The Carboniferous Period – Land of the Giants

360 to 300 Million Years Ago

 

Giant dragonflies, swamps filled with towering trees, lush plant growth in every direction: these are the quintessential images of the paleo world and the reality for the denizens of the Carboniferous. A warmer climate and higher oxygen levels created this tropical world filled with large insects of frightening size: millipedes up to three feet in length, spiders with 18-inch legs and scorpions weighing in at fifty pounds.

During this period, the advancement of Life is moving at an astonishing pace. Amphibians lose their fish-like appearance and become more familiar to our modern eyes. Reptiles arrived on the scene, preferring dry land to the wet environments of their amphibious relatives. By the end of the Carboniferous, three great lines of reptiles have emerged: the diapsids, ancestors to dinosaurs, lizards and birds; the synapsids, ancestors to mammals; and the anapsids, ancient ancestors of turtles.¹ 

The new repoductive innovation of the amniote egg enabled the reptiles to lay their eggs on land.  They no longer had to rely on a body of water in which to lay their eggs, they could be independent of the water and lay their eggs anywhere on land, safe within a shell, for within this perfect little incubator, the reptile embryo need not fear starvation or dehydration.  (Even today the amphibians depend upon water to lay their eggs, but not so for reptiles.)

As plants thrived in the tropical climate they began to produce lignin, a bark-like substance that gave them the structural support needed to grow to gigantic proportions. Yet lignin posed a perplexing problem for the environment. Because it was a new substance and a very tough one at that, there were no microbes that could consume it. The normal mechanisms of decay were thrown out of sorts. As trees died or were toppled in storms, their trunks and branches collected on the forest floor, the pile becoming deeper and deeper as millions of years rolled by with no method for for breaking down the lignin. Buried by time and compressed by tectonic forces, these layers would become the vast coal deposits that would later bring about the industrial revolution and our modern age; thus the period acquired its name, carboniferous, meaning ”coal bearing”.

Oxygen, Carbon Dioxide and the Atmosphere

• Oxygen reached its highest level of any age: 35% compared to modern day measurements of 21%.
  
• Carbon Dioxide was the lowest in history (until modern times). The vast amounts of vegetation pulled CO2 from the air and it remain locked in their stalks and stems even upon death. The microbes crucial in the process of decay were overwhelmed, the plants did not decompose and huge amounts of CO2 remained trapped within the plant matter. As the plants were turned into coal, the  CO2 would remain buried for eons until released in our modern age by the burning of coal.
  

Geography – Continents and Oceans

• The supercontinent Gondwana continued to cover the South Pole; but in the first half of the period, it collided with North America as the great supercontinent of Pangea began to form.
  • The collision of North America with Gondwana is the last of three phases that built the Appalachian Mountains. This mountain-building episode is called the Allegheny Orogeny.
• The oceans of Panthalassa and Paleo-Thethys were confined within Pangea’s early circular form, but as the circle tightened and the continents drew together these seas would slowly disappear.
  
• By the late Carboniferous, as the continents began to merge to form Pangea, the marine environments disappeared, replaced by swamps and arid climates.
  
• The continent of Euramerica (North America and Europe) was covered in vast swamps setting the stage for future coal deposits.
  

Climate

• There continued to be extensive glaciers at the South Pole.
• Warm and tropical in many regions.
  
  • In these regions the fossilized trees lacked growth rings signifying that the climate was stable and unchanging.
  • On a longer time scale the rock formations of this period often alternate between coal and shale indicating there was cyclic melting of glaciers that inundated dry land producing the shale deposits. As the glaciers once again grew and water was trapped in the ice, the land once again dried allowing the vegetation (and the coal deposits) to return.
• Growing areas of aridity occurred in the central areas of the continents.
  

Plant Life

• Lignin first appeared, allowing plants (especially trees) to grow strong and tall while it inhibited decomposition.
  • Most of the world’s coal deposits (especially in North America and Europe) come from this period.
• Earliest relatives of the conifers (pine trees, etc) appeared.
• Seed-bearing plants became more sophisticated.
  

Animal Life

• The appearance of the amniotic egg freed Life from dependence on water.
  
  • The high levels of oxygen possibly played a considerable role in this innovation.
    
• Giant forms of dragonflies and other insects inhabit the land.
  
  • One theory gaining new ground is that the high oxygen levels of the period promoted the gigantic growth of these creatures whose respiratory system allowed them to take advantage of higher oxygen levels.
    
  • The high oxygen levels also played another important role: enabling the giant insects to fly. Higher oxygen provides greater air pressures to support such giant bodies in the air.
    
• Reptiles appeared. While they remained inconsequential during this time, they diversify into three major lines: diapsid, synapsids and anapsids.
• Sharks became abundant and diversified.
  

Extinction Event

• There were two minor extinctions during the Carboniferous: one in the middle of the period and another near the end.
  
• Depending on the plant species, the extinction rates ranged from 40% to 85%.
  
• Some animal group extinctions reached as high as 70%.
  
• Extinctions possibly due to climate change since specific groups of both plants and animals were impacted more than other groups.

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1. The more advanced members of the synapsids were called therapsids and include both mammals and the mammal-like reptiles such as dicynodonts and cynodonts.  Therapsids would not appear on the scene until the Early to Middle Permian and would include only the mammal-like reptiles (proto-mammals) since true mammals would not arrive until the Triassic.  As always, there is disagreement about these classifications and where both ancient and modern animals belong, but these are usually on the finer points that probably won't be of great concernn to most non-scientists.