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Megafauna Extinction

Evolution, Distribution, Extinction & and Preservation

 

WEATHER REPORT:  Colder tonight, and tomorrow.....and for the next million years (W.W. Reed, Geologist, 1930)

 

Introduction 

Recognition of the value of megafauna fossils in the context of geo-conservation requires an understanding of the global biological diversity of megafauna during the Cainozoic.  Equally, it is important to appreciate why these animals are not present today.

Megafauna Evolution And Diversity

The term megafauna, mega from the Greek meaning large, has been applied as a generic term to refer to the diverse fauna that evolved during the Cainozoic.  The earth during the Pleistocene supported an impressive range of large mammals on all continents, except perhaps Antarctica.  North and South America were home to species of sabretooth cats (inset left: Smilodon populator), ground sloths, mammoth and mastodons, while woolly rhinoceroses, woolly mammoths, cave bear and giant deer roamed across Europe and northern Asia (Martin & Klein, 1984).

Pleistocene fauna of Australia included mammals, birds and reptiles.  The mammals were characterised by Diprotodon spp., Zygomaturus spp. and Palorchestes sp. These large mammalian herbivores reached mammoth proportions and weighed in excess of 1500 kg. They were similar in size to the extant African hippopotamus.  A number of megafauna species were forest browsers and graziers, including several species of very large kangaroos weighing between 100 – 1000 kg.  Many Pleistocene birds were also grazers and reached exceptional size and weight.  The largest of these birds was Genyornis newtoni, an emu-like species which weighing around 100 kg (Flannery, 1997). 

In addition to herbivores, a contingent of mammalian carnivores such as the Thylacine were also present during the Pleistocene. Although terrestrial vertebrates are the more conspicuous megafauna, an assortment of invertebrates also evolved in association with these larger animals; many, such as the dung beetle developed complex symbiotic relations with these larger animals (Porch, 1993).

Several families of reptiles are also considered to be megafauna.  The varanid Megalania prisca, a 7 metre long monitor lizard, inhabited the drier inland parts of Australia and would have been a  formidable predator and scavenger.  Two large crocodile species also lived during this period: Quinkana fortirostrum was a terrestrial crocodile which weighed approximately 200 kg, pocessed teeth which were sabre-shaped and laterally flattened, and grew to a length of 3 metres. Quinkana's running mate was  Pallimarchus pollens which reached 10 meters in length, had a very broad snout, and inhabited inland waterways and lakes (Flannery, 1990).  A giant snake, Wonambi narracourtensis is known to have inhabited southern Australia and reached approximately 5 meters in length and weighed 50 kg plus.  W. narracourtensis had numerous small teeth suggesting that it preyed upon moderate-sized mammals (Flannery, 1990).

Gigantism, a product of natural selection, favoured many species during the Pleistocene, however, some species were small, or of similar size to extant species.  One striking aspect of the megafauna is that many extant fauna are similar to their Pleistocene descendants with an exception of a size decrease.  For example, the extant red kangaroo Macropus rufus is a direct descendant of the giant megafauna kangaroo Macropus titan (Flannery, 1997).

Reptiles - Apex Predators in the Pleistocene

Of interest is the evolution in Australia of reptiles as apex predators during the Pleistocene.  This is in contrast to large mammalian predators such as large cats on other continents.  M. prisca as stated above grew to an estimated length of 7 meters, and was probably living contemporaneously with early Australian aboriginals.  Current evidence suggest it became extinct ~45,000 ka., although this is really only an educated guess as Pleistocene deposits in Australia are notoriously difficult to accurately date.   The nearest "living" relative M. prisca, and next largest monitor lizard reaching a length of 3 meters, is Varianus komodoesis (komodo  dragon) indigenous to several islands in the Indonesian archipelago.      

Smaller carnivorous predators in Australia were the now extinct Propleopus oscillans – nicknamed by palaeontologists as the “killer kangaroo”.   This carnivorous marsupial had pre molars similar to robust blades suitable for grasping and cutting very tough muscle and sinew, whilst the lower incisors were suitably adapted to stabbing (Vickers-Rich, 1993). 

