The Pennsylvanian Era: The Age of Amphibians

The Pennsylvanian Era: The Age of Amphibians

The Pennsylvanian Era, lasting between 323.2 to 298.9 million years ago, is the second interval of the Carboniferous Era, the first one being the Mississippian Era. The Pennsylvanian Era also has three major divisions- the Bashkirian Era, the Moscovian Era, and the Kasimovian and Gzhelian Era, in chronological order.


Most of the geochronological units are identified and their timelines are defined by the rock bed formations, and the same was done with the Pennsylvanian Era, but the timeline remains uncertain by a couple of thousand others. And these rock beds are vastly spread in the U.S. state of Pennsylvania, hence the era being named as such.

Most artists conceptualize Pennsylvanian Era as being swampy, and it is true, but only for the equatorial regions. In the southern hemisphere, which then consisted of the Gondwana landmass, there were a series of ice ages, as a result of which, the water would continuously freeze and melt, causing irregular rising and lowering of the sea level. Even after hundreds of millions of years, researchers can study and find out about those changes in the sea levels from the sedimentary rock layers, that were subjected to the continuous lashes of the sea.


The sedimentary rocks have a similar composition throughout, containing sandstone, shale, coal, limestone, and again sandstone. Each of one these rock formations forms a unit, called cyclothem. When the ice in the Gondwana melted, the sea levels rose globally, and the sand from rivers and lakes was deposited in the coastal lowlands. Over time, these sand deposits grew in quantity and formed sandstones.


As the water of the seas became stagnant and formed a coastal swamp over the sandstones, the dead leaves and plants, tree trunks, and all forms of other organic material also got deposited, and decomposed under pressure to form coal. When the swamp finally submerged, the remains of shelly marine animals were also left, which became limestone. After the ice began to form again in Gondwana, the absence of the water layer led to the erosion of the limestone layer. This whole process formed one cyclothem and recorded the climatic cycle of the period. As many as 90 cyclothems have been found in just one area, one on top of the other.


Talking about life in this phase, the Pennsylvanian Era witnessed the birth of the first reptiles. These were only about a foot long and were outgunned by the amphibians, both in size and quantity. Insects were also present in this age, such as the dragonfly, with a wingspan of 2.5 ft, and cockroaches.

This era records about 1000 different species of cockroaches, thus gaining the informal title of the “age of cockroaches”. Fungi of all modern classes have also been found.  Plant life flourished in the coastal swamps near the equator, and was mostly dominated by ferns, horsetail rushes, lycopods, and conifers, among others. Some trees even reached heights of up to 100 feet, such as the Lepidodendron and the Sigillaria.


The subdivisions of this era are done in two ways- the ICS, which follows the Russian subdivision into four parts, the Bashkirian, the Moscovian, the Kasimovian, and the Gzehlian, in order of oldest to youngest.

The North American subdivision, however, follows five parts- the Morrowan, the Atokan, the Desmoinesian, the Missourian, and the Virgilian, in order of oldest to youngest.

The Europeans, on the other hand, follow a different division. According to them, the Carboniferous Epoch, under which the Pennsylvanian comes, is divided into two parts, the Early or Dinantian Era, and the Late or Silesian Era. And it is in this Silesian Era, that the Pennsylvanian lies. The divisions goes as follows- the Namurian, corresponding to early Bashkirian; the Westphalian, corresponding to the Late Bashkirian, Moscovian, and Kasimovian ages; and the Stephanian, corresponding to the Gzehlian age.


The Pennsylvanian Era ended around 299 million years ago and was marked by a dry climate, the gradual disappearance of the coastal swamps, changes in the flora and fauna of the time, and a retreat in the shallow oceans from the interior part of the continent. The formation of the supercontinent Pangea is what these changes are a result of, which marked the start of yet another interesting era, the Permian Period.

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