Evolution of Australian biota

Transcription

1 Gill Sans Bold Biology Preliminary Course Stage 6 Evolution of Australian biota Part 3: Changing environments and biota in Australia IncorporatingOctober2002 AMENDMENTS

2 Contents Introduction... 2 Geological time scale and biota... 4 Evidence of changing environments...5 The last 100 million years... 9 Environmental change...9 Adaptive radiation of marsupials Origin of marsupials...15 Origin and radiation of monotremes The megafauna Additional resources Suggested answers Exercises Part Part 3: Changing environments and biota in Australia 1

3 Introduction In Part 2 of this module you became familiar with the theory of plate tectonics and saw how it is related to the biota found on the separate continents which made up Gondwanaland (Gondwana), especially the Australian continent. In this part of the module the nature of changing environments during the northerly drift of Australia and the resultant effects on the evolution of ancient and modern flora and fauna are discussed. Most emphasis is given to the marsupial and monotreme mammals, as they are typically Australian biota. Although a large number of marsupial species also occurs in South America, the species are mainly very different from the Australian marsupials. Monotreme fossils have also been found in South America, but living monotremes are restricted in their distribution to Australia and New Guinea and most fossil monotremes have been found here. In this part of the module you will need to read and work carefully through the material provided, to practice important skills especially gathering, processing and analysing information from secondary sources. This means that you need to be able to research reference material, understand it and present conclusions based on your research. If you have access to a library, CD ROM or the Internet, you can read further than the material provided here. However, there is sufficient research information in the material provided for you to carry out the exercises and assignments and to achieve the competencies required. In Part 3 you will be given opportunities to learn to: discuss current research into the evolutionary relationships between extinct species, including megafauna and extant Australian species identify and describe evidence of changing environments in Australia over millions of years identify changes in the distribution of Australian species, as rainforests contracted and sclerophyll communities and grasslands spread, as indicated by fossil evidence 2 Evolution of Australian biota

4 discuss current theories that provide a model for these changes. In Part 3 you will be given opportunities to: gather information from secondary sources to describe some Australian fossils, where these fossils were found and use available evidence to explain how they contribute to the development of understanding about the evolution of species in Australia. perform a first-hand investigation, gather information of named Australian fossil samples and use available evidence to identify similarities and differences between current and extinct Australian life forms. Extract from Biology Stage 6 Syllabus Board of Studies NSW, originally issued Revised November The most up-to-date version can be found on the Board's website at Part 3: Changing environments and biota in Australia 3

5 Geological time scale and biota As you saw in Part 2, Australia was once part of the huge landmass of Pangaea and then of the southern supercontinent, Gondwanaland (Gondwana). Even before Gondwana split from Pangaea most of the major groups of plants and animals known today existed on the Earth. Although the fossil record shows that mammals, birds and probably the flowering plants had not yet risen to any prominence by the time Gondwana separated from the rest of the Pangaean landmass. The table in the Additional resources shows the categories of geological time for the Earth but these are not drawn to scale. All periods are shown as fairly similar sized boxes, but the time span of each period is given, along with the major plant and animal groups from each. The table also indicates when it is thought each group evolved, reached prominence in the fossil or living record and when some became extinct. You should not be concerned if other sources do not match exactly the one given in the table which has actually been constructed from several sources of information. The generally accepted progression of evolution, as deduced by various types of information (especially fossil evidence), will be the same or very similar in most of these sources. However, you may find small discrepancies. These may be due to differences in opinion or interpretation by authors or even to how recently the source has been written or updated. In Part 2 you saw that Australia s association, firstly with the other land areas making up Pangaea, and then those bound up into Gondwana, resulted in the biota of the modern Australian continent showing some similarities to those of other areas of the world. However, after its rifting from Antarctica and its movement north with the drifting of the Indo-Australian Plate, Australia experienced considerable changes in both climate, geography and geology so that species of organisms which were present on the continent were isolated 4 Evolution of Australian biota