 Other Australian megafauna predators include the Thylacine (Tasmanian Tiger) and Thylacoleo carnifex.  The later was a marsupial lion which sported 2 long tusks. 

Quick List of Representative Australian Megafauna

Zygomaturus trilobus was a bullock-sized relative of Diprotodon that may have had a short trunk. 

Palorchestes azael was also the size of a bullock, with long claws and a longish trunk. Imaginative writers have suggested it as the inspiration for the Aboriginal bunyip. 

Procoptodon goliah was the largest kangaroo ever, and had a shortened flat face and forward-looking eyes. 

Thylacoleo carnifex, the so-called 'Marsupial Lion', was a leopard-like animal, and was almost certainly carnivorous and a tree-dweller. 

Zaglossus hacketti, a sheep-sized echidna whose remains were discovered in Mammoth Cave in Western Australia, was probably the largest monotreme ever.

Genyornis newtoni was a flightless bird about the height of an ostrich. It was the last survivor of a group of large flightless birds more closely related to ducks than emus and ostriches. A relative, Dromornis stirtoni, which was three metres tall and weighed half a tonne, was probably the largest bird ever. 

Megalania prisca was an enormous, carnivorous, goanna-like (monitor) reptile, at least 7 metres long, and with a weight of about 600 kilograms (line image above is an artist's recreation of M. prisca)

Megafauna Distribution In Australia

Megafauna are known to have existed across most of the Australian continent, although the relative paucity of fossil locations from central and northern Australia limit knowledge of fauna distribution in these regions.  The total distribution of megafauna in Australia may never be fully known, however, interpretation of fossil localities, and distribution patterns of extant fauna, suggest that megafauna were abundant and widespread (Flannery, 1990). 

LEFT:  Varianus komodoesis (komodo dragon)

 

 

 

 

Megafauna Extinction: A Brief Synopsis Of Current Debates

The magnitude of the Late Pleistocene (50 ka - 20 ka) extinction in Australia was a major biological event.   The extinction removed 13 genera of marsupial mammals, comprising as many as 45 species (Ward, 1995).  Large mammalian species were not the only fauna to become extinct, a wide variety of terrestrial animals of a variety of sizes also disappeared. Mammals that became extinct had a body weight greater than 10 kg and reptiles had a body weight greater then 50 kg (Flannery, 1990).  The extinction also removed several families of birds with the exact number of species unknown, as some species are known only from undated “Pleistocene” deposits in central Australia (Flannery, 1990).  

Table 1-1 (bottom of this page) lists mammalian species that became extinct in the Late Pleistocene. The table also shows the extinct species’ probable distribution and habitat.  Actual numbers may vary somewhat as the number of extinct species currently recognised is probably within +- 20% of the real number (Flannery, 1990).  This is because of the paucity of the fossil record in some areas of Australia. 

Extinctions occurred worldwide during the Late Pleistocene, with each continental landmass having its own history of events and dates.  There currently is no unifying theory that can be applied on a global basis to explain the megafauna extinction. Initial hypotheses centred on climatic change, however, the synchronicity of extinction events and the arrival of Palaeolithic hunters, especially in Northern America, suggests that human predation may have been responsible for the demise of the megafauna in what Martin (1984) refers to as Pleistocene overkill. 

Uncertainty about events and the lack of definite dates has fuelled a number of hypotheses.  In essence, the debate remains in a state of polarisation between those advocating climate change as the primary factor leading to extinction (Beck, 1996; Gillespie et al, 1978 and Flannery, 1997), and those advocating over-hunting (Flannery, 1987 & 1989; Martin, 1984; Spaulding, 1983; Miller & Johnson, 1999 & Krantz, 1970) with some favouring a combination of both factors (Beck, 1996; Gorecki et al; 1984; Flannery, 1987 & Wright, 1986). 

Currently there are three main contending hypotheses which attempt to explain the Late Pleistocene extinction in Australia.  

