8.I: The Mesozoic World

Archaeopteryx_crop

During the Mesozoic Era, mammals and birds both evolved from reptilian ancestors. Archaeopteryx is a famous fossil animal along the dinosaur-bird evolutionary path.

A.  Introduction and Geological Time

B.  Pangaea and the Modern Continents

C.  The Worst Mass Extinction of All Time

D.  Dinosaurs, Mammals, Birds, and Flowers

E.  Oil!

F.  Citations


A.  Introduction and Geological Time

Chapter 8 mostly belongs to what geologists call the Mesozoic or “Middle Life” era.  (See linear timeline).  Chapter 9 left off with the Paleozoic or “Old Life” era.  After the Mesozoic came the present-day Cenozoic or “New Life” era.  These eras are all part of the gigayear Phanerozoic Eon, which is not labeled on the timeline below but was put into broader context at the beginning of Chapter 9.

Recall that eras are divided into periods, based on changing layers of sedimentation and fossil records.  Eras run hundreds of millions of years, and periods are subunits spanning tens of millions of years.  The last period of the Paleozoic Era was the Permian.  The three periods of the Mesozoic Era were the Triassic, then the Jurassic, and finally the Cretaceous.  The first period of the Cenozoic Era was the Paleogene.  It is almost impossible to keep these crazy names straight, so I find the timeline indispensable to summarize these divisions of time.

chapter_08_timeline-2

The Mesozoic Era was dramatically bookended by two major mass extinctions.  The mass extinction that ended the Paleozoic era, called the Permian-Triassic or simply the P-T event, was the worst known mass extinction in world history.  It set the stage for the dinosaurs, which were the dominant and most famous form of life in the Mesozoic.  The dinosaurs themselves, along with other orders of life, went extinct in the Cretaceous-Paleogene or K-Pg event.

Both of these mass extinctions had significant influence on the evolution of our ancestors.  The P-T event was an evolutionary bottleneck, narrowing down the gene pool and leaving only a lucky minority of species to repopulate the Earth.  When dinosaurs perished in the K-Pg event, it cleared the way for mammals to flourish in a variety of niches.

If the age of the universe were represented by a meter stick, Ch. 8 would span the last centimeter.  To follow our three-dimensional gold analogy where one year is a gold flake and the age of the universe is measured by ten pallets of gold, Chapter 8 is represented by the 100 gold bars on the top row of the last pallet.


B.  Pangaea and the Modern Continents

This entire chapter will make more sense in the context of geography.  As explained in Chapter 9, over the course of billions of years, the continents grew and drifted around the world multiple times.  By about 300 million years ago, they happened to coincide in one single land mass.  That supercontinent is now called Pangaea (all-land).  It was centered roughly at what is now the middle of the Atlantic Ocean, and extended from North Pole to South Pole.  The rest of the world was the single ocean, Panthalassa (all-sea), with a few major island arcs to the east of Pangaea.  Evidence of the formation of Pangaea can still be found in the mountains created by continental collisions.  The Appalachian Mountains of North America and the Anti-Atlas Mountains of Morocco were part of the same orogeny.  The Ural Mountains were formed when Europe joined Asia. 1

Pangaea remained intact for most of the Mesozoic Era.  Around 200 million years ago, the North American plate began drifting westward from the rest of the landmass, opening a waterway that would widen to become the North Atlantic Ocean.  The South Atlantic Ocean formed another 100 million years later between South America and Africa.  Meanwhile, Australia and India separated from Antarctica and drifted northward.  By 30 million years ago, the continents were mostly recognizable as they appear today, with the exception that Eurasia was still under formation.  The Atlantic Ocean is still widening while the Panthalassa / Pacific Ocean narrows, at the rate of a few centimeters per year.  Antarctica has been essentially parked at the South Pole the whole time.  The sequence is roughly illustrated below.  My favorite images of world maps from past eras are those posted by the Royal Ontario Museum (they are copyrighted, so I cannot duplicate them here).

Pangaea_to_present_PD

The assembly, consolidation, and breakup of Pangaea all had important consequences for life.  Recall that plants and animals came ashore gradually 500 – 360 MYA.  This was when bygone continents were converging to form Pangaea.  All terrestrial life forms were then in a position to conquer the same landmass.  If continents had been widely separated at that time, some of them might have remained lifeless.

