Amber deposits are quite rare in Australia; Miocene-Pliocene ambers
are known from coal deposits in Victoria and Eocene ambers are known from
similar deposits in Tasmania. A number of beach deposits in Australia also
contain amber thought to have been reworked from Southeast Asian deposits
(amber floats and can be transported long distances on ocean currents). To date
Mesozoic amber has not been described from Australia, and is rare in the
Southern Hemisphere. Late Cretaceous amber deposits are known from a range of
deposits across North America and Eurasia, but in the Gondwanan continents
(i.e. the modern continents that made up the Mesozoic southern supercontinent
of Gondwana) amber is known only from Brazil, Argentina, South Africa, Ethiopia
and New Zealand.
In a paper published in the journal PLoS One on 13 May 2015, Annie
Quinney, Chris Mays and Jeffrey Stilwell of the School of Earth, Atmosphere& Environment at Monash University, Darla Zelenitsky of the Department of Geoscience at the University of Calgary and François Therrien of the Royal TyrrellMuseum of Palaeontology describe a newly discovered amber deposit from the
Otway Basin off the southern coast of Victoria.
The amber samples all come from drill cores produced by offshore
exploration for the hydrocarbons industry, and come from the Flaxman andWaarre
formations of the Upper Cretaceous Sherbrook Group, which are judged to be
Turonian in age, or about 90 million years old. During the Late Cretaceous the
Otway Basin was one of a series of rift basins forming as Australia spilt away
from Antarctica during the latter part of the breakup of Gondwana. These basins
were flooded in their centres, but also contained a range of marginal marine
environments, including numerous deltas. The Otway Basin ambers occur in thin
lignite (coal) horizons or sandy siltstones and sandstones, or occasionally
from within organic lenses within these sandstones, or within carbonaceous
(chalky) mudstones. Importantly these deposits are thought to have originated
in a high latitude austral forest, the first known record of amber from such an
environment from the Late Cretaceous, the other known Southern Hemisphere
ambers all having come from low-to-middle latitudes.
(Left) Turonianpalaeogeographic reconstruction of the
continents from a South Polar perspective. Black box over Australia indicates
study area. (Right) Study area map within the Otway Basin (grey box) off the
coast of Victoria, Australia (inset). White dots indicate amber-bearing well
locations including well names. Dark grey shading represents modern land; light
grey represents continental crust. Quinney et
al. (2015).
The majority of the amber recovered comprised small flecks, 0.5-5 mm
in diameter, though a few larger nodules of 1-3 cm were also found; many of
these larger nodules burst on being freed from the rock due to the release of
pressure. The amber ranged from light yellow to dark red in colour, with orange
ambers comprising about 65% of the sample and yellow about 15%. Most of the
samples were very clear, though a few brown or cloudy brown opaque ambers were
also found.
Colours of Australian amber. (A) Burst piece of yellow
amber, some still embedded in matrix; (B, top): Partial nodule of orange amber
embedded in matrix; (B, bottom left): Cross-section through orange amber nodule
containing organic matter; (B, bottom right): Cross-section through orange
amber droplet; (C) fragments from burst piece of darkorange amber; (D)
fragments from burst piece of red amber; (E) fragments from burst piece of dark
brown; (F) fragments from burst piece of milky brown amber. Scale bar is 4 mm.Quinney et al. (2015).
Only two identifiable fossils were found in the Otway Amber, both
being plant spores, the first identified as a Lycophyte of the genus Kraeuselisporites, and the other as Cyathidites minor, a widespread Fern
spore thought to have been produced by a member of the Cyatheaceae or
Dicksoniaceae. Kraeuselisporites spores
have previously been recorded from Late Cretaceous amber from East Africa, and
from Middle to Late Cretaceous deposits from New Zealand. Cyathidites minor is the most abundant known fossil spore in
Mesozoic deposits from Gondwana, and in particular is known from the Cretaceous
of Antarctica, New Zealand, and southern and eastern Australia, though this is
the first time it has been recorded as an inclusion in amber. Thus these spores
shed little light on the precise age or palaeoenvironment of the Otway Amber,
but their presence does leave the hope of more useful spores being found in the
future.
The deposits also include a number of filamentous micro-organisms.
Similar organisms have been interpreted as either Bacterial or Fungal filaments
in other deposits, however the inclusions from the Otway Amber fall within a size
range that could imply membership of either group, and in the absence of any
diagnostic reproductive features Quinney et
al. prefer not to make any assumptions about their origin. A large amount
of fragmentary degraded organic material is also present in many samples,
sometimes not completely encased within the amber.
Examples of bioinclusions from Otway amber. (A)
Probable specimen of Kraeuselisporites.
(B) Cyathidites minor. (C) Randomly
oriented network of filamentous microorganisms. (D) Mutually aligned
filamentous microorganisms pictured with ovoid and fusiform inclusions. (E)
Degraded organic matter exhibiting cracks along outer margin. (F) Degraded
organic matter with attached filamentous microorganisms. Scale Bars: 100 μm
(A–D); 50 μm (E); 200 μm (F). Quinney et
al. (2015).
As well as the identifiable inclusions there are a number of
non-identifiable inclusions, which Quinney et
al. describe as ‘pseudoinclusions’, an interpretation which allows for
these to be preservational artefacts instead of external material encased
within the amber.
Firstly there are small
spherical inclusions 5-200 μm in diameter, which range in colour from pale
yellow through orange to dark orange-brown, and which are coloured
independently of the amber in which they are included. Similar inclusions from
other amber deposits have previously been described as testate Protists
(shelled single-celled micro-organisms), however Quinney et al. can find no evidence of organelles, vacuoles or other
structures within the spheres, which could be considered indicative of a
biological origin, and feel that the wide range of sizes present does not
reflect a true Protist ecology (which would tend to produce clusters of size
values).
