Chilodontidae

Herbert, D. G., 2012, A revision of the Chilodontidae (Gastropoda: Vetigastropoda: Seguenzioidea) of southern Africa and the south-western Indian Ocean, African Invertebrates 53 (2), pp. 381-381 : 397-400

Family Chilodontidae 1 Wenz, 1938

The use of generic names now considered to belong to the Chilodontidae has been complicated by a lack of precision. Doubtlessly, in many cases this is due to the fact that the type species are poorly known and thus the diagnostic characters of the genera are not clearly identified. Genera such as Turcica (sensu stricto, a calliotropid genus) and Euchelus have been employed as hold-all taxa that in reality represent ill-defined assemblages of dubiously related species. Some clarity is beginning to emerge as more material is studied, but there remain problematic areas where shell characters, when considered across a range of taxa, seem to intergrade between genera, as for example between Euchelus and Herpetopoma . Even within Herpetopoma there is a puzzling diversity of shell form, which suggests that it may be a composite taxon (see below). I propose two new genera for taxa which exhibit distinctive combinations of characters not present in the existing genera. A third new genus is proposed for extant species currently referred to Agathodonta , a fossil genus that I consider is not appropriate for these Recent forms.

The genera are not treated alphabetical order, but are grouped loosely in terms of similarity in shell form. However, this should not be taken to indicate any suggestion of phylogenetic relationship. Such inferences must await analysis of molecular data and a more complete dataset of morphological characters. Species treatments do not contain diagnoses as these tend to be repetitive of information given in the description. Instead, I provide keys to species which summarise the characters important for species discrimination and represent a more useful tool for identification. Further discussion of distinctive features and comparison with similar extralimital species is provided under ‘Remarks’. In addition to the Recent species discussed, I draw attention to a fossil taxon Chilodonta (Agathodonta) africana described by Rennie (1930) from Upper Cretaceous deposits on the Pondoland coast, Eastern Cape. The holotype (SAMC 8630) somewhat resembles Ascetosotoma providentiae (see below) and it seems correctly referred to the Chilodontidae .

1 Chilodontinae Wenz, 1938 (Mollusca) is homonymous with Chilodontidae Macalister, 1876 (Ciliophora) and Chilodontinae Eigenmann, 1910 (Pisces).An application has been submitted to the International Commission on Zoological Nomenclature (Case 3555) to emend the spelling of Chilodontidae Wenz, 1938 to Chilodontaidae ( Herbert & Bouchet 2011) . While such a case is under consideration by the Commission, prevailing usage is to be retained (ICZN 1999: Art. 82.1).

Excluded taxa

Leptothyra alfredensis Bartsch, 1915 , and Cyclostremella alfredensis Bartsch, 1915 , both from Port Alfred, Eastern Cape, superficially resemble Vaceuchelus , but show much greater similarity to the skeneid genus Parviturbo Pilsbry & McGinty, 1945 ( Hickman & McLean 1990; Warén 1991). Unfortunately, this cannot be confirmed by examination of the radula, as neither species has ever been found alive. Both names may quite possibly apply to the same species. Likewise Vitrinella agulhasensis Thiele, 1925 , from the Agulhas Bank, resembles Vaceuchelus , but is also probably a species of Parviturbo .

Stomatella cancellata Krauss, 1848 , the type species of Hybochelus Pilsbry, 1890 , was erroneously described from Table Bay, Cape Town. The species is not part of the southern African fauna and occurs instead in the central Indo-West Pacific ( Wilson 1993; Herbert 1996; Herbert & Warén 1999; Poppe et al. 2006; Poppe & Tagaro 2008).

Material referable to Tibatrochus Nomura, 1940 has been dredged off southern Mozambique and north-eastern South Africa, but is not included in this review as I strongly suspect the genus belongs in the Calliotropidae .