A world with one supercontinent and one super ocean had distinct climate patterns.  Pangaean coasts were probably buffeted by monsoons. 2 Monsoons are associated with the meeting of a large ocean with a large landmass in tropical regions, such as the Indian Ocean today.  The continental interior was arid with extensive deserts.  The ocean acts as an “insulator” because it does not change temperature quickly.  On a large continent with a deep uninsulated interior, desert temperatures fluctuated from low to high extremes on a daily basis.  Additionally, there was no way for warm ocean currents to circle the equator.  The land mass diverted those currents toward the poles.  This contributed to a climate that was more globally uniform than today.  There were no ice caps during the Mesozoic, and even Antarctica thrived with life.

Some scientists have attributed both global cooling and global warming to Pangaea.  It is true that temperatures fluctuated greatly while Pangaea formed and grew, but these changes can mostly be explained by atmospheric conditions. 3 High carbon-dioxide content causes a greenhouse effect and global warming.  During high-oxygen periods, temperatures fall.  The deepest cold spell was the Carboniferous-Permian Ice Age, which covered the southern continents with a large polar ice cap for 90 million years.  This was the cooling period that played a role in the extinction of the carboniferous coal-producing forests.

Pangaean animals had an unlimited range of latitude, so they could spread out to ideal locations all across the supercontinent.  During the Carboniferous-Permian Ice Age, animals could migrate toward the tropics.  When the climate was hotter, they could retreat toward the poles.  In fact, during most of the Mesozoic, equatorial Pangaea was too hot and dry for very much life at all.

What is remarkable is that all terrestrial animal life evolved together on the same landmass.  The ancestors of today’s amphibians and reptiles, mammals and birds all originated in Pangaea, so they were all shaped by the same environmental constraints.  Eventually, the full diversity of Pangaean life was carried by continental drift in all directions.  By the Jurassic Period, oceans were large enough to almost completely isolate the continents from one another.  From that time forward, life on the various continents diverged on different evolutionary paths.

Sea life suffered its own minor mass extinction after the continents joined together. 4  Obviously, the formation of Pangaea destroyed thousands of miles of inland coastline, and shallow coastal waters are the favored environment for invertebrates.  The marine extinction was caused partly by an oxygen crisis in the ocean.  This may or may not have been related to Pangaea and its effect on ocean currents and climate, but it occurred at about the same time as Pangaea’s coalescing.  During the Permian Period, marine life recovered and thrived.  Pangaea was ringed by rich coral reefs.


C.  The Worst Mass Extinction of All Time

Extinction is the rule, not the exception.  99.9% of species that have ever lived are now extinct, at least as defined by physical appearance.  It is normal for one to ten species to go extinct every year. 5 The exceptional episodes have been a number of mass extinctions, global events that either killed off a high number of life forms or hit some taxa particularly hard.  Obviously, our ancestors survived every one.  It might seem that a history of proto-human evolution could ignore the extinctions of foreign clades.  But mass extinctions played an important role in shaping our lineage.  By determining the course of who lived and who died, extinction events favored some animals over others.  They exerted evolutionary pressures and opened up ecological opportunities when competing species failed.  In other words, we are who we are because our ancestors had what it took to survive these extinctions – even when it was just dumb luck.

Since a mass extinction is global, it usually involves the atmosphere and / or ocean.  The Earth is changing constantly, just like living things.  Sometimes the environment changes too rapidly for life to adjust.  It need not be instantaneous.  Each species goes extinct over many generations, as young adults fail to have enough offspring to replace their numbers.  A mass extinction can occur gradually over thousands or millions of years.

The most devastating mass extinction in world history was the end-Permian extinction, also known as the Permian-Triassic (P-T) Extinction. 1 Scientists are still unearthing the full story of this catastrophe, but it appears to be strongly associated with major ongoing volcanic activity.  250 million years ago, the Siberian Traps volcanoes released enough gas from within the Earth’s mantle to change the entire atmosphere. 6 The main culprit was carbon dioxide, CO2.  CO2 is a greenhouse gas that traps heat at the surface.  An excess of CO2 caused runaway global warming.  Temperatures rose so high that the tropics became virtually uninhabitable. 7 Other gases caused acid rain, which killed plants and eroded soil.  The ocean became acidic and lost most of its oxygen; this was exacerbated because ocean water could not circulate well around Pangaea.

The P-T Extinction is of particular interest in the 21st century.  Not only is this event on the frontier of geological research, but it serves as a warning about today’s man-made carbon dioxide.  The CO2 levels of the Permian reached levels much higher than today.  However, our current annual rate of carbon production is comparable to Permian accumulation. 8 If today’s industrialization continued on track for tens or hundreds of millennia, it would likely have the same effect.

