USGS Open-File Report 94-588
Pliocene Nothofagus of the Sirius Group, Transantarctic Mountains
- Robert S. Hill
- Department of Plant Science, University of Tasmania, Hobart, Australia
Nothofagus leaves which were recovered from glacial sediments of
the upper Pliocene Sirius Group in the Dominion Range, next to the upper
Beardmore Glacier in the Transantarctic Mountains will shortly be
described as a new species, tentatively named N. beardmorensis.
The current altitude of the site is about 1800 m, but there is evidence
for substantial post-depositional uplift. The age of the fossil flora is
constrained by the presence of recycled (reworked) mid-Pliocene marine
diatoms, which must be older than the Nothofagus fossils, which
are in situ. If this conclusion is correct, and it is assumed that N.
beardmorensis did not re-invade Antarctica, then we must accept that
all pre-Pliocene glaciations must have been at least 15-20°C warmer than
today, with higher values during deglacial periods.
The taxonomic affinities of the fossils
There are three different types of fossil which have been assigned to
Nothofagus at this site. The pollen has been assigned to
Nothofagidites lachlanae, which today is produced by both deciduous and
evergreen species. This pollen may belong to the fusca a group (subgenus
Fuscospora) or the fusca b group (subgenus Nothofagus). The wood
has been compared favorably with N. betuloides (evergreen,
subgenus Nothofagus) and N. gunnii (deciduous, subgenus
Fuscospora). The pieces of wood which have been recovered to date have a
small diameter, suggestive of a shrubby growth form, possibly even a
ground-hugging habit. The leaves are very close in morphology to several
extant deciduous species from southern South America.
The deciduous nature of the leaves
There are three lines of evidence supporting the deciduous nature of the
leaves. First, their taxonomic affinities, mentioned above. Second, the
leaves have plicate vernation, which is unique to the deciduous species,
where the leaves are folded like a fan in the bud. Third, the fossil
leaves occur in dense, but very thin mats, which look very much like a
short term event, and probably an autumn leaf fall. Only one of these
mats has been discovered to date. These three lines of evidence provide
the strongest evidence ever presented for the winter deciduous habit of a
fossil plant, and place the issue beyond reasonable doubt.
A local source for the fossils
There are several lines of evidence which support a local source for the
fossils. The leaves, as already mentioned, occur in a thin but dense
mat. They have organic preservation, but are very thin (as are all the
extant deciduous species) and could not possibly be re-worked and left in
this condition. Therefore, there is no possibility that they have been re-
worked from older sediments. Furthermore, it is highly improbable that
the leaves have been transported more than a very few metres, since:
- The very dense leaf mat would not be expected if they had been
transported far, especially by wind. The probable low growth habit, as
evidenced by the wood, also argues against mass transport of leaves by
wind.
- There is no evidence of damage to the very delicate laminas (either
mechanical or due to decay), suggesting that there was little wind or
water transport and that the leaves were incorporated into the sediment
soon after they were shed.
- Three different organs of what is almost certainly the same species
occur in the sediments, which is very suggestive of a local source. It is
unlikely that long distance dispersal would bring three such dissimilar
organs into close proximity. All evidence points overwhelmingly to the
conclusion that the source plants for these fossils were growing nearby,
and probably directly over the site of deposition.
What climatic constraints are imposed by the probable physiology of
N. beadmorensis?
Nothofagus is a very conservative genus, and the vast literature
on living and fossil species suggests that they evolve very slowly, both
morphologically and physiologically. Therefore, we can use the
physiological response of living species to infer the physiology of the
fossil species. There are two aspects which are of particular interest:
- Frost resistance. In winter, a deciduous species loses its leaves, but
its wood and dormant buds must still survive the low winter temperatures.
The available data on frost resistance in Nothofagus suggests that
minimum temperatures below about -20°C will place plants in jeopardy.
Although the data is limited, it is interesting that some of the evergreen
species appear to be as frost resistant as the deciduous species (e.g.
N. cunninghamii cf. N. gunnii), and the reason that a
deciduous species has survived rather than an evergreen species may be due
to the photoperiod rather than frost resistance. Snow cover may have been
of some assistance in surviving winter extremes, but this is difficult to
judge.
- During summer, the plants must reach a temperature substantially above
0°C in order to grow, and more importantly, reproduce. Temperatures in
excess of 5°C were probably required for a prolonged period of time. It
is also notable that the fossil leaves are substantially larger than some
of the deciduous species, which again may be an effect of the photoperiod,
but also suggests milder conditions than those experienced for example in
alpine Tasmania.
- In order to survive there must have been a plentiful supply of liquid
water during summer (also suggesting temperatures above 0°C). The open
Ross Sea may have been an important local moisture source and may also
have had an important effect on the local microclimate.
Finally, it is important to note that Nothofagus is notoriously
poorly equipped for long distance dispersal. The seeds rarely fall
further away from the parent plant than its height, and they cannot stand
prolonged immersion in sea water. There are no adaptations for bird or
other animal dispersal. This suggests that not only was Nothofagus
present in Antarctica in the mid-Pliocene, but that it had been present
continuously up until that time. However, there is some published
evidence for rare long distance dispersal of Nothofagus during
the Cenozoic, so the option that this species re-invaded Antarctica after
a prolonged cold period cannot be dismissed.
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