Spirolaxis cornicula ( Boettger, 1902 )

Harzhauser, Mathias & Landau, Bernard, 2023, The Architectonicidae and Mathildidae (Gastropoda, Heterobranchia) of the Miocene Paratethys Sea-victims of the Miocene Climatic Transition, Zootaxa 5370 (1), pp. 1-74 : 42-44

Spirolaxis cornicula ( Boettger, 1902)

Figs 21A–C

* Discohelix (Pseudomalaxis) corniculum n. sp. — Boettger 1902: 116.

Discohelix quinquangularis n. sp. — Boettger 1902: 116.

Discohelix (Pseudomalaxis) corniculum Boettger —Boettger 1907: 138.

Pseudomalaxis corniculum (Boettger) — Cossmann 1916: 143, pl. 12, figs 19–21.

Pseudomalaxis corniculum (Boettger) —Zilch 1934: 218, pl. 7, figs 19–20.

Pseudomalaxis quinquangularis (Boettger) —Zilch 1934: 219, pl. 7, figs 21a–b.

Pseudomalaxis (Discosolix) corniculum Boettger — Korobkov 1955: plate captions, pl. 13, figs 26–28.

Pseudomalaxis corniculum Boettger — Csepreghy-Meznerics 1956: 431.

Pseudomalaxis corniculum Boettger, 1901 —Strausz 1966: 119, pl. 57, fig. 20.

Pseudomalaxis (Spirolaxis) corniculum (O. Boettger 1902) — Bieler 1984: 77, pl. 3, fig. 21.

Pseudomalaxis (Spirolaxis) quinquangularis (O. Boettger 1902) — Bieler 1984: 77, pl. 3, fig. 22.

corniculum (Discohelix) Boettger, 1902 — Bieler & Petit 2005: 30.

quinquangularis (Discohelix ) Boettger, 1902 — Bieler & Petit 2005: 60.

non Discohelix (Pseudomalaxis) corniculum (Boettger)—Hornung 1920: 85, pl. 2, fig. 15 [= Spirolaxis clenchi Jaume & Borro, 1946].

Type material. Lectotype (designated by Zilch 1934: 218): SMF 372697 (XII 2373a), SL: 0.7 mm, MD: 2.8 mm, Senckenberg Museum, Frankfurt/Main, Germany, Coşteiu de Sus ( Romania), illustrated in Zilch (1934: pl. 7, fig. 19), Figs 21A 1 –A 3 .

Illustrated material: Holotype of Discohelix quinquangularis Boettger, 1902 , SMF 372694 (XII 2374a), SL: 0.5 mm, MD: 1.7 mm, Senckenberg Museum, Frankfurt/Main, Germany, Lăpugiu de Sus ( Romania), illustrated in Zilch (1934: pl. 7, fig. 21), Figs 21B 1 –B 3 . NHMW 2016 /0177/0342, SL: 1.2 mm, MD: 4.4 mm, Lăpugiu des Sus ( Romania), Figs 21C 1 –C 2 .

Additional material: Paralectotype XII 2373c, Senckenberg Museum, Frankfurt/Main, Germany, Coşteiu de Sus ( Romania), illustrated in Zilch (1934: pl. 7, fig. 20) .

Revised description. Small, delicate, glossy, translucent, open-coiled, almost planispiral shell. Protoconch small, of 2.25 whorls, planispiral (diameter 0.75 mm). Smooth with distinct protoconch lip. Teleoconch of 1.24 whorls. Dorsal side with faint growth lines. SSC reduced to smooth, narrow spiral thread. First half teleoconch whorl with weak central angulation on dorsum; later whorl profile weakly convex on dorsal side. Periphery with two finely crenulated keels (upper one corresponding to LPC). Nearly straight sided between peripheral keels. Lower keel (on basal area sensu Bieler 1993: 322) strengthens with ontogeny. Base weakly convex with delicate growth lines. Columellar side of whorls convex resulting in subcircular aperture with two weak peripheral angulations. UC reduced to faintly crenulated spiral thread. Apertural cross-section changing from sub-circular towards markedly rectangular during ontogeny.