The Permian Period was the grand finale of Paleozoic “old life”.  Land and sea alike were rich with diverse communities of plants and animals.  The crown jewels of shallow coastal waters were the coral reefs.  Reefs sheltered familiar animals like shellfish, snails, sea urchins, shrimp, and lobsters.  Bony fish and some marine reptiles now swam with rays, sharks, armored fish, and jawless fish.  The seafloor was home to burrowing worms, sponges, and other sessile animals rooted in the ground.  One of the most distinctive Paleozoic animals was the trilobite, the three-lobed relative of the crab.

Reptiles ruled the continent.  Many of them were moderately large (bear-sized) and are sometimes wrongly called dinosaurs.  The most charismatic creatures, at the top of the food chain, included the sail-backed Dimetrodon and the saber-toothed Gorgonopsian.  Amphibians were less dominant but still holding their own in wetlands.  Terrestrial plants made a major breakthrough in the Permian Period as well, with the evolution of seed-bearing plants.  Forests of conifer trees, like pines, covered vast areas.

After all that evolutionary success, the impact of the climate change is hard to believe.  The P-T extinction killed off almost all forms of life, 90% of the species living throughout the world!  Most species seem to have died out relatively suddenly, maybe within 100,000 years, concurrently with the lava flows that created the Siberian Traps.  Panthalassa was impacted worse than Pangaea.  Marine invertebrates took the hardest hit of all, with 95% species loss. 9 A primary reason for this was the interdependency of ecosystems.  Many forms of plankton, the very base of the aquatic food chain, died out. 10 Most forms of coral were severely reduced too.  Coral reefs shelter myriad species and slow down coastal erosion.  After the extinction, sea floors were almost bare, with just a few sturdy shellfish remaining.  Iconic sea creatures like armored fish and trilobites were gone forever.

On Pangaea, the landscape was also laid bare with decimation of the forests.  It was a period of heavy continental weathering.  Many large reptiles and amphibians were killed off, including gorgonopsians and fin-backed reptiles.  This was the only mass extinction that even put a dent in insect populations.  Global temperatures remained so high for so long that there was almost no life near the equator for millions of years. 11 Living things migrated toward the poles and adapted to the hot, low-oxygen environment.

The P-T extinction event defines the transition from the Paleozoic to the Mesozoic.  With nine out of ten species wiped out worldwide, the planet became a desolate wasteland.  It took tens of millions of years for life to fully recover.  It was truly the end of an era.


D.  Dinosaurs, Mammals, Birds, and Flowers

In the barren epochs of the early Triassic Period, ecosystems started over almost from scratch.  One important fact remained:  reptiles were still the dominant life form on land.  The Mesozoic Era was truly the age of reptiles.  Over the course of hundreds of millions of years, these successful survivors came to dominate all realms of the Earth.  They begat lines of descent that went on to inherit their thrones.

An early reptilian breakthrough was conquest of the ocean.  Turtles and sea serpents were already swimming before the extinction.  Some of them, like ichthyosaur in the mid-Triassic, grew as large as great white sharks.  Their bodies adapted to the marine environment to become streamlined with flippers.  They assumed such a strongly fish-like appearance that it is easy to forget that they were reptiles.  This is a good example of convergent evolution.  A common environment such as the shallow sea can shape entirely different creatures into similar physical forms.

Another case of convergent evolution is winged flight.  After insects, the next animals to take to the air were flying reptiles, pterosaurs, which evolved in the late Triassic Period.  Pterosaurs were not ancestral to birds, but just happened to evolve flight earlier.  Pterosaurs grew gradually larger over time.  The first known specimens were a meter long.  By the end of the Mesozoic, the monstrous quetzalcoatlus stood as tall as a giraffe and had a wingspan of 10 meters – a real-life dragon!

When we think of Mesozoic land reptiles, we are inclined to think of dinosaurs.  Of course, not all reptiles at the time were dinosaurs.  There were tortoises, lizards, snakes, crocodiles, and other varieties that don’t exist anymore.  The most important difference was that dinosaurs walked upright.  Instead of sprawling crocodilian legs, dinosaurs had erect legs directly beneath their bodies.  Like columns, upright legs are good at supporting weight.  This is one of the main anatomical features that allowed many species of dinosaurs to grow so large.  Upright legs also provided speed and agility.  Many dinosaurs were bipedal, walking on two legs, which freed up the front legs to be used as arms.  Dinosaurs dominated the landscape with these major advantages.