Climate change hypothesis:   In this model the harsh climatic conditions experienced during the peak of the last glaciation ca 20 ka/BP are thought to be the catalyst towards faunal extinction.  Climate change is a plausible reason for the extinction event, as the biological, isotopic and geological evidence of cyclic Quaternary temperature and precipitation has been accurately dated (Williams, Dunkerley, De Deckker, Kershaw & Stokes, 1993).  Climatic patterns were altered markedly by glaciation which, apart from causing ambient temperatures to fall ca. 5.2 to 7.9 degrees cooler than the present (depending on which glaciation; event), also caused a decrease in precipitation and an increase in aridity (Colhoun et al., 1982, Kirkpatrick, 1997).  

Groundwater levels fell and inland waterways began to dry out with a subsequent change in vegetation type.  Formally wooded areas became arid causing a loss in plant biomass and diversity.  Xeromorphic species, which are less nutritious to animals, replaced much of the original flora biota (Hill, 1998). 

Interpretation of the fossil record suggests that megafauna became restricted to areas surrounding shrinking inland lakes, or migrated to more favourable, moist areas closer to the eastern Australian seaboard (Horton, 1984).  Large mammalian fauna perished during drought conditions causing a flow on effect to other predatory animals, leading eventually to trophic collapse of the ecosystem and further extinctions.  Deposits unearthed from a megafauna site at Lancefield Swamp, Victoria provide evidence in support of this hypothesis (Gillespie et al., 1978).

A major weakness in this hypothesis is that it does not explain why megafauna survived several earlier, more severe climatic perturbations before becoming extinct during the last glaciation, by which time humans were present in Australia.

Of interest, are geomorphological observations which provide evidence to suggest that El Nino events began to occur in Australia at a similar time to stepwise extinction events.  The severity of El Nino cycles which causes short term climatic swings within a slowly changing climatic model, and the inability of large fauna to adapt quickly enough to climatic change  may be partly responsible for megafauna extinction.  Further study is required in this area to determine whether El Nino was the catalyst to the beginning of stepwise extinction.

Overkill hypothesis:    Proponents of the overkill hypothesis claim that there is a strong correlation between the arrival of humans in Australia and the extinction of the megafauna in the Late Pleistocene (Flannery, 1989). The overkill hypothesis proposes that early aboriginals preferentially hunted the megafauna, eventually lowering breeding numbers and pushing the animals to extinction. Martin (1984) argues that humans caused the extinctions of large animals worldwide and Flannery (1989) suggests that humans altered the environment by their use of fire, causing catastrophic changes in the ecosystem.

Extinctions in Northern America occurred in a short time span and have a very strong correlation with the arrival of Palaeolithic hunters (Martin, 1984); however, extinctions in Australia are stepwise and occurred over a longer time period (Flannery, 1997).  Megafauna “kill sites” although common in Northern America are comparatively rare in the Australian fossil record, and dating any Australian megafauna site is exceptionally difficult due to faunal remains being close to the limit of radiocarbon dating methods. If aboriginal hunting did cause the megafauna to become extinct, the time window for this event to be preserved within the fossil record would be exceptionally small.  Consequently, it may not be possible to uncover actual evidence for a human-induced extinction within Australia.

The lack of evidence for direct killing by humans does not suggest that aboriginal groups did not have an effect on the megafauna.  Radiometric dates derived by van Huet et al., (1998) from Lancefield indicate that aboriginals and megafauna overlapped for at least 15,000 years, and, if the latest dates of aboriginal colonisation of Australia are applied, this figure may stretch to 90,000 years (Roberts et al., 1990).  This opens up the debate as to whether megafauna became extinct before aboriginal occupation, or survived into the Holocene before becoming extinct.

The Late Pleistocene-Early Holocene stratigraphic record contains an increase in charcoal, attributed to an increased use of fire by aboriginals to clear grassland to improve hunting.  Vickers-Rich (1993) claims any subtle change over an extended time frame, such as the use of fire, would have a pronounced effect on the ecosystem, and annual burning would have fundamentally altered the vegetation and the community structure of animals based upon it.  

Climate-predation model:    A third hypothesis suggests that a combination of climate change and hominoid predation may be the cause for the extinctions (Gillespie et al; 1978).  This would explain the reason as to why Australian extinctions are stepwise, rather than cataclysmic as in Northern America.  Further research, especially the dating of known megafaunal sites, is needed to substantiate a duel climate-predation extinction model. 