6 and exposed to different natural selection pressures, so that they changed and evolved into the groups of organisms present today. The mechanisms of this speciation and adaptive radiation were discussed in detail in Part 1. If you need to, return briefly to your notes and assignments from Part 1 just to make yourself familiar again with these mechanisms and how they are thought to work. Evidence of changing environments No one was around to witness the changes occurring in world environments, but a good deal of information on palaeogeography (pre-human geography) and palaeoclimatology (pre-human climate) can be obtained by a variety of methods including: fossils tree rings dating methods and glaciers. Fossils The remains of organisms can be preserved in a number of ways, including: the preservation of bodies or body parts in amber (gum), peat or tar where bacteria are prevented from breaking down the organic material replacement of body tissues by minerals (eg. opal) impressions in mud which subsequently turns to rock the preservation of whole organisms in ice (eg. woolly mammoth). These fossils often give clues about the environment or climate. For example, finding the fossil remains of marsupials which had skeletons adapted for climbing and teeth adapted for feeding on leaves and fruits suggest that the area in which they lived at the time was probably a forest, not a shrubland or grassland. Part 3: Changing environments and biota in Australia 5

7 A fossilised trilobite indicates a past marine environment. (Photo: LMP Tim Reid). If fossilised leaves of rainforest species were also found it might indicate that the region was part of a rainforest rather than a sclerophyll forest. Similarly, the finding of fossils of fish, turtles and frogs would indicate an aquatic rather than a terrestrial environment. Pollen and spores from plants can be quite distinctive and are frequently used to deduce the nature of the past vegetation in an area. Tree rings Tree rings represent seasonal growth cycles. This means that the ages of trees can often be calculated. The rates of growth each season can also be estimated from tree rings. Various environmental factors, including rainfall, aspect (sunshine) and altitude can affect tree growth. In this way the study of tree rings can indicate climate and geography of the place where a tree grows, or where its fossil remains are found. 6 Evolution of Australian biota

8 Petrified wood from the Permian showing large annual growth rings. This indicates that there were warm conditions at the time that this tree grew. (Fossil Andrew West). (Photo: LMP Thomas Brown) Dating methods The age of rocks which contain radioactive minerals can be determined by calculating backwards from the amount of the radioactive element remaining in the rock and the amount of the element into which it decays (changes). For example uranium-235 decays into lead-207. The rate at which radioactive decay occurs is fixed and is known as the half-life of the element. That is, the time taken for half of the original element present (parent element) to decay into the other element (daughter element). In the case of uranium-235 changing into lead-207, the half-life is 713 million years. This method is called radiometric dating. If fossils, or the rocks in which they are found, can be given a date of formation, then other fossils and rocks can be dated relative to them. If they are found higher in layers (strata) of the Earth, then they must have been formed later and so are younger, while lower ones are older. Once a fossil type has been dated, then it can be used to establish the dates of other fossils found with it. This is called relative dating. There can be problems with the method of relative dating, as it is known that the positions of rock strata can move in various ways, so that the younger fossils or rocks may be found under rather than above older Part 3: Changing environments and biota in Australia 7

9 ones. However, other geological evidence can often identify this disruption to the layering. Glaciers As the ice sheets of glaciers move over the surface of the Earth they erode rocks and deposit material in a characteristic way. During ice ages glaciers advance and then retreat during warmer periods. Studying the occurrence of features of glacial erosion can enable scientists to map glaciers and determine a good deal about the climate of the time. Ice cores Cores of ice taken from glaciers and from polar ice caps can also be used to determine climate. The higher levels of carbon dioxide in air bubbles trapped in ice can indicate greenhouse global warming. Ash trapped in ice can show the presence of volcanic activity at the time the ice was formed. In an ice core the most recently formed ice is at the top and the older at the bottom. Complete Exercise 3.1 now. 8 Evolution of Australian biota

10 The last 100 million years Environmental change In this section you will become more familiar with the sequence of changes in geography, climate and vegetation of Australia prior to its separation from Gondwana and during its drift northward towards south- east Asia. It was these environmental changes which almost certainly gave rise to the evolution of the diverse flora and fauna of this continent. The information that follows outlines the changes in climate and vegetation during different periods in Australia. You will be using this information to complete a table to summarise these changes. Cretaceous period The beginning of this period in Australia was cold. The sea level was high and much of the land mass consisted of islands in a shallow inland sea. The vegetation was dominated by non-flowering vascular seed plants (conifers, cycads, horse tails and seed ferns) but the first flowering plant fossils also appeared during this period. The climate warmed later in the Cretaceous, the land of Australia was uplifted and the angiosperms began to dominate the vegetation. By the end of the period mass extinctions, which included the Australian dinosaurs, occurred. This was possibly due to a cooling effect brought about by the collision of an asteroid with the Earth. This is thought to have caused dust in the atmosphere and a resulting reduction in the heating of the Earth s surface by heat from the Sun. Part 3: Changing environments and biota in Australia 9

