Pontiothauma, E. A. Smith, 1895

Criscione, Francesco & Hallan, Anders, 2024, Integrative taxonomy of Gymnobela and Pontiothauma (Conoidea: Raphitomidae) from Australian waters provides more evidence of transoceanic distribution in deep-sea gastropods, Journal of Natural History 58 (37 - 40), pp. 1429-1459 : 1451

Genus Pontiothauma E.A. Smith, 1895

Type species: Pontiothauma mirabile E.A. Smith, 1895 by monotypy.

Diagnosis

Shell fusiform to biconical-fusiform, moderately thick. Protoconch eroded. Teleoconch cream (white when eroded), suture impressed. Whorl profile medium- to very broad, with wide, oblique subsutural ramp, weakly demarcated from whorl periphery. Lower portion of whorl weakly shouldered or convex. Subsutural ramp sculpture of dense arcuate growth lines, reflecting shape of anal sinus. Teleoconch with axial sculpture of ribs below subsutural ramp; spiral sculpture of fine cords or shallow grooves; microsculpture of growth lines. Last adult whorl markedly convex, clearly demarcated from rather straight, broad siphonal canal. Aperture elongated, about half of total shell length; outer lip thin, unsculptured. Inner lip with distinct whitish callus. Anal sinus wide, moderately shallow, U-shaped. Radula hypodermic, loosely rolled, up to 500 µm in length (AMS C.519375; not figured), relatively straight, bearing two blunt barbs; adapical opening extremely long (about half length of tooth) situated posterior to barb most proximal to tip; basal swelling moderately broad, texture rough; basal opening large.

Remarks

Criscione et al. (2021b) redefined Pontiothauma based on its position in their molecular phylogeny and a distinctive combination of the following morpho-anatomical characters: (a) large stout-fusiform shell with shouldered whorls, (b) shell sculpture of few wide and prominent ribs and numerous, thin and dense cordlets and (c) radula of hypodermic, loosely rolled, double-barbed teeth. Pontiothauma currently encompasses four species, all from the central Indian Ocean: the type species, Pontiothauma mirabile E.A. Smith 1895 from (off) the Malabar coast in India ( Alcock et al. 1892, pl. 21, fig. 2–2a; Smith 1895, p. 2, pl. 1, fig. 1; Pace 1903), P. pacei E.A. Smith 1906 from off Kanyakumari (southern India) and off Sri Lanka ( Alcock et al. 1892, pl. 21, fig. 1–1a; Smith 1906, p. 159–160), P. minus E.A. Smith 1906 off Sri Lanka ( Alcock et al. 1892, pl. 21, fig. 2–2a; Smith 1906, p. 159) and P. abyssicola E.A. Smith 1895 off Kistna, Bay of Bengal ( Alcock et al. 1892, pl. 5, fig. 2–2a; Smith 1895, p. 3–4, pl. 1, fig. 2). As the genus name was declared to be of neuter gender ( Smith 1895, p. 2), the corrected name P. abyssicolum is henceforth used for the latter species. None of these species can be tested through integrative taxonomy as there is no molecular data or anatomical information available (except for the type species). Photographs of the shell of the holotype of P. mirabile ( Figure 5I ) – previously known from its published images only (drawings in Alcock et al. 1892, pl. 21, fig. 2–2a; Smith 1895, p. 2, pl. 1, fig. 1) – were generated and made available for this study. Based on these images and on the available literature data, we here attempt to test the current attribution of species to Pontiothauma , by assessing the presence or absence of genus-level diagnostic shell features. A drawing of its shell ( Alcock et al. 1892, pl. 21, fig. 2–2a) is the only image available for the holotype of P. pacei . However, the species’ current placement is maintained, due to the typical Pontiothauma features exhibited by the shell of its paratype ( Figure 5K ). The shell of the holotype of P. abyssicolum is significantly divergent from the typical Pontiothauma shell, due to its nearly-turriform shape, subcylindrical walls and short siphonal canal ( Alcock et al. 1892, pl. 5, fig 2a). Substantial differences between the holotypes of P. abyssicolum and P. mirabile were also reported based on anatomical observations, such as the presence in the former of eyes ( Smith 1895, p. 3; Pace 1903, p. 459). While these data are inconsistent with the attribution of P. abyssicolum to Pontiothauma , more data are required to propose a new generic placement. A superficial resemblance of the shell of the holotype of P. minus (drawn in Alcock et al. 1892, pl. 21, fig. 1a) to the shells in Figure 5D–H is insufficient to speculate on the species’ correct genus placement. With regard to a comment by Criscione et al. (2021b) stating that Pontiothauma has a paucispiral protoconch: upon further examination, we wish to retract this statement, as we do not consider there to be sufficient evidence. The only specimen examined that has remnants of a protoconch (AMS C.482289) is too worn to arrive at any conclusion as to whether the genus possesses paucispiral or multispiral protoconchs. Furthermore, neither Smith (1895) nor Pace (1903) provided information about the protoconch morphology. The original description by Smith (1895, p. 2) mentioned the absence of a radula. However, Pace (1903, p. 459) describes a hypodermic, barbed radula for the same specimen (holotype). Kantor and Taylor (2002) figured a radula of what they identified as P. mirabile . While the illustrations of the radula by Pace (1903 pl. 42, figs 5–7) are comparatively simple line drawings, his description of the barbs is instructive, and the scanning micrographs provided by Kantor and Taylor (2002, fig. 3E) are entirely consistent with our radular investigation of this genus. The hypodermic teeth of Pontiothauma are rather distinct, and not observed in any other raphitomid genus, as they possess an extremely long adapical opening situated posterior to the barb most adjacent to the tip, as opposed to the barb most distal from the tip in other genera such as Austrobela , Theta and Typhlosyrinx (see Criscione et al. 2021b).

Figures

Figure 5. Shells of Pontiothauma primary species hypotheses (PSHs)/species. (A) P1/ P. sp. AMS C.519343; (B) P1/ P. sp. Australian Museum (AMS) C.571758; (C) P1/ P. sp. AMS C.482298; (D) P1/ P. sp. AMS C.519375; (E) P2/ P. transregna sp. n. holotype AMS C.571641; (F) P2/ P. transregna sp. n. paratype AMS C.519398; (G) P2/ P. transregna sp. n. paratype AMS C.593511; (H) P2/ P. transregna sp. n. paratype AMS C.482289; (I) P. mirabile holotype E.A. Smith 1895 Zoological Survey of India (ZSI) M179/1; (J) P. pacei E.A. Smith 1906 paratype Natural History Museum of the United Kingdom (NHMUK) 1906.7.21.1; (K) P. sp. AMS C.547959; (L) Miniatures of shells drawn to scale. Scale bars = 10 mm.