Key to genera of Chilodontidae in the south-western Indian Ocean (applies only to specimens with mature apertural dentition)

1 Columella lacking any form of dentition or with at most a low rounded bulge at its base.........................................................................................................................2

– Columella with one or more denticles (mature specimens)...................................3

2 Shell small (adult diameter <10 mm) and predominantly white, turbiniform; sculpture cancellate or foveolate......................................................... Vaceuchelus

– Shell larger, depressed turbiniform to ear-shaped, last whorl expanding rapidly and aperture consequently large; interior highly nacreous .............................. Granata

3 Aperture lacking a U-shaped notch at junction of basal and columella lips..........4

– Aperture with a distinct U-shaped notch at junction of basal and columella lips..6

4 Periphery angled and marked by a well-developed, often keel-like, spiral cord, sometimes stellate; suture strongly channelled; spiral sculpture inside outer lip extending deep within aperture................................................................. Perrinia

– Periphery more or less rounded.............................................................................5

5 Adult shell generally exceeding 10 mm in length; operculum oligospiral .............. .................................................................................................................. Euchelus

– Adult shell less than 10 mm in length; operculum multispiral ................................ ................................................................................................ Herpetopoma (part)

6 Columella with a single tooth above U-shaped basal notch..................................7

– Columella with 2 teeth above U-shaped basal notch.............................................8

7 Shell trochiform and with coarsely cancellate sculpture; periphery almost stellate; outer lip with subterminal external varix; parietal callus expanded beyond aperture ..................................................................................................... Pholidotrope

– Shell turbiniform-conical and with finely cancellate or beaded sculpture; periphery more or less evenly rounded; outer lip lacking external varix; parietal callus not expanded ................................................................................ Herpetopoma (part)

8 Exterior of outer lip with a strong, rib-like, subterminal varix; apex truncate and protoconch sunken ...................................................................................... Danilia

– Exterior of outer lip lacking such a varix, at most with a broad, low thickening behind outer lip; apex not truncate and protoconch exsert....................................9

9 Umbilicus present, lined and apically plugged with callus................ Ascetostoma

– Umbilicus absent............................................................................... Clypeostoma

Genus Clypeostoma gen. n.

Etymology: From Latin clypeus (a shield) and Greek stoma (a mouth); in reference to the shield-like inductural callus. Gender neuter.

Type species: Turcica salpinx Barnard, 1964 .

Diagnosis: Shell moderately elevated; spire whorls flat-sided to weakly convex; suture indented, level with subperipheral cord; sculpture of spiral cords and axial pliculae, cords ornamented with squamose beads where crossed by pliculae; aperture subcircular to obliquely D-shaped; outer lip conspicuously flaring beyond an internal collabral thickening; columella stout and bearing 2 rounded teeth when mature; thickened interior of outer lip with ridge-like denticles, that nearest columella largest and separated from basal columella tooth by deep U-shaped notch; umbilical and parietal region covered by glossy inductural callus shield; shield raised and flaring basally where it joins basal lip; umbilicus absent; circumference of aperture and inductural shield more or less in one plane, obliquely tangential to base of last adult whorl; outer lip not thickened externally; protoconch exsert and with a distinct sinusigera-like projection on terminal lip.

Remarks: Extant species belonging to this genus have previously been referred to Agathodonta Cossmann, 1918 (McLean 1984; Neubert 1998; Vilvens 2001; Vilvens & Héros 2003; Poppe et al. 2006). The type species of Agathodonta , by original designation, is Trochus dentiger d’Orbigny, 1843 (an unjustified emendation of Trochus dentigerus d’Orbigny, 1843 ) from the Lower Cretaceous (Neocomian) of north-eastern France, which has recently been shown to be a junior synonym of Littorina elegans Deshayes in Leymerie, 1842 ( Kollmann 2005). Whilst extant species undoubtedly exhibit some similarity with Agathodonta , examination of the lectotype (designated by Kollmann 2005: 70, pl. 9, fig. 6) and topotypic material of Trochus dentigerus (UCBL, Fig. 7A, B and MNHN, Fig. 7C , respectively) indicates that its shell has more strongly convex whorls, does not develop a flaring aperture or expanded inductural callus shield, has a broad, thickened external varix behind the outer lip, with additional varices on the spire whorls (arrowed in Fig. 7A, C ), shows little evidence of any denticles or ridges inside the outer lip, and the columella teeth, instead of being discrete teeth (which develop only at maturity), appear to be the ends of ridges spiralling up the columella. In addition, the sculpture is considerably less coarse, the spiral cords being finely and regularly beaded, and without axial pliculae in their intervals. Since these characters are at variance with those of Recent species, I have chosen not to refer extant material to Agathodonta . The original figure of Trochus dentigerus provided by d’Orbigny (1843) is misleading in not showing the varices ( Fig. 7D ), but Cossmann’s figures clearly do so, though strangely he does not mention them in his description of the genus ( Cossmann 1918: pl. vii, fig. 9). Such varices also occur in Calliovarica Vokes, 1939 , which is also probably referable to the Chilodontidae ( Beu & Raine 2009) .

