Devonian Period: The Age of Fishes and the Greening of Earth
Spanning approximately 60.7 million years, the Devonian Period stands as one of the most transformative eras in Earth's history. Occurring during the Paleozoic era, this period lasted from roughly 419.62 million years ago (Ma) to 358.86 Ma. It is famously nicknamed the "Age of Fishes" due to the incredible diversification of aquatic life, but its impact extends far beyond the oceans, marking the era when life truly began to claim the land.
Named after the county of Devon in South West England, where its defining rock layers were first studied, the Devonian represents a pivotal chapter in the Phanerozoic eon. From the rise of complex marine predators to the first sprawling forests, the world of the Devonian was a place of radical biological and geological change.
Key Facts
- Time Span: Approximately 419.62 to 358.86 million years ago.
- Nickname: Often referred to as the "Age of Fishes."
- Major Biological Milestones: The appearance of the first ammonoids, the rise of early forests, and the evolution of the first tetrapods (four-limbed vertebrates).
- Geological Context: The fourth period of both the Paleozoic and Phanerozoic eras.
- Mass Extinctions: The Late Devonian was marked by three significant extinction episodes.
Chronology and Subdivisions
The Devonian is divided into three distinct stages by the International Commission on Stratigraphy: the Early, Middle, and Late Devonian. Each stage represents a unique phase of evolutionary and environmental progression.
Early Devonian
Lasting from 419.62 to 393.47 Ma, the Early Devonian includes the Lochkovian, Pragian, and Emsian stages. This era saw the emergence of the first ammonoids—a class of cephalopod molluscs that descended from bactritoid nautiloids. These early ammonoids were relatively simple but would eventually evolve into dominant marine predators that ruled the seas until the Mesozoic Era.
Middle Devonian
The Middle Devonian served as a bridge of increasing complexity. During this time, the biological diversity of both land and sea continued to expand, setting the stage for the massive terrestrial shifts that would follow.
Late Devonian
The Late Devonian began with the Frasnian stage (382.31 to 372.15 Ma) and concluded with the Famennian stage. This was a period of profound transition. The Frasnian saw the formation of the first true forests, while the Famennian recorded the appearance of the first tetrapods—vertebrates that began the transition from water to land. However, this era was also defined by instability, ending with significant extinction events.
Paleogeography: A Shifting World
The map of the world during the Devonian looked vastly different from the one we recognize today. The supercontinent Laurussia (also known as Euramerica) was a major landmass, and the massive supercontinent Gondwana occupied much of the southern hemisphere. The movement of these plates and the closure of ancient oceans, such as the Rheic Ocean, fundamentally reshaped global climates and marine habitats.
The Evolution of Life
The Devonian is a masterclass in evolutionary biology, showcasing the rapid development of complex life forms across multiple environments.
Marine Life: The Age of Fishes
In the oceans, the Devonian was dominated by a massive variety of fish. This included early sharks, heavily armored placoderms (such as the giant Dunkleosteus), and various lobe-finned fishes. These lobe-finned species were particularly important, as they provided the evolutionary blueprint for the transition to land.
![Spindle diagram for the evolution of vertebrates[36]](/images/00/b6/00b617892a56a8de5068315edff254d8f7cf4efa977d4a2be186f5ee0bae6360.png)
Terrestrial Life: The Greening of the Land
On land, the Devonian witnessed a biological revolution. The period marked the beginning of extensive land colonization by plants. As large land-dwelling herbivores had not yet evolved, vast forests of shrubs and early trees grew unchecked, fundamentally altering the Earth's atmosphere and soil composition.
Even more mysterious were organisms like Prototaxites milwaukeensis, a massive fungal-like organism that existed during the Middle Devonian, representing a unique branch of life that shaped the early terrestrial landscape.
Summary of the Devonian Period
| Feature | Details |
|---|---|
| Time Range | 419.62 – 358.86 Ma |
| Major Landmasses | Laurussia, Gondwana, Siberia |
| Key Marine Life | Placoderms, early sharks, ammonoids, trilobites |
| Key Terrestrial Life | Early forests, vascular plants, first tetrapods |
| Climate Trend | Relatively steady sea levels, gradually falling from 189m to 120m |
Late Devonian Extinction Events
Despite the biological flourishing, the end of the Devonian was marked by turmoil. The Late Devonian is characterized by three distinct episodes of mass extinction. These events, including the Kellwasser and Hangenberg events, significantly reduced marine biodiversity and reshaped the evolutionary trajectory of life on Earth.
Frequently Asked Questions
Why is the Devonian called the "Age of Fishes"?
The period saw an unprecedented diversification of fish species, including the rise of armored placoderms, early sharks, and the lobe-finned fishes that would eventually evolve into land vertebrates.
What was the most significant change on land during this time?
The most significant change was the "greening of the land," where plants began to colonize terrestrial environments, leading to the development of the first large forests.
Where does the name "Devonian" come from?
The period is named after the county of Devon in South West England, where the rock strata defining this geological period were first studied.
Did humans exist during the Devonian?
No. The Devonian occurred hundreds of millions of years before the evolution of humans. During this time, life was primarily composed of fish, early plants, and primitive invertebrates.
What caused the Late Devonian extinctions?
While the exact causes are still being studied, the Late Devonian is characterized by three major extinction episodes that impacted marine life significantly.