The Devonian Period – The Age of Fishes

 415 to 360 Million Years Ago

 

The Devonian period was a time of many “firsts”: the first amphibians, the first sharks, the first plants with roots, leaves and stomata. Despite such significant innovations, the Devonian is best known for its diversity and abundance of fish. Not only did fish with cartilage first appear (such as sharks), but fish with bony skeletons developed and it is this type of fish which today comprises a majority of our modern fishes.

Other fish also appeared whose fins were made of strong nodules (lobes) of bones that could be used as legs. These lobed-finned fish forever changed Life on Earth as they became the first amphibians that crawled onto land and breathed in oxygen with their newly-formed lungs.

It was a period of great drama as oxygen levels fell and carbon dioxide levels rose causing chaos among all forms of life striving to adapt to the ever-changing conditions. By the Middle Devonian, one of the Five Great Mass Extinctions occurred devastating the marine communities and temporarily impeding the development on land and sea.

Oxygen, Carbon Dioxide and the Atmosphere

• Oxygen levels plummeted during this time, leaving marine environments with little or no oxygen, decimating or stunting marine communities.
  
• Carbon dioxide levels rose to their highest levels since the Ordovician.
  

Geography – Continents and Oceans

• The continental masses shifted southward. North America and Australia sat along the equator while most of the other continents were either south of the equator or resting at the South Pole.
  
• The supercontinent Pangea began to form at the end of the Devonian.
  

Climate

• The climate was predominantly warm with shallow seas.
  

Plant Life

• This period is referred to as the "Devonian Explosion" because of the significant advances in plants, their reproduction techniques as well as the plant structure
• The first seed plants appear in the form of seed ferns, a primitive version of the later gymnosperms of the Carboniferous period.
  
  • The advantages of these seed-bearing plants over their spore-bearing contemporaries are significant for the changing environment.  Seeds are able to withstand a variety of climate conditions while spores rely on water or watery conditions.
    
• Early Devonian plants were still small and restricted to shores and river banks.
  • They lacked roots, leaves and structural strength to grow taller than a few feet; but all these features would be present by the end of the period enabling the growth of large trees and other plants.
  • The Devonian witnessed the appearance of bark, but it wouldn't be until the Carbniferous that lignin would appear which would serve as a tough inner lining of the bark, giving even greater strength to trees and other plants.
    
  • The production of lignin in plant tissues provided the strength plants needed to grow straight and tall.
    
    • Lignin appeared late in the Late Devonian or Early Carboniferous. It will play a significant role in the Carboniferous period.
      
  • The stomatal cells of plants allowed them to absorb carbon dioxide and exhale oxygen. Plants thrive in the high carbon dioxide and low oxygen levels that held sway during much of the Devonian.
    
• By the end of the Devonian, the CO2 levels had crashed, causing devastation among the plant life.
  

Animal Life

• Amphibians appear at the end of the Devonian.
  
• Fish thrived in the Devonian in both marine and freshwater habitats; so much so that the Devonian is termed the “Age of Fishes”. The important innovations were as follows:
  
  • Fish with cartilage and those with rayed fins (fins supported by bony structures) appeared for the first time. Both types of fish are extant today: modern cartiliginous fish include sharks and stingrays while the rayed-fin fish is the predominant type of fish today.
    
  • Lobed-finned fishes eventually moved onto land becoming the first amphibians.
    
    • Another significant aspect of the lobed fishes was their ability to breathe oxygen using lungs and nostrils making them perfect candidates for living on dry land.
      
  • Jawed fish dominated the seas by the Late Devonian. (The first jawed fish appeared in the Silurian, but by the Devonian they were dominant - having jaws is a definite advantage over those without.
• Ammonites first appear, descending from the existing bactrites with their straight-coned shells.
  

Extinction Events

• The mass extinction in the latter portion of the Devonian ranks as one of the Five Great Mass Extinctions.
  
• As much as 56% of marine life was wiped out.
  
• Some marine areas show a complete lack of oxygen and unable to support life.
  
• Theories for the cause of the mass extinction include glaciations, meteor impact and low oxygen levels.