Since no other genus-rank name is available for these extant species, I propose the new genus Clypeostoma , with Turcica salpinx Barnard, 1964 , as type species. Other described taxa referable here are Agathodonta elongata Vilvens, 2001 , A. meteorae Neubert, 1998 , A. nortoni McLean, 1984 , Euchelus townsendianus Melvill & Standen, 1903 and perhaps Perrinia cecileae , P. docili and Herpetopoma barbieri all of Poppe, Tagaro & Dekker (2006). Whereas Clypeostoma elongatum , C. reticulatum and C. salpinx are distinctive, the others are confusingly similar and exhibit intergrading variability, suggesting that some at least may prove to be synonyms (see below). When a good series of samples is available for one taxon, e.g. C. salpinx , it is clear that allowance must be made for some individual variation in size at maturity, L/D ratio, spiral cord number and strength of apertural dentition. In the absence of such information for the other taxa, it is not possible to make informed comment on potential synonymies within the genus. In some cases the differences evident between the species are small and perhaps of limited significance. C. meteorae and C. townsendianum were both described from the mid to outer continental shelf of the north-western Indian Ocean and may well be synonyms, even though the holotype of former is smaller than the figured syntype of Euchelus townsendianus (length 7.5 vs 10.4 mm), which I here refigure and designate as lectotype ( Fig. 12F, G ).

Clypeostoma differs from Danilia in lacking an external varix behind the outer lip and in not having a sunken region median to the thickened edge of the columella pillar. In Danilia the shell apex is also truncate and the protoconch flat or a little sunken. In Ascetostoma gen. n. the apertural dentition is more complex and the umbilicus is retained and lined with callus. Some species referred to Herpetopoma Pilsbry, 1890 , also have a strong U-shaped notch at the base of the columella, but such species have only one distinct columella tooth, lack an expanded inductural callus and often retain an open umbilicus.

Figures

Fig. 7. Agathodonta Cossmann, 1918: (A, B) cast of lectotype of Trochus dentigerus d’Orbigny, 1843 (Dupin coll’n, UCBL, EM-32041-1, originally in l’École des Mines de Paris), designated lectotype by Kollmann (2005: 70, pl. 9, fig. 6), specimen also illustrated by Cossmann (1918: pl. vii, fig. 9), Neocomian, Marolles, Aube, France, length 18.3 mm (cast in NMSA L2816); (C) topotype of T. dentigerus d’Orbigny, 1843, Neocomian, Aube, Marolles, length 11.7 mm, diameter 8.1 mm, (d’Orbigny coll’n, MNHN 4927); (D) original figure of T.dentigerus provided by d’Orbigny (1843: pl. 177, fig. 9). Arrows indicate varices on spire whorls.

Fig.12.Clypeostomaspecies:(A, B) C.meteorae (Neubert,1998),SW of Cap d’Ambre,Antsiranana,northern Madagascar, length 6.9 mm (MNHN); (C–E) C. meteorae, small specimen, off Mahajamba Bay, northern Madagascar, length 5.6 mm (MNHN); (F, G) Euchelus townsendianus Melvill & Standen, 1903, figured syntype, length 10.4 mm (NHMUK 1903.12.15.119); (H, I) C. cf. nortoni (McLean, 1984), west of Nosy Be, northern Madagascar, length 9.6 mm (MNHN); (J–L) C. cf. nortoni, large specimen, Sofala Bank, central Mozambique, length 14.6 mm (J. Rosado coll’n).