Devonian Period
Introduction
The Devonian Period, often referred to as the "Age of Fishes," is a significant era in the Paleozoic Era that spanned from approximately 419.2 million to 358.9 million years ago. This period is notable for its extensive diversification of fish, the first appearance of amphibians, and the establishment of vast terrestrial ecosystems. The Devonian is named after Devon, England, where rocks from this period were first studied.
Geological Setting
The Devonian Period is divided into three epochs: the Early Devonian (419.2–393.3 million years ago), the Middle Devonian (393.3–382.7 million years ago), and the Late Devonian (382.7–358.9 million years ago). These epochs are further subdivided into stages based on significant events and fossil records.
Early Devonian
The Early Devonian epoch saw the continuation of the Caledonian Orogeny, a mountain-building event that began in the Silurian Period. This epoch is characterized by the formation of the Old Red Sandstone continent, which included parts of present-day North America, Greenland, and Europe.
Middle Devonian
During the Middle Devonian, the Acadian Orogeny began, leading to the formation of the Acadian Mountains in what is now the northeastern United States. This epoch also witnessed the widespread development of coral reefs and the first significant appearance of terrestrial plants.
Late Devonian
The Late Devonian is marked by the Kellwasser Event, a series of extinction events that significantly impacted marine life. This epoch also saw the rise of the first forests and the diversification of early amphibians.
Paleoclimate
The Devonian climate was generally warm, with high levels of atmospheric carbon dioxide. This greenhouse climate contributed to the extensive development of shallow marine environments and the proliferation of coral reefs. However, the Late Devonian experienced a series of cooling events, possibly linked to the expansion of terrestrial vegetation and the drawdown of atmospheric CO2.
Marine Life
The Devonian seas were teeming with life, particularly fish. This period saw the emergence of several major fish groups, including:
Placoderms
Placoderms were armored fish that dominated the Devonian seas. The most famous placoderm is Dunkleosteus, a large predatory fish with powerful jaws and sharp, bony plates.
Lobe-finned Fish
Lobe-finned fish, or Sarcopterygii, are significant for their evolutionary link to terrestrial vertebrates. The most well-known lobe-finned fish from this period is Eusthenopteron, which exhibits features that foreshadow the transition to land-dwelling animals.
Ray-finned Fish
Ray-finned fish, or Actinopterygii, also diversified during the Devonian. These fish are characterized by their fin rays, which provide support and flexibility.
Terrestrial Life
The Devonian Period is crucial for understanding the colonization of land by plants and animals.
Early Land Plants
The first vascular plants appeared in the Silurian, but it was during the Devonian that they diversified and spread across the land. Lycophytes, ferns, and the first seed plants, or gymnosperms, began to dominate terrestrial ecosystems.
Early Forests
The development of early forests had a profound impact on the Devonian landscape. These forests consisted primarily of large Archaeopteris trees, which could grow up to 30 meters tall. The establishment of forests contributed to soil formation and altered the carbon cycle.
Early Terrestrial Animals
The Devonian also saw the emergence of the first terrestrial vertebrates. Ichthyostega and Acanthostega are among the earliest known amphibians, exhibiting both aquatic and terrestrial adaptations.
Extinction Events
The Devonian Period experienced several extinction events, the most significant being the Kellwasser Event and the Hangenberg Event.
Kellwasser Event
The Kellwasser Event occurred during the Late Devonian and is characterized by a series of extinctions that affected marine life, particularly reef-building organisms and brachiopods. The causes of this event are still debated, but hypotheses include global cooling, anoxia, and changes in sea level.
Hangenberg Event
The Hangenberg Event marks the end of the Devonian Period and led to the extinction of many fish and marine invertebrates. This event is associated with a rapid drop in sea level and significant climatic changes.
Economic Importance
Devonian rocks are economically significant due to their rich deposits of fossil fuels, particularly oil and gas. The organic-rich shales of the Devonian, such as the Marcellus Shale in North America, are important sources of natural gas.