Mammoth Extinction

The extinction of the Pleistocene megafauna is a heavily debated topic, was it over hunting by man or was it climate change? We now have the answer thanks to the University of Adelaide.

By analysing ancient DNA, using radiocarbon dating and other geologic analysis methods, the University of Adelaide has shown that short rapid warming events, known as interstadials, experienced during the past ice age at the end of the Pleistocene coincided with major extinction events even before man became dominant.

Professor Alan Cooper says that "abrupt warming had a profound impact on climate that caused marked shifts in global rainfall and vegetation patterns". It was therefore sudden warming not extreme cold that killed the Woolly Mammoths in Eurasia.

However, Professor Chris Turney believes that "man still played an important role in the disappearance of megafauna".

The culminating factors of rapid warming and the constant pursuit of man pushed the Mammoth's over the edge as they were already under extreme stress with a lack of tundra shrubs and grasses available as the ice retreated further north, this would have lowered reproduction success and limited the sizes of the herds as the food could not support the animals.

Let me know what you think of this, do you think the Mammoth's extinction was a result of the rapid warming of the climate or did early man hunt the Mammoths to the point of extinction?

http://www.sciencedaily.com/releases/2015/07/150723181113.htm?utm_source=feedburner&utm_medium=email&utm_campaign=Feed%3A+sciencedaily%2Ffossils_ruins%2Fpaleontology+%28Paleontology+News+--+ScienceDaily%29

The Pleistocene Extinction

This extinction is not as catastrophic as the Big Five that I discussed in a separate post, but here we see a massive 73% of large mammal genera disappear from Earth's ecosystems. There are two theories that have been put forward to explain this extinction; climate change and the Prehistoric Overkill Theory. It is possible that both of these worked in tandem to push the mammals to extinction. Due to the disappearance of the megafauna there is a shift in the planet's flora, grasslands becomes woodland, this increases the number of forest fires.

Theory 1 - Climate Change: This is the more popular theory. The warming of the the climate would have melted the vast ice sheets that covered a lot of the Northern Hemisphere, the sea levels would have risen preventing the migration of mammals to southern latitudes.

A warming climate would also alter the plant life; we see a transition from grasslands and confier forests that withstood the cold of the Ice Age to deciduous woodland. The herbivorous megafauna depended on the grasslands as their primary source of food, thus when the grasses disappear the mammals have to move further North following the receeding grasslands. This explains why the last populations of mammoths are found in the Arctic Circle in Siberia.

In part this change was natural, part of the fluctuation between glacial interglacial periods that characterise Ice Ages. The change to interglacial was exacerbated by the increase in forest fires, they contribute to an increase in greenhouse gases and therefore allow for the warming of the climate.

Humans managed to survive the extinction as we had the ability to quickly adapt our lifestyles to the changing climate.

Early humans hunting a woolly mammoth. The bones and skin would be used to
make shelters and the meat would be eaten. But did early man hunt to often?
Image credit sciencemag.org

Theory 2 - Prehistoric Overkill Theory: This theory was put forward by Paul Martin of the University of Arizona. Martin noticed a chronological and casual link between the appearance of humans and the disappearance of the megafauna mammals. The theory suggests that when humans first entered areas such as North America, the megafauna did not recognise humans as a threat as they had not come into contact with humans before. This meant that hunting the large mammals was much easier initially and therefore the humans exploited this through overhunting. The loss of the megafauna is also believed to have been the reason behind the extinction of smaller species as there would have been a major ecological disruption.

This extinction event lasted for 1,000 years. In comparison to the K/T Extinction it was a very rapid event, it is believed that the extinction of the dinosaurs took close to 55,000 years from the impact of the meteorite in the Gulf of Mexico.

There is evidence to support this theory. For instance, some Mastodon bones are found with the scarring of tuberculosis. This is not seen in fossils before the appearance of humans in the area, indicating that these animals were susceptible to new diseases that humans were carrying. Also in Africa where large mammals and humans had coevolved and coexisted for millions of years, there were very few extinctions, only two out of twenty three large mammal genera went extinct.