11 Paleocene-Eocene periods Australia had broken away from Antarctica by about the end of the Eocene period and began drifting north. Australia s climate went from being wet and warm to being cool. Large inland lakes were part of the landscape and rainforests covered much of the continent. Ferns and conifers were still found, but flowering plants were certainly the dominant land plants, including species belonging to the genus Nothofagus or southern beech trees (see Part 2). Oligocene period This period was also cold and wet in Australia. Rainforests, including Nothofagus trees, dominated and swamps were common. The first Eucalyptus trees appeared during this period. Miocene period In the early Miocene period sea levels varied and the climate fluctuated between being hot and cold. But, it was always wet and rainforests were still dominant. However, by the late Miocene sea levels fell and the climate was cold and was becoming drier. Rainforests were beginning to give way to forests of trees with hard dry leaves (sclerophyll), especially Eucalyptus species, which could tolerate the drier conditions. Casuarina or she oak species of trees and shrubs were also common angiosperms of this later period. Pliocene period This was a time of raised sea levels with rainforests present in the wet conditions at the beginning of the period, but giving rise to sclerophyll forest, woodland and even grasslands later, especially in inland Australia. Pleistocene period Both temperatures and sea levels fluctuated during the Pleistocene, as Australia experienced alternating icehouse and greenhouse conditions. However, in general the climate became more arid and plants and animals able to cope with such conditions evolved. The giant forms, often called the megafauna evolved early in this period but became extinct by the end of it. 10 Evolution of Australian biota

12 The table below uses a format similar to the table in the Additional resources showing the Earth s geological time periods. Some of the information in the table has been filled in but you will need to carefully read the descriptions given for each period. Complete the worksheet. Period Time span (million years) Geography Climate Vegetation Early Cretaceous Late Cretaceous islands in shallow inland seas cold, moist cycads and conifers, horsetails and seed ferns Paleocene- Eocene rifting from Antarctica began; large inland lakes; northern drift of Australia rainforests containing ferns and conifers but mainly angiosperms, especially Nothofagus (southern beech trees) Oligocene first Eucalyptus trees Early Miocene Late Miocene Pliocene Pleistocene Recent (holocene) arid inland, north dominated by summer rainfall and south by winter rainfall.; temperatures increase from south to north Check your answers. Part 3: Changing environments and biota in Australia 11

13 The environmental conditions of Australia varied considerably as the continent moved north, but the most notable trend in the last 15 million years was one of increasing arid conditions. The overall change in the climate and vegetation from being cool, wet and forested to being warm and with dry mainly woodland, shrubland and grassland ecosystems. cool and wet Climate warm and dry today Time (millions of years from present) Climate change in Australia over the past 15 million years. The dark shading shows the increase in warmer and drier conditions. You will draw on this information in the next part when you will examine vegetation patterns and the impact on Australian biota over time. Complete Exercise Evolution of Australian biota

14 Adaptive radiation of marsupials The changes in environments and vegetation in parts of Australia led to the evolution of very different animals under the influence of different selection pressures. The first marsupial mammals are thought to have come from South America. The likely ancestor of Australian marsupials was probably a small species like the small South American marsupial called Dromiciops australis, which is found in the beech forests of southern Chile. This animal has many features like the small possums of Australia. Dromiciops australis a South American marsupial which is thought to belong to the group (Microbiotherids) from which Australian marsupials evolved. (Photo: LMP T R Grant) This species is quite similar in appearance to the Australian eastern pygmy possum (Cercartetus nanus). All other American marsupials have sperm which are attached together in pairs, while Dromiciops australis and the Australian species have single sperm. There is also considerable similarity in the DNA between this little creature and Part 3: Changing environments and biota in Australia 13

