Jökull - 01.12.2007, Side 16

Símonarson and Leifsdóttir
apparently similar to that of the present day, except
when the Búland Member was deposited. The pres-
ence of a considerable number of Portlandia arctica
indicates polyhaline conditions brought to the coast
by currents in addition to a considerable fresh water
input from land near retreating glaciers.
Marine fossils are found from two horizons in the
Máná Formation. At the base of the siltstone in the
TorfhóllMember, horizon 14 of Bárðarson (1925), the
dominating species in the assemblage are the infau-
nal bivalves Macoma calcarea and Nuculoma tenuis.
They are both deposit feeders but do not compete be-
cause they inhabit different levels in the substratum.
Higher up Serripes groenlandicus and Ciliatocardium
ciliatum become more common than N. tenuis as the
siltstone gradually goes over to fine-grained sandstone
(Vilhjálmsson, 1985). At the same time suspension
feeders become more common. The depth range indi-
cates that the assemblages lived sublittorally in shal-
low water near the coast and isotope analyses indicate
a sea temperature and salinity very similar to those
measured for recent shells in Breiðavík (Vilhjálms-
son, 1985). TheMacoma assemblage with N. tenuis is
considered to represent a life assemblage, whereas the
upperMacoma assemblage with S. groenlandicus and
C. ciliatum as more prominent species is supposed
to be an indigenous death assemblage (Vilhjálmsson,
1985). The epifaunal Chlamys breidavikensis assem-
blage has been found only in a 10-15 cm thick unit
in Stapavík, 200 m north of Torfhóll (Table 2). It
is named after an extinct petinid C. breidavikensis
closely related to the well known living C. islandica.
The shell population, encrusting epifauna as well as
the substrate indicate a life habit similar to the latter
(Vilhjálmsson, 1985). The assemblage is subarctic in
thermal requirements and epifaunal suspension feed-
ing organisms dominate, while infaunal molluscs are
not in life position and often fragmented. This indi-
cates a sublittoral assemblage living at relatively shal-
low depth at normal salinity.
MIGRATIONAL KEY SPECIES
Five molluscan species found in Icelandic Early Pleis-
tocene sediments are especially sensitive to changes
in sea temperatures as well as other ecological pa-
rameters. These are the three prosobranch gastro-
pod species Littorina littorea (Linné, 1758), Nucella
lapillus (Linné, 1758), and Tachyrhynchus erosus
(Couthouy, 1838) and the two bivalve species Port-
landia arctica (Gray, 1824), and Tridonta placenta
(Mørch, 1869) (Figure 4). Three of them, viz. L. lit-
torea, T. erosus, and T. placenta have only been found
in Iceland in the Búlandshöfði Formation. They do
not live in Iceland today. The same is true for P. arc-
tica, found in several Icelandic localities, both older
and younger, in sediments from glacial periods such
as the uppermost part of the marine isotope stage 2.
The oldest occurrence of N. lapillus in Iceland is in
the 1.5 Ma Threngingar Formation in Breiðavík, but
it is rarely found in interglacial sediments in Iceland
(Vilhjálmsson, 1985).
Littorina littorea and Nucella lapillus are boreal-
lusitanian species with subarctic outposts, whereas
Tachyrhynchus erosus, Portlandia arctica, and Tri-
donta placenta are arctic with subarctic outposts
(Table 1). The species are classified according to their
northern penetration along the European coasts. The
biogeographical terms are used in an oceanographic
sense, based on water mass characteristics (Símonar-
son et al., 1998; Funder et al., 2002). The zoogeo-
graphical distribution of the species is based on in-
formation from Mossewitsch (1928), Thorson (1941,
1944), Jensen (1942), Ockelmann (1958), Macpher-
son (1971), Bernard (1979), Lubinsky (1980), Fretter
and Graham (1980), Scarlato (1981), Graham (1988),
Poppe and Goto (1991), Reid (1996), and Petersen
(2001).
The heterodont bivalve species Tridonta placenta
belongs to the family Astartidae (Figure 4f-g). It
is closely related to Tridonta borealis (Schumacher,
1817) and usually considered as a subspecies or va-
riety of that species (Figure 4h). Here it is consid-
ered to be a distinct species in accordance with Pe-
tersen (2001) as it differs from T. borealis in several
characteristics (Table 3). Actually, he considered T.
placenta as two distinct species: Astarte jenseni, dis-
tributed around Iceland, and A. nuuki living in West
Greenland. However, further studies might clear up
the complex systematics of these species.
The five species live mainly in shallow water
14 JÖKULL No. 57