, Buenos Aires Province, Argentina.

In shell morphology and sculpture “ Cirsotrema ” magellanicum resembles “ Cirsotrema ” georgeanum nomen novum, “ Cirsotrema ” ctenodentatum n. sp. and “ Cirsotrema ” strebeli n. sp. (see distinguishing characters under these species).

Cirsotrema douvillei Fenaux, 1937, described on the basis of a single, broken, large fossil specimen (95 mm L, according to the original reconstruction) from “rivière Santa Cruz ”, Santa Cruz Province, Argentina, was regarded as a synonym, and as a possible synonym of “ Cirsotrema ” magellanicum, by Clench & Turner (1952) and by Brown & Neville (2015), respectively. However, the large size reached by C. douvillei, the age of its record, and the fact that in this species the spiral sculpture crosses the axial ribs —a condition absent in “ Cirsotrema ” magellanicum — preclude us from accepting this synonymy.