![]() |
||||||||||||||
Australia: The Land Where Time Began |
||||||||||||||
Chalk quarries near Bardeh and Soukhneh have
produced more than 50 marine vertebrate species. This snapshot of life
in the central gyres of the Tethys display a mixture of modern and
archaic forms. In these deposits there are many sharks and rays, and
other teleosts, essentially modern fish, but there were other top
predators such as marine reptiles that included plesiosaurs and
mosasaurs that became extinct not long after. Mosasaurs were very large
marine reptiles growing to more than 15 m and had rows of pointed teeth.
These are not the same as the mesosaurs from the Parana Basin of Brazil
that were smaller and more elegant that had died out at the Permian
Extinction Event. The author3 suggests that the mosasaurs
appear to have been flourishing at the close of the Cretaceous as most
of the other marine reptiles were in decline. In this deposit there were
also some of the more ancient crocodiles and many modern marine turtles
that had first appeared in the fossil record in the Late Cretaceous,
some of which grew to almost 4 m in length. Syria at this time would have been nearly 2500 km
from the nearest land that the author3 suggests would have
been on the central African continent. The most abundant fossils were
found in the phosphatic sediment that were typical of outer shelf
conditions, where there was extremely high levels of productivity but
where oxygen levels were generally low. The indications are that the
continental shelves extending into the ocean basin were very wide. These were what the author3 describes as
‘golden days in a truly greenhouse world’, both average temperatures and
mean sea levels were extremely high around the world. The surface waters
of the ocean were ideal for plankton to flourish, making them probably
the most productive that have ever existed in the history of the world. End of an
Era At the close of the Cretaceous the dinosaurs and
the related reptiles of the sea and the air, had been the dominant life
forms for about 180 My. At about 65 Ma they had all gone, as well as
most marsupials and birds, and about ¼ of all species of crocodiles,
turtles and fish, about 20 % of known families and an estimated 50 % of
all species became extinct. It is known that the extinction event at the
KT boundary occurred over a short period, though not instantaneously. There have been hundreds of theories proposed to
explain the extermination of the dinosaurs, most being considered
fanciful by the author3. Though the extinction of dinosaurs
at this time is what is nearly always talked about, a much more
significant extinction at the KT boundary was that of the many
coccoliths and related microscopic organisms of the plankton from the
open ocean, including the Tethys and all its extensions. Prior to the
extinction event of the KT boundary they had been proliferating wildly,
as is indicated by the many chalk cliffs, uplands and mountain crags
from this time and place. Their significance lies more in their place at
the bottom of the food chains and food webs of the ocean, not just their
sheer numbers. As mentioned previously the continents covered no
more than 18 % of the Earth’s surface at the close of the Cretaceous 65
Ma, not much more than half the area covered by the continents of the
present, and that was mostly low-lying and peneplained (flattened by
long periods of erosion). The main mountain ranges were along the west
coast of North and South America, much as the Rockies and Andes of the
present, and along the eastern edge of South East Asia and northwest
Australia. The Pacific (the Panthalassic Ocean) was being subducted
beneath the outer margin of the continents. Most palaeogeographic
reconstructions replace the name Panthalassic Ocean with Pacific Ocean
after either the Triassic or the Jurassic. At this time there were no
landmasses at either the North Pole of the South Pole. The author3
suggests it was equally significant that what small continental areas
that existed had steadily drifted apart throughout the Cretaceous so
were well separated geographically. North America was in the form of 2
or 3 segments that were separate from South America. Africa was in 2
parts and India was on its own in the middle of the Tethys. Most of
Europe and South and Central Asia were completely submerged. Siberia was
joined to South East Asia and Australia was still joined to Antarctica.
Australia was unique among the landmasses (as usual for this unusual,
ancient continent) in not being submerged when everywhere else large
parts of the continents were submerged at this time of extremely high
sea levels. According to the author3 a tectonic process that
is still not understood was uplifting the entire land area of Australia. The result of this biogeographic separation was
that isolation and biozonation occurred rapidly. In the oceans this was
also true of coastal and shallow marine groups where the separation was
not too wide for some exchange to take place been the margins of
adjacent landmasses. The dinosaurs and pterosaurs that were already
showing signs of environmental stress would also have been affected by
this separation. The groups from Africa, Asia and South America during
the Early Cretaceous didn’t survive until the Late Cretaceous. The final
survivors were confined to North America, but they still had high levels
of diversity 75 Ma, but as the KT boundary approached they were much
reduced. The author suggests that the end of the dinosaurs came as a
whimper rather than a bang as expected by the catastrophist theories. It has been estimated that ¾ of birds and
marsupials and about ¼ of the crocodiles went extinct at the KT
boundary. The author3 suggests a confusing fact about the
mass extinction was that lizards, snakes and amphibians, as well as
a great majority of placentals remained more or less unscathed.