15 Australian marsupials. Dromiciops itself is not the actual ancestor but may belong to the same ancestral group as the original Australian marsupials. Once such a marsupial ancestor (or ancestors) reached Australia via Antarctica, the group radiated by adaptation to the different environments of the continent to produce the range of unique marsupials found in Australia today. Despite what some books say, there are still lots of species of marsupials in South and Central America today (around 75 species). American marsupials are called opossums, which are completely different from the Australian group of marsupials which are called possums. One species is found in North America, but it almost certainly moved there quite recently in geological terms. There are no kangaroos, wombats, koalas or bandicoots on the American continent, and most American marsupials are fairly small creatures, the largest of which is about the size of the Australian brushtail possum. During the adaptive radiation of marsupials in Australia a number of species became extinct as conditions changed. There were presumably no variations within their populations which allowed these marsupials to survive the changes. The major groups of modern Australian marsupials are summarised in the table below. Adaptation to different habitats has occurred during their evolution and the varied diets of some are shown in the table. Order Common names Example Diet Dasyuromorphia dasyurids marsupial mice Tasmanian devil native cats Peramelemorphia bandicoots bandicoots bilby carnivores insectivores Diprotodontia koala wombats macropods (kangaroos, wallabies, potoroos, bettongs, rat-kangaroos) possums pygmy possums gliders koala common wombat grey kangaroo swamp wallaby brush-tailed bettong tree kangaroo ringtail possum, brushtail possum eastern pygmy possum sugar glider leaf eater grazer grazer browser fungus, seed, tuber and insect eater leaf and fruit eater leaves, flowers, fruit eaters nectar, pollen eater nectar, pollen, insect, sap eater 14 Evolution of Australian biota

16 Optional You could try to use either the Internet, CD ROMs or go to your local library and have a look at pictures of the members of these groups of marsupials. Find out what they look like, what they eat and where they are found. Origin of marsupials As was mentioned earlier, the currently most accepted theory of the origin of the Australian marsupials is that their ancestors came from South America, probably around mya in the late Cretaceous or early Paleocene period. If this was a reasonable theory, then the following might be reasonable expectations (hypotheses) from the theory. There should be older marsupial fossils in south America than in Australia. Marsupial fossils should be found in Antarctica At least some South American marsupials should show affinities (similar characteristics) with Australian species. Most modern South American marsupials appear to be more closely related to the earliest marsupial fossils which were found in the Cretaceous period in North America (around 100 mya) than to modern Australian marsupials. The ancestors of the modern South American forms quickly disappeared from the north of their range, but their descendants persisted in South and Central America during the Cretaceous period. It should be noted that the one species of marsupial now found in North America, the Virginian opossum, has more recently invaded from Central America. A marsupial fossil has been found on an island off the Antarctic Peninsula, adjacent to the tip of South America. The living species Dromiciops australis has DNA similarities to Australian marsupials. Dromiciops australis has unpaired sperm, while the sperm of all other American forms swim together in pairs. These pieces of evidence suggest that the theory has at least some support. An alternative theory that the marsupial ancestors entered Australia from the north has little support, as no marsupial fossils have yet been found in Asia. It was not until around 15 mya that Australia was close enough to south-east Asia for mammals to have reached the mainland from the north. Part 3: Changing environments and biota in Australia 15

17 Origin and radiation of monotremes Monotremes are mammals which lay eggs but, like all other mammals, feed their young on milk. There are currently only three living species of the Monotremata group, two species of echidnas (or spiny anteaters) and one species of platypus. The short-beaked echidna occurs both in Australia and Papua New Guinea; the long-beaked echidna occurs only in Papua New Guinea; the platypus occurs only in Australia. There was once a greater abundance and distribution of monotreme species in Australia than currently occurs. Fossils of this group show that it has been here for longer than the marsupials. The oldest monotreme fossils are from the Lightning Ridge area in western New South Wales. These appeared to have been from platypus-like species which had teeth possibly adapted for eating crustaceans (eg. shrimps, crayfish). It would seem then that they were probably aquatic, so that during the Cretaceous period in which they occurred (around mya) the environment of the Lightning Ridge area would have been much wetter than it is today. Until recently monotreme fossils had only been found in Australia, but in 1991 and 1992 several fossil monotreme teeth (which are quite distinctive) were found in Patagonia in the south of Argentina. These were about 62 million years old. Can you remember in which geological period that date falls? No doubt you would have decided the Paleocene period. Fossils of three ancient species of platypuses have been found that are from million years old. A number of fossil echidnas have also been found on mainland Australia, indicating greater species abundance and distribution of the group. The early age of monotreme fossils, the greater abundance of fossil forms, and the occurrence of living forms in Australia has led some palaeontologists to hypothesise that the group may have evolved in Australia, before it broke away from Antarctica, and that the South American forms descended from an Australian ancestor. At present there are insufficient data to support or reject this hypothesis. 16 Evolution of Australian biota

