C. Middle Life
About 300 million years ago, the continents just 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, extending from pole to 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 Ural Mountains were formed when Europe joined Asia.1 The Appalachian Mountains of North America were part of the same orogeny as the Anti-Atlas Mountains of Africa. They recorded the joinder of two continents that are now an ocean apart.
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. 2
The assembly, consolidation, and breakup of Pangaea all had important consequences for life. Recall that plants and animals came ashore gradually 300 – 500 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. 3 Monsoons occur in tropical coastal zones such as today’s Indian Ocean. 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.
Differing 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. 4 High carbon-dioxide content causes a greenhouse effect and global warming. During high-oxygen periods, temperatures fall. The deepest cold spell was a late-Paleozoic 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 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 Paleozoic 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 most living things.
What is remarkable is that all terrestrial animal life evolved together on the same landmass. The ancestors of today’s amphibians, reptiles, mammals and birds 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.
1. Extinction and mass extinctions
Nothing is inevitable in life but death, taxes, and the death of taxa. 99.9% of species that have ever lived are now extinct, at least as defined by physical appearance. It is normal for a micro-percentage of all species to go extinct every year. 5
Occasionally, Earth purges her life at exceptional rates. A mass extinction is a global event that terminates a major cross section of species or that hits 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. A mass extinction can occur over thousands or millions of years, as populations gradually wither away.
2. Permian paradise
The late Permian Period was the grand finale of Paleozoic “old life”. Land and sea alike were rich with diverse communities of plants and animals. Coral reefs were the crown jewels of shallow coastal waters. Reefs already sheltered animals we’d recognize, 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. Sounds like paradise, doesn’t it?
3. The Siberian Traps
Don’t buy that time-travel vacation package yet. Those coral reefs were about to face the most devastating mass extinction in world history, 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 millennia, it would likely have the same effect (though we’d probably run out of gas well before that).
I hope you haven’t gotten too attached to armored fish. The impact of this climate change was more brutal than any genocide. The P-T extinction killed off almost all forms of life, 90% of the species living throughout the world! It’s safe to say that we narrowly avoided complete catastrophe and the end of all life on Earth. 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 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 Era 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.
1. Reptiles of every realm
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.
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 long extinct. 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.
3. Birds, mammals, fruits, and grains
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 evolved from a non-dinosaurian line of reptiles. 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 grains such as wheat and corn.
4. Oh no, not again
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 survived for an astounding 300 million years.
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. Most of it, though, was deposited in the Mesozoic Era. 19
Like coal, oil can only form in a low-oxygen environment where dead matter does not decompose completely. Since oxygen content decreases with water depth, the ocean floor is a good environment to preserve biofilm. 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. 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 process is finicky. If conditions are not just right, oil will not accumulate and / or endure. 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. 20 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! 21 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 thick layer of solid cap rock. The oil reserve was never disturbed by earthquakes or harmful bacteria. Today, this region is located in Southwest Asia. 22 This streak of geologic good luck is what makes the Arabian Peninsula the richest source of petroleum exports in the world today.