He says that a fundamental change that had taken place, beginning along
the continental edges and along river banks, but eventually spread
across the continents was the evolution of flowering plants,
angiosperms), that replaced the gymnosperms in many places. This change
took place in the Middle Cretaceous along the fringes of continents that
were vegetated by ferns and conifers, as the biota of the seas was being
devastated by the Black Death. At first there were no recognisable
flowers, though it has been suggested that there were eventually white
flowers. The evolution of coloured flowers has been suggested to have
come later as the angiosperms began to co-evolve with the pollinators
that were mostly insects. Once the angiosperms had become established on
the land that was above the sea level their radiation and
diversification increased very rapidly. Continental sands, silts and variegated muds full
of the remains of early flowering plants, in the form of black
carbonaceous material, were deposited along the margin of the North
American continent on the shores of the Tethys Ocean. There are many embryonic angiosperm fossils in a
well-hidden series outcropping beneath the Maryland coastal plain near
Washington and Baltimore. The diversification of new pollen and types of
leaf has been preserved in immaculate detail in deposits covering a span
of 10 million years. Angiosperms surpassed the gymnosperms in variety
and adaptability between that time and the KT event. The KT mass
extinction event was survived by up to 50 complete families, including
sycamores and plane trees, hollies, oaks, walnuts, birches and alders.
By the Late Cretaceous forests were beginning to look familiar. The
author3 suggests it was almost like a mass extinction in
reverse, this radiation seeming on a geological time scale to be almost
instantaneous, it was considered by Charles Darwin to be ’an abominable
mystery’. Open, unforested areas would have looked unfamiliar as grasses
had yet to evolve and these open spaces were vegetated with primitive
forms of heathers, mallows, myrtles, spurges and nettles. The seeds of angiosperms are fertilised within an
encased ovary, and a ready supply of nutrients is provided by the
surrounding fleshy fruit. As a result a much more rapid reproductive
cycle was possible than occurs in gymnosperms, providing the germinating
seed with a secure start and therefore the opportunistic colonisation of
a changing landscape. Insects, birds and other animals were enlisted by
the development of flowers and fruit to spread the pollen more widely
and target it to flowers of the same species further afield and the
dispersal of the resulting seeds. By providing nectar and fruit to
entice the pollinators and distributors, respectively, of their seeds
they gained an advantage over their competitors that depended on the
random spread of their pollen by the wind, a much less efficient
mechanism. Also developed at this time along the shores of the Tethys
were edible nuts and seeds. These new traits of angiosperms were
immediately successful, suggesting that at this time the environment was
changing and becoming stressful for the established plants and animals
of the time, and the angiosperms took advantage of this period of change
where their new traits could enable them to outcompete the gymnosperms. Stress
among the seagrass During the Cretaceous the seagrass, an angiosperm,
colonised the marine environment, demonstrating just how adaptable this
group of plants were. As well as adapting to living in seawater, they
also developed a mechanisms for subaqueous pollination and seed
dispersal. Sea grasses of the present are eelgrass that is distributed
widely throughout the North Atlantic and Pacific, and as turtle grass
and manatee grass in the Caribbean. They are the main food of green
turtles, dugongs and manatees, and provide a habitat for many fish,
crustaceans and benthic organisms.
The dugongs and manatees, of the order Sirenia, are
herbivorous marine mammals, the oldest known fossils date to 50 Ma,
since which they have followed a similar path of evolution as the
cetaceans, that includes whales and dolphins. Sea turtles had evolved by the Cretaceous,
including the giant
Archelon that the author3
suggests was probably a forerunner of the green turtles of the present.
Living among the seagrasses and the roots of mangroves along the shore,
as well as in crevices and cavities in nearby reefs, were new types of
bivalve and gastropod. In rocks of North Africa, of Cretaceous age,
pollen has been found of the mangrove palm
Nypa that grew along the
southern shores of the Tethys. There were small brightly coloured sea
snails grazing on coralline algae or seagrass fronds, others preyed on
animals much larger than themselves by digesting them externally.
