Australia: The Land Where Time Began
Tethys Ocean Life - Old and New
Tethys Ocean Life - Old and New greenhouse Conditions
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: email@example.com Sources & Further reading|