18 Answer the following questions, then check your answers at the end of this part. 1 The most accepted theory is that Australian marsupials evolved from ancestors which originally dispersed here from south America. Discuss the evidence which supports this theory. 2 On the other hand it has been suggested that the fossil monotreme species found in South America may have evolved from an Australian ancestor. Briefly describe evidence for such a suggestion and comment on its acceptance by palaeontologists. Check your answers. Complete Exercise 3.3 where you will be comparing a fossil form of a platypus to a current form. Part 3: Changing environments and biota in Australia 17

19 The megafauna Gigantism occurred in mammal groups in many parts of the world during the Pleistocene period and those in Australia were no exception. The term megafauna refers primarily to large marsupials, such as giant kangaroos (eg. Procoptodon goliah, Macropus giganteus titan) and huge four legged herbivores (eg. Diprotodon optatum, Zygomaturus trilobus). There were also very large echidnas (eg. Zaglossus hacketti) and a few large carnivores, like the marsupial lion (Thylacoleo carnifex). As well, there were also known to be at least four giant reptiles, including a goanna and constrictor snake, several carnivorous birds and a horned land tortoise. During this period such large forms co-existed with smaller species of the same groups and all the giant forms had died out before the end of the period (about years ago.) Several suggestions have been made to explain the selection pressure acting on ancestral populations to favour large size. The most common suggestions are outlined below. The climate during the Pleistocene period was often very cold and so the small surface area to volume ratio of large animals which regulated their body temperature would reduce body heat loss and increase survival. In Australia, soils were often poor in nutrients and so plant growth may have been low and food of poor quality (more indigestible). Large digestive systems, especially those with fermentation chambers full of micro organisms, could be carried by large animals, allowing them to make use of the poor quality of vegetation available. Along with this, the large animals would also lose less body heat and therefore, require less food anyway. However, there were also large reptiles (which probably did not regulate their body temperature) at the time. The climate during the Pleistocene was not always cold and similar, small species also successfully survived and reproduced during the period. These things all highlight the 18 Evolution of Australian biota

20 difficulties of research into the environments, flora and fauna of Australia in the past. Australian megafauna. Drawings of hypothetical reconstructions of some Australian megafauna species. A species of giant kangaroo is shown along with a species of giant herbivorous diprotodont. The human is included for a size reference. There is also controversy over the reason for the demise of the megafauna. Some scientists maintain that they were hunted and killed by Aboriginal peoples, who were in Australia by between years ago, or that Aboriginal use of fire eventually changed the vegetation and brought about their extinction. Another theory is that humans killed off the megafauna and this resulted in a change in the vegetation. As large herbivores were no longer eating the vegetation, very intense bushfires occurred due to the accumulation of fuel. The Aboriginal peoples were thought to have subsequently adjusted the situation by regular burning, maintaining grasslands and woodlands, where forests had occurred previously. Again, because history was not recorded at the time, the theories are based on indirect evidence, some of which is highly speculative. Many botanists believe that the vegetation changed as a result of climate change and it seems that no one really knows why the megafauna died out in Australia. As part of your syllabus requirements you are asked to gather information from secondary sources to describe some Australian fossils, where these fossils were found and use available evidence to explain how they contribute to the development of understanding about the evolution of species in Australia. Part 3: Changing environments and biota in Australia 19

21 If you visit the LMP Science online website there are some good starting points for your investigation. Below are some famous Australian fossil locations that you could use as a basis for your research. Riversleigh Bluff Downs Murgon Lightning Ridge Naracoote Canowindra Choose two of these sites, describe two fossils found at the site and give the age of the fossils. If possible say how each fossil contributes to the development of understanding. Some examples are given in the Additional resources section. Write your answers to the above in Exercise Evolution of Australian biota