Hesperornis, a flightless
diving bird that evolved in the Late Cretaceous, is known from a near
complete fossil skeleton. It had adapted to living in the sea by the
reduction of its wings to a size which would have made flying impossible
and developing large paddle-like feet, and it sharp, backward-pointing
teeth appear to have been adapted for catching fish. For a short time in the Late Cretaceous
rudist-bryozoan reefs completely usurped reefs of corals and algae, a
combination that had by the Late Cretaceous flourished for an estimated
95 % of the Mesozoic. The cone-shaped lower valves of the rudists were
greatly thickened. Large numbers of rudists cemented themselves to each
other, as well as to other giant bivalves of the oyster family that had
also become common in the Late Cretaceous. The author3 suggests the success of the
rudists was due to an effective partnership between them and a symbiotic
organism, either a symbiotic alga or other micro-organism the lived in
the tissues of the rudist where it multiplied. He suggests that it is
unlikely fossil evidence of such symbiosis will be found among fossils,
it has been assumed that as this type of symbiotic relationship exists
between a variety of corals, giant clams, as well as other bivalves that
together form coral reefs of the present it probably existed in the
Cretaceous. They are the main reef building organism, and also the main
primary producers at the base of food webs that are richly complex. This
role is made possible by the close symbiotic relationship between
individual coral polyps and zooxanthellae that line the inner wall of
the polyp tissue, a single-celled species of dinoflagellate that is
photosynthetic. As each dinoflagellate cell is 0.01 mm wide it allows as
many as 1 million cells in a square centimetre of coral tissue. By
living in the tissue of the coral polyps they are protected by their
hosts rows of stinging cells, as well as being in an environment that
suits them, and the coral can survive on the food it receives from the
dinoflagellates. Each coral polyp extracts calcium carbonate from
seawater and constructs a lime cement cell that surrounds it. Coral bleaching is a significant problem for coral
reefs of the present. When their environmental conditions change, as
occurs when the water temperature rises as a result of global warming,
the coral polyps expel their symbionts, following which both die,
resulting in white, dead reef, coral bleaching. The author3 suggests the same probably
happened to the coral reefs of the Late Cretaceous when they were
replaced by the rudists, and eventually happened to the rudists as the
environment changed again. Coral covers about 0.2 % of the seafloor
though the reefs support about 25 % of all known marine species, and the
same probably applied to the rudist and coral reefs of the Tethys, the
entire complex community being affected significantly by a knock-on
effect when something changes that affects the basis of those
communities. It is not easy to determine how the seagrass meadows of the
Cretaceous were affected as the seagrass does not easily fossilise. What is known is that between 10 and 20 My prior to
the KT boundary most species of coral died out. With most of the coral
gone the rudists proliferated to take their place, though only a few
species survived until the end of the Cretaceous as they also went into
a marked decline over the last 2 My before the KT boundary. The
gastropods that had flourished in the Tethys in the seagrass meadows and
the reefs were badly affected. All the species from the region of North
Africa went extinct, after which cooler-water species migrated from
Greenland to replace them. There was a very rapid decline in the final
years of the Cretaceous of most brachiopods that had adapted to the warm
chalky water of the Tethys Ocean. Alarm in
the Open Ocean There is not enough fossil material of the marine
reptiles of the Tethys Ocean to determine exactly how they were
affected, though it appears that the Jurassic ichthyosaurs had gone well
before the end of the Cretaceous. The mosasaurs took their place for a
short time. Ammonites and belemnites (ammonoids) were very common during
the high point of the flooding of the Late Cretaceous. Near Zumaya in
northern Spain there is a sequence of deep-sea sediments that is very
well exposed containing ammonite fossils. A cyclic alternation of
turbidites and pelagites spanning the KT boundary is recorded in the
sediments, fossils are best found in the pelagites. According to the
author3 there is no doubt the ammonites were in gradual
decline over a period of about 4-5 My, the last known fossil being found
about 12 m below the uppermost Cretaceous strata. It can be estimated
from the average rate of accumulation of these sediments that the last
of the ammonites died about 100,000 years before the KT boundary. The
author3 suggests the same story applies elsewhere around the
world. There were about 40 species of ammonoids known 4 My before the KT
Boundary, but in the last 2 My before the end of the Cretaceous there
are 20 known species. Among the ammonites changes were taking place in
their forms, some developed partially or completely uncoiled shells,
some grew to giant sizes, though most got much smaller. The author3
suggests these were responses to stressful conditions. The changes
taking place near the bottom of the food chain were the more important
ones. The author says that the succession at Zumaya indicates that the
foraminifera were in decline before the KT boundary, though many species
disappeared abruptly at the end. The Deep Sea Drilling Program and its successors
have drilled more than 100 boreholes through the KT boundary beneath the
floor of the ocean. What they show is that there was a stepped decline
in some species of microflora leading up to the KT boundary, and in
part, transitional replacements of Cretaceous microflora with microflora
from the Tertiary, but no clear single extinction. The results from the seafloor drilling have found
the same lack of abrupt killing of planktonic species at the KT boundary
from sites around the world. The most abrupt changeover of species
occurred over a period of about 100,000 years, warm water foraminifera
declined and cool water species migrated in to replace them. Being all
warm water species, the coccoliths mainly died out over the same
interval of time. The base of the food chain had been decimated with the
inevitable result being a catastrophic loss all the way up the food
chain, thermal stress again being an important factor.
Stow, Dorrik, 2010, Vanished Ocean; How Tethys Reshaped the World, Oxford University Press.
|
|
|||||||||||||
|
||||||||||||||
Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |