The Devonian period, spanning 419.2 to 358.9 million years ago, was a crucial time in Earth's history. Known as the "Age of Fishes," it saw rapid diversification of marine life, the emergence of the first forests, and significant changes in the planet's atmosphere and climate.

This period witnessed the formation of supercontinents, widespread shallow seas, and a generally warm climate. The Devonian's most notable developments include the evolution of vascular plants, the rise of the first forests, and the appearance of early tetrapods, setting the stage for vertebrate colonization of land.

Devonian period overview

• Occurred approximately 419.2 to 358.9 million years ago, following the Silurian and preceding the Carboniferous periods in the Paleozoic Era
• Characterized by the rapid diversification of fish, the appearance of the first forests, and significant changes in the Earth's atmosphere and climate
• Marked by two major extinction events at the end of the period, which significantly impacted marine life

Age of fishes

• The Devonian is often referred to as the "Age of Fishes" due to the remarkable diversification and evolution of fish during this time
• Placoderms, lobe-finned fishes, and ray-finned fishes all underwent significant adaptive radiations
• The period also saw the emergence of the first tetrapods, four-limbed vertebrates that would eventually give rise to amphibians and all other land vertebrates

Temporal range

• The Devonian spans a time interval of approximately 60.3 million years
• Subdivided into three epochs: the Early Devonian (419.2 to 393.3 Ma), Middle Devonian (393.3 to 382.7 Ma), and Late Devonian (382.7 to 358.9 Ma)
• Each epoch is further divided into stages based on the appearance and disappearance of specific fossils and changes in the global environment

Devonian paleogeography

• The Devonian was characterized by the continued breakup of the supercontinent Gondwana and the formation of the supercontinent Euramerica (also known as Laurussia)
• The collision of Laurussia and Gondwana began in the Late Devonian, eventually leading to the formation of the supercontinent Pangaea in the Permian

Laurussia vs Gondwana

• Laurussia formed from the collision of the Laurentian and Baltic cratons during the Caledonian Orogeny in the Early Devonian
• Gondwana, which included modern-day Africa, South America, Australia, Antarctica, and India, was located in the southern hemisphere
• The two supercontinents were separated by the Rheic Ocean, which progressively closed during the Devonian due to the convergence of Laurussia and Gondwana

Shallow seas

• Much of Laurussia and parts of Gondwana were covered by shallow, warm seas during the Devonian
• These epicontinental seas provided habitat for diverse marine life, including reef-building organisms like stromatoporoids and corals
• Fluctuations in sea level throughout the Devonian led to the repeated flooding and exposure of continental shelves, influencing sediment deposition and fossil preservation

Devonian climate

• The Devonian climate was generally warm and humid, with atmospheric CO2 levels estimated to be several times higher than present-day levels
• The period began with greenhouse conditions but experienced a gradual cooling trend, culminating in a brief glacial episode near the end of the Devonian

Greenhouse conditions

• The Early Devonian was characterized by a greenhouse climate, with high atmospheric CO2 levels and global temperatures
• Lack of significant continental glaciation and the presence of tropical flora and fauna at high latitudes provide evidence for these warm conditions
• Greenhouse conditions likely facilitated the expansion of vascular plants and the development of the first forests

Glaciation evidence

• Despite the overall warm climate, evidence suggests a brief glacial episode occurred during the Late Devonian
• Glacial deposits have been found in South America and Africa, indicating the presence of ice sheets on parts of Gondwana
• The onset of glaciation may have been triggered by a drawdown of atmospheric CO2 levels, possibly linked to the rise of vascular plants and increased weathering rates

Devonian flora

• The Devonian saw the rapid evolution and diversification of vascular plants, which had first appeared in the Silurian
• Early Devonian flora was dominated by small, herbaceous plants, but by the Late Devonian, the first forests had emerged, transforming terrestrial ecosystems

Early vascular plants

• Early Devonian vascular plants were small and lacked true roots and leaves, relying on rhizoids for anchorage and sporangia for reproduction
• Genera such as Cooksonia and Rhynia represent some of the earliest known vascular plants
• As the Devonian progressed, plants evolved more complex vascular systems, true roots, leaves, and secondary growth, enabling them to grow taller and colonize new habitats