22 Additional resources Geological time Era Period Epoch Time spans (million years) Plant life Animal life Cenozoic Quaternary Recent (Holocene) Pleistocene today 2 mill first Homo sapiens, megafauna Tertiary Pliocene Miocene Oligocene Eocene Paleocene Australia drifts north (35-45 mya) dominance of flowering plants (Angiosperms) land dominated by mammals and bird Cretaceous Gondwana begins to rift (approx. 150 mya) major extinctions flowering plants expand nonflowering seed plants (eg. conifers, cycads, seed ferns) decline major extinctions last dinosaurs adaptive radiation of insects Mesozoic Jurassic Pangaea rift (approx. 170 mya) angiosperms evolve nonflowering seed plants dominant age of dinosaurs, first birds Part 3: Changing environments and biota in Australia 21

23 Triassic non-flowering seed plants dominant first mammals first dinosaurs Paleozoic Permian major extinctions, especially primitive plants Carboniferous primitive land plants then non-flowering seed plants and ferns dominant expansion of reptiles major extinctions, especially amphibians and trilobites age of amphibians first reptiles first insect radiation Paleozoic Devonian dominance of primitive vascular plants first seed plants age of fishes first amphibians first insects Silurian first vascular land plants Ordovician marine algae abundant Cambrian marine algae cyanobacteria adaptive radiation of fishes first land arthropods first fish most invertebrate phyla known today Neoproterozoic 1, cyanobacteria early cnidaria, annelids, arthropods and echinoderms Proterozoi c Mesoproterozoi c cyanobacteria first animals Paleoproterozo ic atmosphere rich in oxygen cyanobacteria Archaean Evolution of Australian biota

24 Australian fossils Riversleigh Thylacine The Thylacine or Tasmanian tiger became extinct in the 1930 s. The fossils at Riversleigh show that the animal was not restricted to Tasmania but used to live on the mainland of Australia. The fossils also show that there was more than one type of thylacine. This information came too late to save this organism from extinction but it does show that the remnant species in Tasmania should have been preserved. Thingodonta This fossil marsupial is from the Miocene period. It is unlike any living marsupial. It lived in the rainforest and may have eaten caterpillars or eggs. Canowindra Fish fossils A chance discovery during road works exposed the rich Canowindra fish fossils. The first find contained the complete fossils of 100 fish from the late Devonian. There were four types of fish present. Three of these were armoured fish and the fourth was an air-breathing lungfish. Naracoote cave Marsupial lion The marsupial lion was part of the Australian megafauna. It was the largest marsupial carnivore to live in Australia. It would have hunted other members of the megafauna such as Diprotodon during the Pleistocene. Giant short faced kangaroo This animal lived during the Pleistocene. It was up to 3 metres in height. It was double the size of the largest kangaroos living today. It had front paws that were like hooks. Its back legs had one toe. Part 3: Changing environments and biota in Australia 23

25 The Naracoote fossil site included the period when humans first arrived in Australia. The climate was changing becoming increasing cooler and drier. There were however, periods of warmer wetter climate corresponding to the glacial and interglacial periods. If you are interested in reading more about the changing environments and palaeontology of Australia the following books are a good place to start. Be warned that some are extremely detailed making the text difficult to read, but all have great pictures and are interesting to skim through to see pictures of fossils and reconstructions of fossil species. Long, Vickers-Rich and Rich and White also present information on plate tectonics with good coloured illustrations. Archer, M, Hand, S and Godthelp, H Uncovering Australia s Dreamtime. Surrey Beatty and Son, Sydney. Archer, M, Hand, S and Godthelp, H Riversleigh. The Story of Animals in Ancient Rainforests of Inland Australia. Reed, Sydney. Augee, M and Fox, M The Biology of Australia and New Zealand. Pearson Education, Sydney. Gould, E and McKay, G Encyclopedia of Mammals 2nd Edition. Weldon Owen, Sydney. Long, J A Dinosaurs of Australia and New Zealand and Other Animals of the Mesozoic Era. NSW University Press, Sydney. MacDonald, D The Encyclopaedia of Mammals. Volume 2. George Allen and Unwin, London. Strahan, R The Mammals of Australia. Reed Books, Sydney. Vickers-Rich, P. and Rich, H T Wildlife of Gondwana. Reed, Sydney. White, M The Greening of Gondwana. Reed, Sydney. White, M After the Greening. The Browning of Australia. Kangaroo Press, 24 Evolution of Australian biota