The first known dinosaurs were modest, no larger than dogs or cats.  They originated in the southern hemisphere in the middle Triassic Period, about 240 million years ago. 12 Their range eventually expanded across Pangaea; dinosaur fossils are now found on all continents.  Dinosaurs flourished especially in the Jurassic and Cretaceous Periods, when the famously large species appeared.  The largest land animal of all time, the brachiosaurus, was a Jurassic dinosaur.  This four-legged herbivore weighed about 40 tons and could eat leaves from treetops ten meters high.  “Jurassic Park” notwithstanding, the famous Tyrannosaurus, Velociraptor, and Triceratops came much later in the Cretaceous Period.

Some of the smallest bipedal dinosaurs went through significant transitions in the Jurassic Period.  They evolved feathers, probably for body insulation at first.  This suggests that they were becoming warm-blooded. 13 Their arms and hands evolved into wings, and wing feathers became specialized for flight.  The skull features of embryonic and baby dinosaurs were extended into adulthood. 14 The result of these transformations was the bird, a whole new class of vertebrates.  The Cretaceous Period saw an explosion of bird diversity and the refinement of body features for effective flight.  By 30 MYA, most modern bird orders had appeared, from songbirds to ostriches.

Mammals also evolved from a line of reptiles (not dinosaurs).  This speciation occurred in the Triassic Period.  The early appearance of mammals will be discussed further in the next section, in relation to human ancestors.  Most mammals remained small through the Mesozoic Period, and were relegated to the role of nocturnal burrowing insectivores.  They were largely unable to compete with dinosaurs or other large reptiles for dominance of the land.  There were exceptions – some Cretaceous mammals were large enough to feed on small dinosaurs! 15  The great mammal radiation occurred shortly after the dinosaur extinction.  Just ten million years into the Paleogene Period, mammals numbered 4,000 species, ranging from bats to whales. 16

In the plant kingdom, the headline of the Cretaceous Period was the success of angiosperms – plants that produce fruit and flowers.  These features serve valuable reproductive purposes.  Fruit encloses seeds, providing them with nutrients and allowing dormancy for seasonal growth.  Flowers and fruits serve to attract animals, which can carry seeds or pollen from one plant to another.  It is no coincidence that birds and flowers evolved aggressively together during the same time period, as they enjoyed a mutually beneficial symbiotic relationship.  The last major plant breakthrough was grasses, which appeared about 40 MYA.  Grasses include not only the short green blades in your front yard, but also bamboo and major grains such as wheat and corn.

The Mesozoic Era formally ended 66 million years ago.  Strong evidence links the end-Mesozoic extinction to at least one asteroid collision. 17  Scientists still debate whether this was the sole cause or the knockout punch after a period of climatic stress. 18 The second-worst mass extinction of all time, it spelled the end for dinosaurs, pterosaurs, and an entire sister order of birds.  Sea serpents vanished along with ammonites, shelled squid-like animals that had been around for 300 million years.


E.  Oil!

Petroleum is the liquid form of fossil fuel.  It formed mostly from the remains of plankton and algae at the bottom of the sea.  These organisms have been around for billions of years, and a modest amount of oil dates back to the Proterozoic Eon.  One interesting hypothesis suggests that these pre-petroleum layers of dead algae may have played a role in the end-Permian extinction!  A certain bacteria may have evolved the ability to decompose this matter 250 million years ago.  If so, the decomposition released untold megatons of pent-up carbon in the form of methane, a greenhouse gas more potent than carbon dioxide. 19

Formation of oil is a finicky process.  If conditions are not just right, oil will not accumulate and / or endure.  Certain times and places have been more productive than others.  Most oil was deposited in the Jurassic and Cretaceous Periods. 20

The formations of coal and oil have important similarities and differences.  Coal forms from forests on land, while oil forms mostly from single-celled organisms and biofilm underwater.  Both processes require a low-oxygen environment so that the dead matter does not decompose completely.  Since oxygen content decreases with water depth, the ocean floor is a good environment to preserve dead matter.  In a properly anoxic region of seafloor, the topsoil gets enriched with organic sludge.  Over millions of years, new sediments of sand and rock accumulate on top of it, and this layer of topsoil ends up meters or kilometers beneath the surface.  A combination of bacterial activity, heat, and pressure combine to eventually transform the sea sludge into oil.  This process is a breakdown of organic macromolecules, mostly lipids, into smaller and simpler hydrocarbon chains.  The bonds between the carbon and hydrogen atoms are high in potential energy, which is now released when people burn it.

The recipe demands great precision.  First, dead matter must build up more quickly than it decomposes, which is relatively rare.  After sedimentation, oil forms only within certain windows of temperature and depth. 21 If the wrong kind of bacteria finds the oil, it will be compromised.  In fact, most of the world’s oil has been degraded to some extent.  The overall process is pretty inefficient.  It took 100 tons of dead sea life to form each gallon of gasoline in your car! 22 Because this process is so lengthy, fine-tuned, and wasteful, oil is non-renewable over any span of time less than millions of years.  That is, once it is consumed, it cannot be replaced.

Since oil is liquid, its distribution depends on the geology of the bedrock.  A reservoir of oil requires a layer of rock that is porous like a sponge, so the oil can flow and pool.  It must be surrounded by non-porous rock so that it does not sink to lost depths or spill out onto the surface.  Natural gas is often trapped in a reservoir above the liquid petroleum.  Oil and gas can seep from one location to another when earthquakes or tectonic shifts dislocate the surrounding layers of rock.

Finding a large amount of useful oil in one place, then, is a rarity.  In fact, there was only one spot in the world that met all of the ideal circumstances.  In the Mesozoic Era, it was on the East Coast of Pangaea along the Tethys Sea, a large region of the Panthalassa Ocean ringed by islands.  This tropical coastline was rich in plankton and minerals.  Ocean currents deposited a large amount of dead matter onto the long continental shelf, producing a rich source rock.  The oil sank into particularly porous and permeable reservoir rock, and was then covered by a very thick layer of solid cap rock to hold it in place.  No major earthquakes upset the reservoir or its cap, and it was not contaminated by harmful bacteria.  Today, this region is located in the Middle East. 23 There is definitely oil to be found elsewhere throughout the world, but Middle Eastern oil, especially Saudi Arabian, is in a class of its own.  It is high not only in volume, but also in quality.  This streak of geologic good luck makes the Middle East the richest source of petroleum exports in the world today, with all its attendant political consequences.

