Extinction & Seed Plants

This time in Earth's history begins (in the Permian Period) with low carbon dioxide levels and high oxygen levels

This global cooling event created an ice-age which began in the Pennsylvanian Period (~320 mya). 

In the Permian (~270 mya), the Earth quickly shifted, from cold, to warm and dry. 

• Oxygen levels dropped during this time, and carbon dioxide levels soared, creating a greenhouse effect.

• The change in carbon dioxide may have been due to immense volcanic eruptions

During the Permian, and into the Triassic, seed plants became more dominant, and spore-bearing plants decreased

The Earth became warmer and drier, which was disadvantageous to spore-bearing plants, since they need standing water so sperm can externally swim to eggs.

• The advent of the seed avoids the necessity of standing water for fertilization

To answer this we would have to go back to the Late Devonian, when the first seed plants arise.

The earliest seed plants were fern-like with frond-like leaves; they were probably small herbs, vines, or short trees

The seeds of these plants were surrounded by a cup-like branch system, possibly for protection, aid in pollination, or both

The first complete understanding of "seed ferns" during the Carboniferous, paints a picture of plants that looked like tree ferns, with seeds hanging from the bottom of frond-like leaves

One prominent plant during this time was Glossopteris, or the tongue-fern

• This plant occupied many ecological niches during this time, existing as shrubs, trees, and even herbaceous plants 

• Glossopteris has distinct tongue-shaped leaves, and the amount of leaves in the fossil record also hints that the plants were deciduous, which helped scientists discover the concept of plate tectonics

• These tropical leaves are found in Antarctic rocks, so researchers proposed that the continents must have had different positions from the modern arrangement

Also during this time, seed plants like Cycads, and early conifer-like plants (Voltziales) flourish, but many spore-bearing plants and some early seed plants go extinct

Extinction is the loss or termination of a species in Earth's history. 

During certain periods during geologic history there have been mass extinctions, in which large percentages of species all go extinct at the same time

Most of the time, these mass extinctions are caused by relatively sudden environmental changes: warming, cooling, drought, increased nutrient cycling, etc.

The environmental disturbance can be caused by plants/animals changing nutrient cycles, volcanoes, meteorite impacts, etc.

The end of the Permian (~251 million years ago) is a lesser known extinction event in history, but represents the largest catastrophe for organisms in Earth's history. 

• The event saw the extinction of 96% of all marine animals, and 70% of land vertebrates

• Many of the spore-bearing plants (scale trees and arborescent horsetails) and ancient seed-bearing plants (Medullosids and Cordaites) also go extinct. 

The Permian extinction divides the Paleozoic Era from the Mesozoic Era.

There are several factors that all lead to the loss of species during the Permian extinction.

1. The Earth's land masses formed into one large super-continent called Pangaea. This affected ocean circulation patterns and sea-levels. The formation of super-continents during geologic history usually leads to extinction events.

2. This time recorded the largest volcano eruptions in Earth’s history; covering 77,000 sq. miles, which is the size of Nebraska and greater than the area of the North East USA. These eruptions emitted massive amounts of sulfur gasses creating large scale acid rain.

3. The volcanic events increased CO2 concentrations to 1,000-1,500 parts per million: current level on Earth is 400 ppm (as of 2013). This produced Greenhouse effect which increased Earth's average temperatures, and promoted drying of the continents.

4. The acid rain increased weathering and erosion of rocks and soils. The run-off of these minerals enter the ocean in large amounts creating blooms of plankton. Boom-bust populations of plankton remove oxygen from the top 100 meters of ocean killing many coastal sea-dwelling species.

A group of seed plants called the Corystosperms, were major plants of the Triassic, surviving until the Cretaceous

• These were small, woody plants living in the understory of forests, that had frond-like leaves (Dicroidium)

• Their seeds (Umkomasia) were born on stalks with helmet-like cupules, a structure surrounding the seed

Conifers and cycads survive into the Triassic

Ginkgoes and a cycad-like group (Bennettitales) appear during this time

Glossopteris disappears half-way through the Triassic