PART TWO: Paleontological Parameters Conducive to Fossil Occurance and Preservation

Introduction

Paleontology is an inexact science, more than often based on circumstantial evidence developed around limited physical material.  It cannot be likened to mathematics, physics or chemistry, which rely on rules that rarely change from one circumstance to another.  There is no equivalent law in paleontology to the law of gravity, nor is there a mathematical equation which can predict exactly where or how fossils will be deposited.  In many instances it is “random chance” as to whether a deceased animal will become fossilised and preserved. 

Fossil Preservation

Fossils are the remains, impressions or traces of organisms that have died and become preserved within sedimentary rocks or unconsolidated sediments.  To preserve organic remains a number of conditions need to be met.  The body of the animal must be covered as quickly, and as gently as possible with sediment or other material to protect the remains from destruction by hydrospheric or atmospheric processes, mechanical damage and biological decomposition.  Over time, under their own weight, the cumulative layers of sediment will become compacted and cemented together to form sedimentary rock. Throughout this process the sediments cannot be disturbed, consequently, metamorphic processes above greenschist facies must be absent from the region, as intense heat and deformation will destroy any preserved organic material.  

The petrographic and chemical composition of the overlying sediment is equally important. Fine-grained impermeable sediments enhance the chance of preservation in contrast to course-grained permeable sediments.  Bone is comprised of calcium carbonate and phosphate; if the overlying sediments are acidic, such as found in some peat bogs and swamp deposits, the bone material will dissolve.  Conversely, if the covering sediments are alkaline, such as in calcareous sands, the chance of fossilisation is improved. 

During burial and mild diagenesis vertebrate bone may retain its original composition, however internal cavities may be infilled with chemical compounds such as carbonates precipitated by percolating ground water.  Alternatively, replacement may occur with cell walls and other solid material chemically replaced by mineral matter such as silica.

Fossilisation can occur only if a number of strict physical requirements are met.  Therefore, paleontology has a strong, built-in bias towards those animals that were found in areas with conditions favourable to fossilisation.  The chances of an animal’s body becoming fossilised are very slim; deceased animals usually become food for scavengers; animal remains decompose, and exposed bone material will eventually disintegrate.  The rate of decomposition will vary depending on the environment and latitude (latitude has a direct effect on precipitation and temperature): carcasses in cold regions decompose at a slower rate than those in tropical areas, and organic remains in regions of low relative humidity may become mummified, preserving both bone and skin material.  

Lithology Constraints

Lithology and sediment genesis act in concert with one another and are significant geological parameters that delineate regions that may contain fossils.  Not all sedimentary strata contain fossiliferous horizons, and certain lithologies have a greater probability of preserving fossils than others.  Fossils can be found in a number of differing sedimentary lithologies, however the deciding variable as to whether a fossil will be found, other than the obvious need for the animal to be present in the first instance, is the depositional sedimentary environment, for this will affect whether a sediment is deposited gently, turbulently, slowly or rapidly.  As already mentioned, fossil preservation is dependent on rapid burial of animal remains and gentle sediment accumulation, without disturbance from other geological processes.  Therefore, lithologies that have been deposited by relatively gentle depositional environments may be more prone to fossil occurrence.  Such lithologies are likely to be silt, mud and to varying extent sand, rather than conglomerate, breccia and glacial diamictite which are deposited in higher energy environments.  Although the probability of fossil occurrence in these latter lithologies is poor, it does not rule them out completely.  Fossil bone material, although highly disarticulated and in poor preservation, has been recovered from basal conglomerates within fluvial channels; this environment being high energy, and having a rapid sedimentation rate (Banks et al., 1978). Sediment Genesis And Depositional Environments

Sedimentary processes are most often responsible for rapid burial, although a lava flow from a volcanic eruption can also cover and bury an animal.  Lacustrine environments are prime locations for fossil preservation, as are fluvial point bar systems, river flood plains and areas that have been covered by aeolian sand dunes. The latter is especially promising if the dune system has covered a lacustrine or fluvial system preserving remains within the lake or along the shoreline.  Similarly a relative rise in sea level may cause low lying areas in which animals have died to be covered by shallow marine mud or sand.  