Up to CHAPTER 8: THE LAST FEW HUNDRED-MILLION YEARS

Next, Section 8.II: Mammals And Primates


F.  Citations

  1. Berzin et al, “Orogenic evolution of the Ural Mountains: Results from an integrated seismic experiment”, Science Vol 274 (10/11/1996), pp. 220 – 221, http://artsandsciences.sc.edu/geol.archive/knapp/Publications/Science/Berzin%20et%20al.pdf (accessed 2/12/16)
  2. Robinson, P.L. (1973).  Paleoclimatology and continental drift: in Tarling, D.H., and Runcorn, S.K. eds. Implications of Continental Drift to the Earth Sciences, I: Academic Press, London, p. 449-476.
  3. Royer et al, “CO2 as a primary driver of Phanerozoic climate”, GSA Today vol. 14 no. 3 (March 2004), p. 8, http://www.geosociety.org/gsatoday/archive/14/3/pdf/i1052-5173-14-3-4.pdf (accessed 2/12/16)
  4. Bond et al, “An abrupt extinction in the middle Permian (Capitanian) of the Boreal realm (Spitsbergen) and its link to anoxia and acidification”, Geological Society of America Bulletin (2015), 127(9/10): 1411, doi: 10.1130/B31216.1, http://eprints.whiterose.ac.uk/85117/7/1411.full.pdf (accessed 2/04/16)
  5. Stearns, Beverly Peterson; Stearns, S. C.; Stearns, Stephen C. (2000). Watching, from the Edge of Extinction. Yale University Press. p. 1921. ISBN 978-0-300-08469-6. (accessed 6/07/15)
  6. The Siberian Traps hypothesis is widely supported but still has loose ends to tie up.  It is the subject of current research.  The most readable and convincing account of the Siberian Traps is Benton, Michael J. (2003), When Life Nearly Died: the Greatest Mass Extinction of all Time.  Thames & Hudson, 336 pp.  ISBN 0-500-05116-X
  7. Bond, D.P.G., and Wignall, P.B., 2014, “Large igneous provinces and mass extinctions: An update,” in Keller, G., and Kerr, A.C., eds., “Volcanism, Impacts, and Mass Extinctions: Causes and Effects” Geological Society of America Special Paper 505, p. 29–55, doi:10.1130/2014.2505(02)
  8. Mason, John, “The cause of the greatest mass-extinctions of all? Pollution”, Skeptical Science, 3/19/15, https://www.skepticalscience.com/print.php?n=2884 (accessed 3/21/16)
  9. Carroll, R.L. 1988. Vertebrate paleontology and evolution. W. H. Freeman, San Francisco.
  10. Feng, Qinglai, Algeo, Thomas J., “Evolution of oceanic redox conditions during the Permo-Triassic: Evidence from radiolarian deepwater facies,” Earth Science Reviews (2014), doi: 10.1016/j.earscirev.2013.12.003, http://973.geobiology.cn/photo/2014021749232577.pdf (accessed 3/05/16)
  11. Sun et al, “Lethally hot temperatures during the early Triassic greenhouse”, Science, 10/19/2012, Vol. 338, Issue 6105, pp. 366-370, http://science.sciencemag.org/content/338/6105/366 (accessed 3/05/16).  You need to create a login account to Science magazine online to view the article; it’s free.  Also see the figure adapted from the article, showing Earth’s Triassic dead zone at https://www.sciencedaily.com/releases/2012/10/121018141844.htm (accessed 3/05/16).
  12. Nesbitt et al, “The oldest dinosaur? A Middle Triassic dinosauriform from Tanzania”, Biology Letters, 12/05/12, DOI: 10.1098/rsbl.2012.0949, http://rsbl.royalsocietypublishing.org/content/9/1/20120949 (accessed 3/20/16)
  13. Chen et al, “An exceptionally well-preserved theropod dinosaur from the Yixian Formation of China”, Nature 391: 147-152, 1/08/1998, http://www.nature.com/nature/journal/v391/n6663/full/391147a0.html (paysite; abstract accessed 3/20/16)
  14. Bhullar et al, “Birds have paedomorphic dinosaur skulls”, Nature 487(7406):223-6, 7/12/2012,  doi: 10.1038/nature11146, http://www.ncbi.nlm.nih.gov/pubmed/22722850 (Paysite, abstract accessed 3/20/16)
  15. Hu et al, “Large Mesozoic mammals fed on young dinosaurs”, Nature 433, 149-152 (1/13/2005), http://www.nature.com/nature/journal/v433/n7022/full/nature03102.html (paysite; abstract accessed 4/17/16)
  16. “The Rise of Mammals”, PBS (2001), http://www.pbs.org/wgbh/evolution/library/03/1/l_031_01.html (accessed 4/26/16)
  17. Renne, Paul R.; Deino, Alan L.; Hilgen, Frederik J.; Kuiper, Klaudia F.; Mark, Darren F.; Mitchell, William S.; Morgan, Leah E.; Mundil, Roland; Smit, Jan (7 February 2013). “Time Scales of Critical Events Around the Cretaceous-Paleogene Boundary”. Science 339 (6120): 684–687
  18. See e.g. McKinnon, “K-Pg Extinction Event: Theories”, 12/21/13, http://www.geomika.com/blog/2013/12/21/k-pg-extinction-event-theories/ (accessed 4/24/16)
  19. Rothman et al, “Methanogenic burst in the end-Permian carbon cycle”, Proceedings of the National Academy of Sciences, vol. 111 no. 15, April 15, 2014, http://www.pnas.org/content/111/15/5462.full.pdf (accessed 4/08/16).  It should be clarified that this hypothesis is still “speculative” even in the authors’ own words.
  20. Smil, Vaclav, Oil: A Beginner’s Guide, Oneworld Publications, 2008, ebook edition 2011, ISBN 978-1-78074-036-8, Location 1169.
  21. Smil, Vaclav, Oil: A Beginner’s Guide, Oneworld Publications, 2008, ebook edition 2011, ISBN 978-1-78074-036-8, location 1065.
  22. Smil, Vaclav, Oil: A Beginner’s Guide, Oneworld Publications, 2008, ebook edition 2011, ISBN 978-1-78074-036-8, location 1101.
  23. Sorkhabi, Rasoul, “Why so much oil in the Middle East?” GeoExPro vol. 7 no. 1 (2010), http://www.geoexpro.com/articles/2010/01/why-so-much-oil-in-the-middle-east-cd8a38e3-b5f5-462e-979e-2b3bb804dcee (accessed 4/08/16). 
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