Náttúrufræðingurinn - 2005, Blaðsíða 30

N áttúru f ræðingurinn
wings. It is noteworthy, however, that
we have found unusually large samaras
within three different species (Table 1),
belonging to three different genera: elm
(Ulmus), wing-nut (Pterocarya), and
maple (Acer). AU the spedes produced
extremely large samaras during the Late
Tertiary in Iceland compared to modem
as well as fossil species of related types.
The elm samara is approximately 40
mm long, with the nutlet placed in the
middle and a conspicuous notch at top.
When compared to modern elm species
that produce large samaras (Table 2),
15-25 mm in length, it is obvious that the
Icelandic Miocene elm is considerably
larger. The only samaras really compara-
ble to its size and form is U. glabra found
in north and central Europe and Asia
Minor, and U. macrocarpa found native
only in China. The nutlet of the late
Miocene Pterocarya samara from Iceland
measures 11 mm in diameter and its
wingspan is close to 60 mm. Only one
modem species is comparable (Table 3),
P. macroptera native to China. The form
of the nutlet is similar and some speci-
mens of P. macroptera even show the
rhombic-shaped wings distinctive for
the Icelandic Miocene type. Maple sama-
ras, Acer askelssonii, that have been found
quite frequently in Icelandic Late
Miocene sediments, are also relatively
large compared to both modem and fos-
sil species. Samaras of A. askelssonii mea-
sure 50-90 mm in length, and the length
of the nutlet is at least 10-35 mm (Table
4). The relation of length of nutlet to
length of wing is even more than 1:3 and
most often around 1:2. The nutlets are
always more than one-third of the length
of the wing, usually around the half of it.
A. askelssonii is most comparable in size
to samaras of the modem maple species
A. saccharinum, and in form to samaras
of the fossil species A. whitebirdense, both
found in North-America.
Although Tertiary plant fossils from
Iceland range in age from 15 to 6 Ma
(from Middle to Late Miocene), these
large samaras are only found in sedi-
ments that are 12 Ma or younger. The
fact that this large size is observed in at
least three different genera indicates a
wide-scale reaction to environmental
factors. They were probably spurred by
competition between the plants for bet-
ter possibilities to survive, especially for
their offspring on the dark forest floor of
the Late Miocene. If the composition of
the 9-8 Ma old flora from Mókollsdalur
is taken into consideration, it becomes
clear that most of the species found there
produce wind-distributed seeds (sama-
ras). During this geological timespan the
distribution of plants in Iceland was
mainly by wind that carried winged
fmits or seeds to new habitats. There
seems to be a rather strong relation
between the seed size within each
species and the amount of imposed
stress or competition, which stimulates
the most competing species to produce
the largest seeds. Nutrition stored in the
seed helps the plant through its early
growth stages in new habitats, where the
plant can grow independently without
supply of nutrition from the mother
plant. Large seeds are generally typical
for closed environments and are consid-
ered to have some advantage in harsh
conditions, such as insufficient light,
heavy leaf fall or droughts. This is con-
sidered to be the main reason for the
development of large fruits or seeds. It is
reasonable to conclude that competition
and fight for survival for the next gener-
ation was rather hard in Iceland during
the Late Tertiary, as indicated by the
unusually large samaras. In Iceland, the
broadleaved forest grew mainly in val-
leys with high ground-water stand out-
side the active volcanic zone, where vol-
canic activity was limited. Thick decidu-
ous forests occupied these areas with
dark floors where light seldom escaped
through the canopies, and the juvenile
plants had severe growth conditions. In
this fight for survival, plants producing
small fruits and seeds will have been at a
disadvantage compared to plants that
were stimulated by natural selection to
produce larger fruits and seeds. This
process produced fruits or seeds of
unusual size, as seen in the Icelandic
Late Miocene flora. The enlargement of
seeds and wings within the three differ-
ent genera in Iceland during the Late
Miocene must be considered as a
response to these environmental factors.
Þakkir
Rannsóknarnámssjóði er þakkað fyrir fjárhagslegan stuðning við Friðgeir
Grímsson, sem nú er í doktorsnámi í Háskóla íslands. Náttúrufræðistofnun
íslands er einnig þakkað fyrir aðgang að steingervingasöfnum
stofnunarinnar. Síðast en ekki síst er Snorra Gíslasyni jarðfræðingi þökkuð
margvísleg hjálp við útivinnu jafnt sem tölvuvinnu.
Heimildir
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during the years 1814 and 1815, 1-2. Oliphant, Waugh & Innes,
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