Glabrocingulum parvum, Foster & Danise & Twitchett, 2017

Foster, William J., Danise, Silvia & Twitchett, Richard J., 2017, A silicified Early Triassic marine assemblage from Svalbard, Journal of Systematic Palaeontology 15 (10), pp. 851-877 : 866-867

Glabrocingulum parvum sp. nov.

( Fig. 11 )

Diagnosis. Moderately low-spired shells with faint spiral ornament without sutural nodes. Broad selenizone in relation to whorl height with sharp edges. Deeply concave selenizone. Funicle absent or weakly formed.

Holotype. Dextral shell, NHMUK PI MG 1531 ; height = 2.5 mm, width = 3.0 mm.

Paratype. Dextral shell, NHMUK PI MG 1518 ; height = 2.0 mm, width = 2.0 mm .

Other material. Ten specimens from LD-04 ( NHMUK PI MG 1468 ; NHMUK PI MG 1471 ; NHMUK PI MG 1495 ; NHMUK PI MG 1500–1501 ; NHMUK PI MG 1513–1515 ; NHMUK PE PEI 5519–5520 ), and 14 specimens from LD-05 ( NHMUK PI MG 1476 ; NHMUK PI MG 1482 ; NHMUK PI MG 1484 ; NHMUK PI MG 1493 ; NHMUK PI MG 1530 ; NHMUK PE PEI 5493; NHMUK PE PEI 5502; NHMUK PE PEI 5514). Fourteen juvenile shells from LD-04 ( NHMUK PI MG 1516–1517 ; NHMUK PI MG 1519–1529 ; NHMUK PE PEI 5486) .

Derivation of name. Latin, parvum (small), referring to its small size.

Description. Shell is dextral, turbiniform, low-spired, with simple sutures. The upper whorl surface is slightly concave and gently sloping, and bears the selenizone between sharply protruding edges, with the lower of the edges on the shell periphery. Selenizone is concave and moderately deep. Narrow, concave band immediately below the lower rib. Whorl profile below the lower rib is gently convex. Basal angulation is relatively sharply defined, but convex; base with a rounded circum-umbilical shoulder; small umbilical chink. Aperture is a rounded trapezoid; inner lip is reflexed; peristome interrupted by a slit in the outer lip. Shell ornamented with closely, irregular spaced fine spiral lirae. Growth lines visible with small knobs at the intersection of spiral ribs, otherwise no axial ornamentation observed. On the top of the keel, near the suture, the growth lines form small nodules.

Protoconch: openly coiled; first two whorls smooth; third whorl possesses ~15 evenly spaced, rounded, spiral threads; peristome uninterrupted ( Fig. 11F ).

Remarks. These specimens resemble Glabrocingulum texanum Batten, 1989 with the selenizone being located in the upper third of the whorl and lacking axial ornamentation. They differ from G. texanum in having a broader selenizone in relation to whorl height; a more concave selenizone with sharper edges; a weakly developed funicle or none at all; and in being moderately low-spired. The uncoiling that has been described for some G. texanum specimens from the Permian of the south-western US ( Batten 1989) was not observed. These specimens are also considerably smaller (max. size: H = 3.6 mm, W = 4.6 mm) than the type material of G. texanum (max. size: H = 8.7 mm, W = 9.9 mm), which may be a consequence of environmental stress in the immediate aftermath of an extinction event and an expression of the Lilliput effect in this genus (cf. Twitchett 2007). Due to their excellent preservation, these specimens reveal the morphology of the larval stages, which show a similar ontogenetic development to other species of Glabrocingulum (e.g. Pan & Shen 2008).

The Vetigastropoda have a diverse range of living habits including being described in association with woodfall communities ( Kiel et al. 2008). The specimens in this study do occur in association with wood; however, no direct relationship was observed. Eotomariidae recorded from Zechstein reefs are described as motile algal grazers that were probably confined to a hard substrate ( Hollingworth & Pettigrew 1988). The lack of evidence of a hard substrate in their depositional environment, however, suggests that these specimens probably had a similar life habit to deep-sea vetigastropods that typically consume sediment ( Hickman 1988).

Mode of life. Surficial, fully motile, slow, surface deposit feeder.

Figures

Figure 11. Glabrocingulum parvum sp. nov. A, B, apertural view; A, holotype, NHMUK PI MG 1531; B, paratype, NHMUK PI MG 1518; C–E, NHMUK PI MG 1531; C, lateral view; D, side view of initial whorls; E, apical view; F, apertural view of a larval shell, NHMUK PI MG 1520. Scale bars = 1 mm, except F.