Which theory do you think is correct? Climate change, Prehistoric Overkill or both? Let me know in the comments.

The Big Five Mass Extinctions

Mounted Brachiosaurus skeleton at the
Naturkunde Museum in Berlin, Germany. The dinosaurs
were the major casualties of the K/T Extinction. Image
credit: Bill Sellers

As I said in my Devonian post yesterday, I am going to cover the Five biggest mass extinctions in the Earth's history. We will look at these from the one with the least impact on global species to the one with the most impact. You will notice that when reading the statistics for what went extinct that the number of species will be higher than the number of families or genera that were lost, this is because you can have vast numbers of species die out without losing a genera if some species are to survive. For instance, the Sarcopterygii that was discussed in the Devonian post largely have died out but the whole class was not lost as it survives with the lungfish and Coelacanth. I am also using marine life families as some of the extinctions took place when there was little or no life on land.

#5: The Cretaceous Tertiary Extinction 
Also known as the K/T Extinction or K/Pg Extinction, Pg being the Palaeogene period that followed the Cretaceous, is famed for being the extinction that wiped out the dinosaurs and closed the Age of Reptiles. Extensive studies, including the drilling of boreholes, of the Chicxulub crater off the coast of the Yucatan Peninsula in Mexico have given a clearer picture of what happened 66 Ma. An asteroid nearly six miles wide smashed into the earth, this caused a chain reaction of events that doomed the dinosaurs. After the impact, mass volcanism, particularly around the Deccan Traps area in India, polluted the atmosphere and caused the Earth to heat up. Sea levels also fell by 150 metres which could be attributed to the loss of the marine life as many would have depended on shallower seas to survive. 
Marine Families Lost: 16%
Genera Lost: 67%
Species Lost: Approximately 76% including non avian dinosaurs, marine reptiles, pterosaurs and ammonites.

Artist's impression of the Chicxulub Crater. Image credit:
Detlev van Ravenswaay

#4: End Triassic Extinction
Little is known about the causes of this extinction that took place 201 Ma, but we do see evidence of falling sea levels which is potentially the reason behind a larger loss of ocean species, including some marine reptiles. There is also sedimentary evidence for volcanic rifting that took place as Pangaea broke apart. Taking place over where North America, Europe and Africa would have been on the supercontinent, the air would have been toxic to nearby ecosystems, another possible reason for an extinction event. 
Marine Families Lost: 22%
Genera Lost: 53%
Species Lost: Approximately 80% including most mammal-like reptiles and large amphibians, surprisingly plants made it through largely unscathed. 

#3: Devonian Period's Two Extinction Events
There were two separate events of extinction in the Devonian; the first being the Kellwasser, in the late middle Devonian, which pushed corals and jawless fish to extinction as well as reducing trilobite species. The second was the Hangeberg, which took place on the Devonian-Carboniferous boundary, was responsible for the extinction of Placoderms and many early ammonite species. The Hangeberg extinction event is believed to have been the result of global cooling due to increased volcanic activity. There is also evidence of eutrophication in the shallow seas. This is where there is an excess of nutrients, usually caused by the run off water from the land, this causes algal blooms. These blooms are disastrous for marine life as it prevents sunlight from penetrating the water, thus there is no replenishment of the oxygen that the algae removes from the water, which in turn kills the marine life that depend on the oxygen rich waters. The result of this is the coral reefs not making a return for another 100 million years.
Marine Families Lost: 22%
Genera Lost: 57%
Species Lost: Approximately 83% 

#2: End Ordovician Extinction
Taking place between 445 and 440 Ma, it is believed that an intense global ice age was the trigger for the event. As the supercontinent of Gondwana moved further South to take it's place over the South Pole, vast glaciers spawned gradually lowering global temperatures and therefore causing a fall in sea levels. It is believed that the sea levels fell by between 70 and 100 metres. 
Marine Families Lost: 26%
Genera Lost: 60%
Species Lost: 85%