26 Suggested answers Environmental change Period Time span (million years) Geography Climate Vegetation Early Cretaceous Late Cretaceous islands in shallow inland seas cold, moist cycads and conifers, horsetails and seed ferns; first flowering plants Paleocene-Eocene rifting from Antarctica began; large inland lakes; northern drift of Australia cool rainforests containing ferns and conifers but mainly angiosperms, especially Nothofagus (southern beech trees) Oligocene swamp cold, wet first Eucalyptus trees rainforests with Nothofagus Early Miocene sea levels varied hot and cold fluctuations rainforests dominant Late Miocene sea levels fell cold, dry reduction of rainforests. Sclerophyll forests increasing casuarina species Pliocene 5-2 raised sea levels wet rainforests then sclerophyll forests woodlands and grasslands Part 3: Changing environments and biota in Australia 25

27 Pleistocene 2 million temperature and sea levels fluctuated alternating icehouse and greenhouse conditions, more arid Recent (holocene) today arid inland, north dominated by summer rainfall and south by winter rainfall. Woodland, grassland and shrubland temperatures increase from south to north Origin and radiation of the monotremes 1 South American marsupial fossils are older than those in Australia. There has been a marsupial fossil found in Antarctica and the living species Dromiciops australis has DNA and sperm similarities to Australian marsupials. These pieces of evidence all give support to the theory. 2 A greater diversity and older monotreme fossils have been found in Australia than in south America. However, the numbers of fossils found so far are quite low and so there are not really enough data yet to support this theory. 26 Evolution of Australian biota

28 Exercises - Part 3 Exercises 3.1 to 3.4 Name: Exercise 3.1: Evidence of changing environments Identify and describe three sources of evidence for the changing environment in Australia over time. Part 3: Changing environments and biota in Australia 27

29 Exercise 3.2: The last 100 million years Describe the general trend in the vegetation of Australia form the nonflowering seed plants in the Cretaceous through to today. Exercise 3.3: Comparison between a fossil and living species of the same group. The two photographs following show the upper skull of a 15 million year old fossil platypus (Obdurodon dicksoni) and the modern platypus (Ornithorhynchus anatinus) taken from the top (dorsal view) and the bottom (ventral view). In the modern platypus the teeth are lost in the adult and replaced by horny grinding pads made up of material a bit like fingernail (one of these is seen in the ventral view picture). These pads are worn away as the food is ground up but they are continuously replaced. The fossil species has teeth, although some are missing in the specimen in the photograph. Reconstructions of the whole skulls of the fossil and modern platypuses showing the whole tooth arrangement are shown on the next page. 28 Evolution of Australian biota

30 A: Ventral view B: Dorsal view Skulls of the extinct fossil Miocene (15 million years old) platypus, Obdurodon dicksoni (top skull), and the modern platypus, Ornithorhynchus anatinus (bottom skull). (Source: Grant, T The Platypus. A unique mammal. NSW University Press, Sydney. Plates 14 and 15) A View from below (ventral) - the sockets for the back teeth are seen in the fossil skull. One horny grinding pad is in place (upper side; dark in colour) in the modern skull. The other is missing but shows the empty socket where the juvenile teeth have been lost. B View from the top (dorsal). In both views the long bones extending forward of the brain case support(ed) the bill. Part 3: Changing environments and biota in Australia 29

31 Drawings made of the fossil (left side) and modern (right side) platypus skulls to include all of the features of the skulls and to show the complete set of teeth in the fossil form (X) and grinding plates in the modern skull (Y). In each drawing the views in a clockwise order are dorsal (upper), ventral (lower) and lateral (side) views. (Source: Musser, A M. and Archer, M New information about the skull and dentary of the Miocene platypus Obdurodon dicksoni, a discussion of ornithorhynchid relationships. Philosophical Transactions of the Royal Society of London B 353) a) Describe the similarities between the two skulls. b) Describe the differences between the two skulls. 30 Evolution of Australian biota

32 c) Changes from the ancestor of Obdurodon dicksoni and Ornithorhynchus anatinus have come about as a result of natural selection acting on variations during the evolution of the two species. i) Suggest possible reasons for both having the similarities you described in the skulls of the two species. ii) Suggest possible changes to the species environments which could have resulted in the evolution of the differences in the skulls which you described. Exercise 3.4: Named Australian fossils Name of fossil site 1: Description and age of fossil 1 Part 3: Changing environments and biota in Australia 31

33 Description and age of fossil 2 Name of fossil site 2: Description and age of fossil 3 Description and age of fossil 4 32 Evolution of Australian biota