Geoscience Reference
In-Depth Information
The lower storey is laterally persistent and domi-
nantly planar bedded. There are minor com-
ponents of lateral accretion, represented here
at the northern channel edge on the western
ridge exposure.
The upper sandstone is planar bedded, sheet-like
and fills large scours at its base.
Plant macrofossils
Plant fossils from the Celsius Bjerg Group are gen-
erally rare, of low diversity and poorly preserved.
They generally do not yield characters suitable for
identifying them to generic level. The macrofossils
are more common in the Stensi
¨
Bjerg Formation.
The best-preserved fossils are found in the
Obrutschew Bjerg Formation lake deposits, but
diversity remains low.
The interpretation preferred here is therefore a
minor channel that is part of, and peripheral to, a
major sandstone body within the Britta Dal
Formation. During at least two major flood events
the Acanthostega sandstone was part of this major
system. Its channelled morphology suggests that,
rather than being a floodplain surface channel, it
was an anastomosing inset low-water channel.
This means that it was more likely to contain
water for longer following flood events, that is, a
waterhole. The tetrapods were found within a unit
with inclined bedding, that is, a scoured depression,
in a small marginal channel peripheral to a major
channel system, and were almost certainly trans-
ported or migrated downstream during or following
a major flooding event. As the flood waters
receded, the individuals would either not been able
to, or not chosen to, return upstream and were even-
tually forced to seek out the remaining water pools
(i.e. refugia) that filled the basal scours. It was
here, trapped by the receding waters level, that they
died. This provides an explanation as to why so
many articulated Acanthostega individuals are
found together: they congregated in the remaining
water in the river that was present in the basal scours.
The alternative to passive transport is less likely
as it requires many intact, that is, fresh carcasses to
be carried into an orientated assemblage where the
other vertebrates are disarticulated bones and
scales (Clack 2002). It is therefore inferred that for
most of the time Acanthostega lived in smaller,
more permanent and likely more upstream parts of
the river system which persisted for the years
between the major flood events. It remains an inter-
esting speculation as to how long Acanthostega
remained in this part of the Britta Dal fluvial
system and whether it could inhabit such waterhole
refugia for long parts of the intervening dry season.
However, such low-water sand-filled channels are
rare within the full thickness of the Britta Dal
Formation on Gauss Halvø and could never have
been the major habitat for Acanthostega.
This new interpretation presented here con-
tradicts that generally portrayed for the Britta Dal
Formation where these sandstones have been inter-
preted as a more permanent meandering fluvial
system. The implication of this new interpretation
means that
Britta Dal Formation plant macrofossils
There have been very few reports of plants from
the Britta Dal Formation (e.g. Larsen et al. 2008,
p. 289). Poorly preserved lycopsids up to 50 mm
in diameter were noted from the channel deposits
of the Acanthostega locality on Stensi
¨
Bjerg
(Bendix-Almgreen et al. 1990, p. 134) and scraps
of non-lycopsids were also noted in 2006. A more
concentrated plant assemblage was found at the
base of the 1174 m Ichthyostega sandstone in the
Profile Ravine measured section. This yielded
numerous axial plant fossils of less than a centimetre
diameter (Fig. 10b, h, i). Most have no distinguish-
ing features (Fig. 10b). One example shows a
swollen base and probable spine bases (Fig. 10h)
and another shows sub-opposite branching with
lateral structures on the daughter axes. These are
suggestive of fern-like plants such as Cephalopteris
(e.g. Schweitzer 2006). This bed is one that should
be noted for future investigation.
Clack and others (Larsen et al. 2008, p. 289) col-
lected a single branched specimen on Central Mount
Celsius at 400 m altitude (Fig. 10c). This poorly
preserved impression shows probable sub-opposite
branching and a main axis (6 mm) impression with
a central ridge. The most proximal branching point
on the left-hand side (line drawing, arrow) is
notable because the surface of the slab reveals that
there are two vertically superposed lateral branches.
These features together suggest a quadriseriate
insertion of laterals [as found in, e.g. Rhacophyton
(fern-like)], rather than an archaeopteridalean
progymnosperm affinity for this structure which
also lacks preserved leaves. Available evidence
therefore suggests the presence of a transported,
poorly preserved mixed lycopsid and 'fern' assem-
blage from the source area of
the Britta Dal
Formation.
Stensi¨ Bjerg Formation plant macrofossils
Fossils from this formation seemed more abundant
in the field. However, all the material found
belonged to lycopsids. These were predominantly
the internal casts of the inner and middle cortex,
with the surface features suggestive of leaf traces
Acanthostega has yet
to be found
within its normal habitat.