- Victor N. Puchkov, “The evolution of the Uralian orogen”, Geological Society, London, Special Publications 327, 161-195 (12/21/2009), https://sp.lyellcollection.org/content/327/1/161 (accessed and saved 8/25/19). ↩
- Maps by Jacquelyne Kious et al., USGS (1996), public domain, https://commons.wikimedia.org/wiki/File:Pangaea_to_present.gif (accessed and saved 2016). ↩
- Pamela Lamplugh Robinson, “Palaeoclimatology and Continental Drift”, in D.H. Tarling and S.K. Runcorn, eds., Implications of Continental Drift to the Earth Sciences, I: Academic Press (London, 1973) pp. 451-476, https://archive.org/details/in.ernet.dli.2015.120209/page/n431 (accessed and saved 8/25/19). ↩
- Dana L. Royer et al, “CO2 as a primary driver of Phanerozoic climate”, GSA Today vol. 14 no. 3 (March 2004), pp. 4-10, http://www.geosociety.org/gsatoday/archive/14/3/pdf/i1052-5173-14-3-4.pdf (accessed and saved 8/25/19). ↩
- Jurriaan M. De Vos et al., “Estimating the normal background rate of species extinction”, Conservation Biology 29(2):452-462 (April, 2015), https://www.ncbi.nlm.nih.gov/pubmed/25159086 (accessed and saved 8/25/19). ↩
- The Siberian Traps were formed during the P-T transition, and geologists have long suspected a connection. Evidence that the eruptions caused the extinction continues to grow. See e.g. S. D. Burgess, J. D. Muirhead, and S. A. Bowring, “Initial pulse of Siberian Traps sills as the trigger of the end-Permian mass extinction”, Nature Communications 8, article no. 164 (7/31/2017), https://www.nature.com/articles/s41467-017-00083-9 (accessed and saved 8/25/19). ↩
- David P. G. Bond and P. B. Wignall, “Large igneous provinces and mass extinctions: An update,” in Gerta Keller and Andrew C. Kerr, eds., Volcanism, Impacts, and Mass Extinctions: Causes and Effects, Geological Society of America Special Paper 505, pp. 29–55 (9/01/2014), https://pubs.geoscienceworld.org/books/book/674/chapter/3807763/large-igneous-provinces-and-mass-extinctions-an (accessed and saved 8/25/19). ↩
- John Mason, “The cause of the greatest mass-extinctions of all? Pollution”, Skeptical Science, 3/19/15, https://www.skepticalscience.com/print.php?n=2884 (accessed and saved 8/25/19). ↩
- Robert L. Carroll, Vertebrate Paleontology and Evolution, W. H. Freeman (San Francisco, 1988), p. 589. Archived at https://archive.org/details/vertebratepaleon0000carr/page/589 (accessed and saved 8/31/19). ↩
- Qinglai Feng and Thomas J. Algeo, “Evolution of oceanic redox conditions during the Permo-Triassic: Evidence from radiolarian deepwater facies,” Earth Science Reviews (2014), https://www.academia.edu/9839104 (accessed and saved 8/31/19). ↩
- Yadong Sun et al., “Lethally hot temperatures during the early Triassic greenhouse”, Science 338(6105):366-370 (10/19/2012), http://science.sciencemag.org/content/338/6105/366 (accessed and saved 8/31/19). ↩
- Sterling J. Nesbitt et al, “The oldest dinosaur? A Middle Triassic dinosauriform from Tanzania”, Biology Letters 9(1) (2/23/2013), http://rsbl.royalsocietypublishing.org/content/9/1/20120949 (accessed and saved 8/31/19). ↩
- Pei-ji Chen, Zhi-ming Dong and Shuo-nan Zhen, “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 (accessed and saved 9/01/19). ↩
- Bhart-Anjan Bhullar et al., “Birds have paedomorphic dinosaur skulls”, Nature 487(7406):223-6 (7/12/2012), http://www.ncbi.nlm.nih.gov/pubmed/22722850 (accessed and saved 9/01/19). ↩
- 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 (accessed and saved 9/01/19). ↩
- “The Rise of Mammals”, PBS (2001), http://www.pbs.org/wgbh/evolution/library/03/1/l_031_01.html (accessed and saved 9/01/19). ↩
- Paul R. Renne et al., “Time Scales of Critical Events Around the Cretaceous-Paleogene Boundary”, Science 339 (6120): 684–687 (2/07/2013), https://science.sciencemag.org/content/339/6120/684 (accessed and saved 9/01/19). ↩
- See e.g. Mika McKinnon, “K-Pg Extinction Event: Theories”, GeoMika (12/21/13), http://www.geomika.com/blog/2013/12/21/k-pg-extinction-event-theories/ (accessed and saved 9/01/19). ↩
- Vaclav Smil, Oil: A Beginner’s Guide, Oneworld Publications (Kindle ebook edition, 2011), Location 1169. ↩
- Smil, ibid., location 1065. ↩
- Smil, ibid., location 1101. ↩
- Rasoul Sorkhabi, “Why so much oil in the Middle East?” GeoExPro vol. 7 no. 1 (2010), https://www.geoexpro.com/articles/2010/07/why-so-much-oil-in-the-middle-east (accessed and saved 9/01/19). ↩
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