#1: End Permian Extinction
The largest extinction event in Earth's history took place at the end of the Permian period and the end of the Palaeozoic Era around 252 Ma. Also known as the "Great Dying", the causes are unknown for certain, however, we already know that Pangaea was incredibly hot and dry which would push a great many of species to the brink. But we also see a spike in greenhouse gases such as Carbon Dioxide and Methane that would have been released from frozen stores in the oceans. Combined with one of the biggest volcanic eruptions ever and we can start to see why so many species were lost at the close of the Permian. 97% of all species that were present on Earth in the Permian were wiped out 252 Ma, all life that followed, the majesty of the dinosaurs, the radiation of mammals and eventually the success of Homo sapiens can all be traced back to the 3% of species that clung to survival at the end of the Palaeozoic Era.
Marine Families Lost: 51%
Genera Lost: 82%
Species Lost: 97%

I hope you find this helpful and interesting, let me know what you think in the comments.

The Devonian Period

An artist's impression of the Devonian landscape. Image credit:
Karen Carr

The Devonian period began 416 Ma and came to a close around 358 Ma. It is commonly referred to as the Age of Fishes, despite plants and insects taking great leaps forward in evolution. 

In the Devonian, Gondwana had begun its drift into lower latitudes, set on a collision course with Euramerica. By the end of the Permian these two continents will collide and form the supercontinent Pangaea. The Caledonian Orogeny was continuing in Euramerica, however, the mountains were being rapidly eroded, this caused vast deposition in shallow ocean basins. The climate was also warming up and so the planet was quite dry. 

Because of the continuation of the formation of shallow sea environments, extensive reef building could be found on the perimeter of the continents, the reefs were continuing to thrive from the Silurian Period. 

The Placoderms that first appeared in the Silurian grew to great lengths, up to 10 metres. This made them the top predators of the oceans. The most famous of which is

Skull of Dunkleosteus. Note the bony extensions at the
front of the mouth, these are not teeth. Image credit:
cmnh.org

Dunkleosteus, this fish did not have crushing teeth instead these were extensions of bone from the armour on the animal's head. The trilobites and brachiopods were joined by the coiled molluscs; these were the first ammonites. By the close of the period we also see the emergence of sharks and rays that diversified from cartilaginous fish. 

Fish also underwent massive evolutionary success. Here ray finned fish (Actinopterygii) and lobe finned fish (Sarcopterygii) had evolved. These fish had evolved true bones, teeth, swim bladders and gills. Actinopterygii have fins supported by thin bones whereas the Sarcopterygii fins were fleshy and had phalanges that were joined to an ulna and a radius on the pectoral fin and a fibula and tibia on the pelvic fins. These two bones where then joined to a humerus on the pectoral fin and a femur on the pelvic fin. You'll notice that these are the same bones that we have in our limbs. This is because Sarcopterygii fish are widely accepted as the common ancestor for all tetrapods. But despite being the more numerous in the Devonian, the Sarcopterygii widely died out, except for the Coelacanth and lungfish that still exist today. 

Plants had taken hold of the land in the Devonian. Ferns, lycophytes and horsetails had evolved from the primitive plants of the Silurian. Plants were evolving incredibly quickly, their size and lifestyle was changing completely. A good example of this is

Artist's impression of Archaeopteris. Note
that the leaves are not true leaves they are
in fact fern-like in appearance. Image credit:
go2add.com

Archaeopteris which grew to a massive 30 metres with a 3 foot diameter. From the fossils of this plant we can see that it shed its fern like branches, a change in lifestyle from the primitive Cooksonia of the Silurian. Archaeopteris was the first deciduous tree. The expansion of greenery across the landscape meant that Carbon dioxide levels fell and Oxygen soared, this was a key characteristic of the following period the Carboniferous. 

The earliest true insect appeared in the Devonian, Rhyniella praecusor was a flightless hexapod that evolved between 412 and 391 Ma. Tetrapods also began to crawl out of the water, the first tetrapods are more closely related to amphibians. Tiktaalik rosae is believed to be the link between the Sarcopterygii and the Tetrapods. This animal was mostly aquatic but had powerful hind limbs that were jointed to a fish-like pelvis. This enabled the animal to propel itself while out of the aquatic environment. It was also able to breathe air through nostrils, an adaptation not previously seen in animals. 