Secondly there are elongate fusiform and ovoid inclusions, which are
again 5-200 μm in size and range in colour from pale yellow through orange to
dark orange-brown, and are coloured independently of the amber in which they
are included.These come in clusters of a few to several hundred, typically
sharing a common orientation, and often are found alongside filamentous
micro-organisms, elongate pieces of decayed organic matter and filamentous
pseudoinclusions, sharing a common orientation with these also. Similar
inclusions to these have again been described as Protists, but again Quinney et al. feel that the lack of internal
structures and random distribution of sizes does not support such a diagnosis.
In some cases these ovoid inclusions have filamentous projections which
resemble germination from Fungal spores or cell plasma escaping from the broken
shell of a Testate Amoeba. Quinney et al. rule
out the Fungal interpretation on the basis of the size of these inclusions,
50-200 μm, which is considerably larger than any modern Fungal spore, typically
2-20 μm. A Testate Amoeba interpretation cannot be ruled out so easily, as
known Testate Amoeba can range from 10 μm to 300 μm in size, though again there
is no real evidence to support this either, and the filaments may simply be
unassociated bodies which have come into contact with the ovoids as they were
entombed in the amber.
Thirdly there are irregular filamentous inclusions of uncertain
origin. These tend to be found individually rather than in clusters, but may be
found alongside filamentous micro-organisms, elongate fragments of degraded
plant matter or fusiform/ovoid pseudoinclusions, and where this occurs will
typically share a common orientation with these. These filamentous inclusions
have well defined borders and lack internal structures. They range from 5 μm to
25 μm in width and from 90 μm to 225 μm in length, and again range in colour
from pale yellow through orange to dark orange-brown, and are coloured
independently of the amber in which they are included. The origin of these filaments
is unclear; they are too large to be of known Bacterial or Fungal origin, but
show no relationship to any other group.
Finally there are amorphous inclusions, lacking any common shape and
ranging in size from 10 μm to 200 μm. These can be elongate, in which case they
tend to be orientated in similar directions to other elongate inclusions in the
amber. The borders of these structures tend to be poorly defined, and they may
include speckles or vesicles within them. Again they range in colour from pale yellow
through orange to dark orange-brown, and are coloured independently of the
amber in which they are included. Similar inclusions in other ambers have been
interpreted as Amorphous Amoebae, but once more Quinney et al. can find no real evidence to support this.
Examples of other inclusions from Otway amber. (A)
Spherical inclusions (Type A pseudoinclusions) with internal vesiculation. (B)
Concentration of ovoid on the left side (dark orange) and no inclusions of the
right side (lighter orange). (C) Mutually aligned fusiform inclusions. (D)
Ovoid inclusion with single vesicle. (E) Ovoid inclusions (black arrows); lower
ovoid inclusion is stacked on top of an irregular filamentous inclusion (grey
arrow); pictured with filamentous microorganisms. (F) Vesicular ovoid inclusion
with projection. (G) Irregular filamentous inclusion (grey arrow) surrounded by
mutually aligned filamentous microorganisms and ovoid inclusions. (H) Amorphous
inclusion. (I) Amorphous inclusions surrounded by mutually aligned ovoid
inclusions. Scale bars: 50 μm (A, F); 200 μm (B); 100 μm (C–E, G–I). Quinney et al. (2015).
Orange amber is the most common type found in the Otway deposits,
accounting for 65% of the samples, and including all known types of
pseudoinclusions, as well as both the identifiable spore fossils, most of the
filamentous micro-organisms and a large quantity of degraded plant matter.
Ovoid and fusiform pseudoinclusions were the most abundant type.
Yellow amber is the second most common type present in the Otway
deposits, accounting for 15% of the Amber present. Inclusions are rare in this
type of amber, and restricted to decayed organic matter and spherical
pseudoinclusions.
Dark orange amber comprises 8% of the total Otway Amber. This amber
contained degraded plant matter and all types of pseudoinclusions, with
spherical inclusions being the most common. Interestingly spherical and
fusiform/ovoid inclusions appear to be concentrated in darker areas within this
amber, but almost absent from the lighter areas.
Red amber comprises about 10% of the Otway material. This amber
frequently includes spherical pseudoinclusions and degraded plant material,
with one sample containing a possible filamentous micro-organism.
Brown amber is uncommon, comprising less than 2% of the total Otway
material. This amber is largely opaque, due to the presence of large amounts of
degraded plant matter, which makes it impossible to determine if other types of
inclusion or pseudoinclusion are present. One single piece of brown amber had a
milky colouration; this was too opaque to detect any inclusions.
Previous studies of amber have suggested that colour may be
influenced by a number of factors, including material included in the amber and
post-depositional chemical weathering. A previous study of amber from Charentes
in France suggested that yellow ambers with few inclusions probably formed from
resin that was extruded and set high in the branches of trees, where little
wind-blown debris reached it, an explanation which Quinney et al. feel is also plausible for the Otway Amber. Under this
interpretation orange amber would have formed lower, on the trunks of the
trees, where they were more easily reached by wind-blown dust and debris. Red
amber from Charentes was found to include aquatic micro-organisms, and is
interpreted as having been extruded underwater and consequently set more
slowly, but no such inclusions were found in red amber from Otway, making
interpretation harder. Brown ambers contain abundant degraded organic material,
which is likely to indicate it was either extruded underground or fell into the
soil before setting. Milky brown ambers have previously been interpreted as
being material extruded from roots, though only a single small piece is
available from Otway, showing no other diagnostic features associated with root
amber, so Quinney et al. refrain from
making any diagnosis on this.
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