Discussion. Boettger (1902) separated two open-coiled species based in the slight angulation on the dorsal side of ‘ Discohelix ’ quinquangularis Boettger, 1902 and based on the narrower coiling and the weaker crenulation of the keels. Placing both type specimens at the same scale shows, that ‘ D ’. quinquangularis fully corresponds to Spirolaxis cornicula at the corresponding growth stage. In our opinion, the differences discussed by Boettger (1902) are related only to different ontogenetic stages. As first revisers we chose Spirolaxis cornicula as name for this species.

Two extant species with an Amphiatlantic distribution have been described: A. centrifuga (Monterosato, 1890) and A. clenchi Jaume & Borro, 1946 (e.g., Bieler 1984: pl. 3, figs 19–20; Negri & Corselli 2016: figs 16o–q). Both have been recorded from the Mediterranean Pliocene. Differences between the two species center around the degree of disjunction of the teleoconch whorls and whether the peripheral keels are bifid or not. It is still unclear whether there is one or two species present in the Atlanto-Mediterranean Pliocene to present day. In any case, the Paratethyan specimens are like S. clenchi in having simple as opposed to the bifid peripheral carinae seen in S. centrifuga , and like S. centrifuga in having finely crenulated peripheral carinae and not coarse as in S. clenchi . The teleoconch is widely uncoiled, whereas in both extant species coiling is tighter. However, specimens ascribed to S. clenchi illustrate a considerable variability in disjunction from the earliest whorls (own data B.L.). The Paratethyan specimens are very similar to one of the specimens from Estepona, but considerably smaller. We consider the Paratethyan specimens to represent a separate species, but this requires confirmation based on a larger number of specimens and a better understanding on the present-day species. The extant Indo-West Pacific Spirolaxis cornuarietis Bieler, 1993 is another comparable species but differs in its prominent beads and additional beaded threads on the ‘umbilical wall’ ( Bieler 1993: figs 276–268).

Paleoenvironment. Extant Spirolaxis species occur from few tens of meters water depth down to bathyal depths ( Bieler 1993). Spirolaxis cornicula seems to have preferred middle to outer neritic environments.

Distribution. Known so far only from the middle Badenian (late Langhian) of the Central Paratethys Sea.

Central Paratethys Sea. Badenian (Middle Miocene): Pannonian Basin: Szob ( Hungary) ( Csepreghy-Meznerics 1956); Făget Basin: Coşteiu de Sus, Lăpugiu de Sus ( Romania) ( Romania) (Zilch 1934).

Figures

FIGURE 1. Descriptive terms used in descriptions of Architectonicidae (modified from Bieler 1993).

FIGURE 2. Whorl cross-sections of Architectonicidae genera. All modified after Bieler (1993) except for Simplexollata, Basisulcata and Ammotectonica. Solatisonax? refers to Solatisonax? kostejana (Boettger, 1907), Bold genera are represented in the Paratethys Sea. SSC: subsutural cord, MC: mid cord(s), UPC: upper peripheral cord, LPC: lower peripheral cord, IPC: infraperipheral cord; UC: umbilical crenae; PUC: proxumbilical cord.

FIGURE 3. Descriptive terms used in descriptions of Mathildidae.

FIGURE 21 A1–A3. Spirolaxis cornicula (Boettger, 1902), lectotype, SMF 372697, Coşteiu de Sus (Romania). B1–B3. Spirolaxis cornicula (Boettger, 1902), holotype of Discohelix quinquangularis Boettger, 1902, SMF 372694, Lăpugiu de Sus (Romania). C1–C2. Spirolaxis cornicula (Boettger, 1902), NHMW 2016/0177/0342, Lăpugiu des Sus (Romania).