The Devonian period was one of the big five extinctions in geological history (I will cover the big five extinctions in a separate post tomorrow). It is hypothesised that this extinction was two prolonged events rather than a single instantaneous eradication of species. Firstly, the Keilwasser Event which took place in the late middle Devonian. This is where great amounts of corals, the jawless fish went extinct, whilst the number of trilobite species were dramatically reduced. The second event, the Hangeberg Event, took place on the Devonian-Carboniferous boundary. Here the Placoderms and many species of early ammonite were pushed to extinction. Despite 70% of invertebrate life going extinct, vertebrates and plants were relatively untouched by these two events. The extinctions are believed to have been caused by global cooling and the first forest fires caused by Carbon dioxide depletion.  

The Ordovician Period

The Ordovician is the second geological time period of the Palaeozoic era, spanning from 485.4 to 443.4 Ma. The Ordovician is named after the Celtic tribe the Ordovices. This period was the stage for a number of revolutions in the world's flora and fauna.

Ordovician oceanscape. Image credit: dustdevil on DeviantArt

For most of the Ordovician, the climate was warm and wet. This caused the sea levels to rise 600 metres above todays levels, this created new habitats such as inland seas and freshwater areas.

The fauna still dominated the planet's oceans. After the Cambrian extinction, coiled cephalopods, called a Nautilus, exploited the empty niche and became successful and effective predators. A straight cephalopod also evolved to become one of the larger predators of it's time, this was the Orthocone. Trilobites and cnidarians also continued to thrive.

Fossil fish became more abundant in the Ordovician. The jawless mouths of these fish are found positioned on the ventral surface of the head. This suggests that they sucked up their food from the sea floor rather than being active and swift predators. We also find the early evolution of armour plating in fish in the Ordovician, the fish depicted above have bony armour plates on their heads. These fish are the ancestors of lampreys and hagfish that we have today.

Crinoids also find their origins in the Ordovician. They pinnules filtered the Ordovician waters for plankton. To read more about Ordovician crinoids, see my post 'My Collection #1' where I discuss a crinoid fossil that I have.

Life also began to make advances on land as well. Hard bodied arthropods; Eurypterids, also known as sea scorpions. could survive on land for short periods of time. This was due to the ability to diffuse gases across their exoskeleton. The living fossil Horseshoe crabs are also believed to have ventured onto land to spawn as they still do today.

Horseshoe crabs spawning, scenes like this would have
been common during the Ordovician. Image credit:
capeandislands.org

The very first terrestrial plants are seen in the Ordovician. They likely evolved from green algae. They appeared as tiny non vascular plants, that resemble Liverworts. Evidence for these plants comes from not only their fossils, but the fossils of their spores that have been identified in Upper Ordovician sediments. This shows that the plants were immediately exploiting the land, using reproductive methods that allowed for rapid expansion across the barren landscape.

The Ordovician was closed with the second largest mass extinction in the Earth's history. It is believed that this event took place between 447 and 444 Ma.

Chronostratigraphical Timescale of the
Ordovician. Image credit:
keyword-suggestions.com

A massive 49% of marine fauna was pushed to extinction, while other phyla saw individual numbers fall dramatically. This is commonly attributed to an ice age. Research by Page et al states that temperate climates did not return until the end of the Silurian. The ice age's trigger is disputed, the more popular hypothesis is that as Gondwana drifted over the South Pole, ice caps formed over where Africa would have been location on the supercontinent. This locked up vast amounts of the Earth's water, sea levels consequently dropped, destroying shallow sea habitats. The falling temperatures also pushed tropical species to extinction. An alternative hypothesis that was put forward by Melott et al in 2004 suggested that a gamma ray burst of a mere 10 seconds destoryed the ozone layer. This exposed the life on Earth to great amounts of radiation. This radiation is believed to have triggered a sharp fall in global temperatures, trigging an ice age.