Helcionellidae Wenz, 1938

Skovsted, Christian B. & Peel, John S., 2007, Small shelly fossils from the argillaceous facies of the Lower Cambrian Forteau Formation of western Newfoundland, Acta Palaeontologica Polonica 52 (4), pp. 729-748 : 734

Family Helcionellidae Wenz, 1938 Genus Mackinnonia Runnegar, 1990

Mackinnonia taconica ( Landing and Bartowski, 1996) Fig. 4A.

1980 Pollicina sp. ; Spencer 1980: 126, pl. 5: 5, 6, non pl. 5: 4, 7 (unidentified helcionelloids).

1996 Stenotheca taconica sp. nov.; Landing and Bartowski 1996: 753, figs. 5.5, 7–9, 10.2–3.

2001 Aequiconus taconica ( Landing and Bartowski, 1996) ; Parkhaev in Gravestock et al. 2001: 138.

2002 Stenotheca taconica Landing and Bartowski, 1996 ; Landing et al. 2002: fig. 8.4.

2004 Mackinnonia taconica ( Landing and Bartowski, 1996) ; Skovsted 2004: 16, figs. 3I–R, 4A–C.

Holotype: NYSM 15529 . Internal mould from the uppermost Browns

Pond Formation (late Early Cambrian), Claverack, New York State,

USA.

Type species: Mellopegma rosratum Zhou and Xiao, 1984 .

Material.— NFM− 682 and nine additional specimens from sample JSP1982−01.

Discussion.—Straight to slightly coiled and laterally compressed helcionelloid molluscs with smooth outer shell surface combined with prominent internal rugae, are assigned to the genus Mackinnonia Runnegar, 1990 . Most specimens of Mackinnonia from the Forteau Formation of Newfoundland have a sub−centrally placed apex and represent Mackinnonia taconica ( Landing and Bartowski, 1996) , also known from the Browns Pond Formation of New York State ( Landing and Bartowski 1996), the “Anse Maranda Formation” of Québec ( Landing et al. 2002) and the Bastion Formation of North−East Greenland (Skovsted 2004). Internal moulds of a similar morphology were reported from the Forteau Formation of Labrador as Pollicina sp. ( Spencer 1980).

A single fragmentary internal mould ( Fig. 4B) with prominent rugae has a more strongly coiled apex and a larger protoconch than typical specimens of Mackinnonia taconica . Large collections of M. taconica from Greenland show considerable variation in apical angle and expression of the internal rugae, but not in the degree of coiling or size of the protoconch (Skovsted 2004). The atypical specimen is most reminiscent of Mackinnonia corrugata (Runnegar, 1990) from Australia in lateral view, but the fragmentary nature of the specimen precludes specific determination.

Stratigraphic and geographic range.—Late Early Cambrian of New York State, Quebec, North−East Greenland, western Newfoundland and southern Labrador.