Littorina obtusata ( Linnaeus, 1758 )

Littorina obtusata ( Linnaeus, 1758) View in CoL

( Fig. 7G View FIG 1-G View FIG 3 View FIG , H 1-H View FIG 3 View FIG )

Turbo obtusatus Linnaeus, 1758: 761 .

Littorina obtusata View in CoL – Harmer 1921 (partim): 661, pl. 52, figs 27-28 (non fig. 29, that corresponds to Littorina littorea ( Linnaeus, 1758)) View in CoL .

Littorina (Neritrema) obtusata View in CoL – Reid 1996: 196, pl. 53, figs 3-5, 76-81.

Littorina obtusata View in CoL – Moerdijk & Janse 2015: 21, figs 4, 5.

For more, see synonymy list in Reid (1996).

MATERIAL AND DIMENSIONS. — Maximum height 8.4 mm, width 9.3 mm. RGM.1365211 (1), leg. ACJ ; RGM.1364983 (6), leg. ACJ ; RGM.1365076 (7), leg. WG ., RGM.1310829 (1), leg. WG ; RGM.1365087 (3), leg AWJ , RGM. 1365286 (24) leg. WG ; RGM.1365086 (c. 90) leg. AWJ ; RGM.1405527 (46), leg ACJ .

CHARACTERISATION. — Relatively small (H = 6 mm) thick shelled, robust, often smooth and shiny Littorina ; The Selsoif specimens are somewhat pointed, higher spired compared to modern specimens (viz. Reid 1996); aperture rounded with particularly thickened abapical margin; well preserved specimens bear very fine, slightly irregular spiral grooves. The ratio between the height of the lower apertural lip and the height of the outerlip ranges between 0.20 and 0.29, which is within the range of Littorina obtusata ( Reid, 1996) ; Littorina fabalis has a ratio over 0.29.

DISTRIBUTION. — The published fossil distribution of L. obtusata may not be entirely correct as previous authors had difficulties in distinguishing the species from L. fabalis (see Reid 1996). The Early Pleistocene North Sea Basin records (Norwich Crag Formation & Bridlington Beds: Harmer 1921) are based on misidentifications ( Reid 1996). The record from Selsoif, NW France (this paper) is possibly the oldest fossil occurrence. The species is common in Late Pleistocene deposits in the entire northern Atlantic region ( Reid 1996). — Present-day: western North Atlantic New-Foundland to NewYork and unconfirmed records from Hudson Bay; eastern North Atlantic from White Sea and Iceland in the north to southern Portugal in the south ( Reid 1996).

REMARKS

Today in western Europe two closely resembling Littorina species, L. fabalis (Turton, 1825) and L. obtusata ( Linnaeus, 1758) occur that can be best distinguished by soft anatomy and ecological differences ( Reid 1996), as well as allozyme differences. Reid however provided a morphological character that leads to 95% success in the separation of the species, viz. the ratio between basal lip height versus the aperture height. The dated phylogenies of Reid (1996) indicate they are sister species that evolved from a common ancestor around 2 Ma, broadly contemporary with the Selsoif material.