Cave environments are particularly suited to fossil preservation.  Caves usually offer low relative humidity; settings conducive to mummification.  Although caves can develop in a number of environments and lithologies, cave development within a karst setting is especially favourable to fossil preservation as limestone and dolomite are chemically similar to bone material. 

The application of several inter-disciplinary parameters will aid in selection of initial areas that may contain fossils.  Figure 1 (below) outlines several factors that assist in determining whether an environment will yield fossiliferous material.

Autochthonous And Allochthonous Fossil Occurrence

Autochthonous fossils are those that are discovered fossilised in-situ.  The animal and surrounding sediments are preserved providing a snapshot of environmental conditions when the animal died.  Fossils of this nature are especially important as information can be inferred relating to the biology of the animal.

Equally, the surrounding rock in which the animal has become fossilised can supply information concerning the animal’s habitat, the climate at the time of death, paleoenvironment, chronology and environmental change.  

In contrast, allochthonous fossils are organic remains that have been transported from their original depositional environment to another location.  Fossilisation may have occurred before transport or after deposition.  Often these micro-sites contain an assortment of fossils, usually sorted according to size, but representing different animal species, that were washed together by a stream after the animal died.  Such deposits, although important in ascertaining species, offer little biological or environmental information. 

 

Summary

The evolution and diversity of megafauna has been discussed with an emphasis on Australian examples. Various hypothesises which aim to explain why the megafauna became extinct have been reviewed, before concluding with a brief discussion on environmental factors necessary for the formation and preservation of fossils.

Table 1:       Mammalian fauna that became extinct in the Late Pleistocene.  The table also indicates the extinct species’ preferred distribution and habitat (source: Flannery, 1989).

 

SPECIES

HABITAT

DISTRIBUTION

Diprotodon minor

Desert-Forest

Eastern Australia

Diprotodon optatum

Desert-Forest

Australia Wide

Euryzygoma dunense

Forest

Northern Australia

Euwenia grata

Forest

Northern Australia

Lasiorhunus angustioens

Forest

Eastern Australia

Macropus thor

Forest

Eastern Australia

Macropus ferragus

Savannah

Niorthern Australia

Macropus pearsoni

Savannah

Northern & Eastern Australia

Macropus piltoneonsis

Forest

Eastern Australia

Macropus rama

Forest

eastern Australia

Nototherium mitchelli

Forest

Northern Australia

Palorchestes azael

Forest

Eastern Australia

Palorcheates parvis

Forest

Eastern Australia

Phascolomys medius

Forest

Eastern Australia

Phascolonus gigas

Desert-Forest

Australia Wide

Procoptodon goliah

Desert-Savannah

Eastern Australia

Procoptopon pusion

Forest

Eastern Australia

Procoptodon rapha

Forest

Eastern Australia

Procoptodon texasensis

Forest

Eastern Australia

Propleopus oscillans

Australia Wide

Eastern Australia

Protemnodon anak

Forest

Eastern Australia

Protemnodon brehus

Forest

Eastern Australia

Protemnodon roechus

Forest

Southern & Eastern Australia

Ramsayia magna

Forest

Eastern Australia

Simosthenurus brownei

Forest

Southern Australia

Simosthenurus gilli

Forest

Southern Australia

Simosthenurus maddocki

Forest

Eastern Australia

Simpsthenurus occidentalis

Forest

Southern Australia

Simpsthenurus orientalis

Forest

Eastern Australia

Simosthenurus pales

Forest

Eastern Australia

Sthenurus andersoni

Forest

Eastern Australia

Sthenurus atlas

Forest

Eastern Australia

Sthenurus oreas

Forest

Eastern Australia

Sthenurus tindalei

Desert

Central Australia

Thylocoleo carnifex

Forest

Australia Wide

Troposodon minor

Forest

Eastern Australia

Vombatis hacketti

Australia Wide

Western Australia

Warendja wakefieldi

Forest

Southern Australia

Zaglossus hacketti

Forest

Western Australia

Zaglossus ramsayi

Forest

Southern & Eastern Australia

Zygomaturus trilobus

Forest

Southern & Eastern Australia

 

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