Archaeopteris forests

• By the Late Devonian, the first forests had appeared, dominated by the tree-like lycophyte Archaeopteris
• Archaeopteris could grow up to 30 meters tall and had a woody trunk, true leaves, and an extensive root system
• The rise of Archaeopteris forests had significant impacts on the global carbon cycle, weathering rates, and atmospheric composition, potentially contributing to the Late Devonian extinction events

Devonian fauna

• The Devonian is renowned for the diversification of fish and the appearance of the first tetrapods, but the period also saw the continued evolution of invertebrate life
• Marine ecosystems were dominated by placoderms, lobe-finned fishes, and ray-finned fishes, while invertebrates such as trilobites, brachiopods, and crinoids remained abundant

Placoderms

• Placoderms were a diverse group of armored fishes that dominated Devonian seas
• They were characterized by bony plates covering their head and thorax, and some species, such as Dunkleosteus, grew to massive sizes (up to 10 meters long)
• Placoderms went extinct at the end of the Devonian, possibly due to competition with other fish groups or the Late Devonian extinction events

Lobe-finned fishes

• Lobe-finned fishes, or sarcopterygians, include the ancestors of tetrapods as well as the coelacanths and lungfishes
• They are characterized by fleshy, lobed fins containing bones homologous to those in the limbs of tetrapods
• During the Devonian, lobe-finned fishes diversified and gave rise to the first tetrapods, which began to venture onto land

Ray-finned fishes

• Ray-finned fishes, or actinopterygians, are the most diverse group of vertebrates today, encompassing nearly all modern fish species
• In the Devonian, ray-finned fishes underwent a major adaptive radiation, evolving a wide range of body forms and feeding strategies
• Notable Devonian ray-finned fishes include the predatory Cheirolepis and the deep-bodied Mimipiscis

Early tetrapods

• The first tetrapods, or four-limbed vertebrates, evolved from lobe-finned fishes during the Late Devonian
• genera such as Acanthostega and Ichthyostega had a mix of fish-like and tetrapod features, including gills, fins, and primitive lungs
• These early tetrapods were still primarily aquatic but had the ability to venture onto land, setting the stage for the vertebrate colonization of terrestrial habitats

Invertebrate diversity

• Devonian seas hosted a diverse array of invertebrate life, including trilobites, brachiopods, mollusks, and echinoderms
• Trilobites, which had dominated Paleozoic seas, began to decline in diversity during the Devonian, possibly due to competition with other arthropods and fish
• Brachiopods and reef-building organisms like stromatoporoids and tabulate corals were abundant in shallow marine environments

Devonian mass extinctions

• The Devonian Period is marked by several extinction events, the most significant being the Late Devonian extinction
• These extinctions had profound impacts on marine ecosystems, leading to the disappearance of many iconic Devonian taxa

Late Devonian extinction

• The Late Devonian extinction was a prolonged series of extinction pulses that occurred over several million years, culminating in the Hangenberg Event at the Devonian-Carboniferous boundary
• It is estimated that up to 70% of marine species went extinct during this time, including all placoderms, many trilobites, and numerous brachiopod and coral species
• The extinction disproportionately affected shallow marine ecosystems, particularly reef communities

Causes of extinctions

• The causes of the Late Devonian extinction are still debated, but several factors likely contributed:
  1. Global cooling and glaciation, which lowered sea levels and disrupted marine habitats
  2. Reduced oxygen levels in the oceans, possibly due to eutrophication from the spread of vascular plants on land
  3. Volcanic activity and associated changes in atmospheric composition and climate
  4. The evolution of land plants, which altered weathering rates and nutrient cycling
• The combination of these factors likely placed significant stress on marine ecosystems, leading to the observed extinction patterns

Devonian reef systems

• Devonian seas were characterized by extensive reef systems, which hosted diverse communities of marine organisms
• Reefs were constructed primarily by stromatoporoids and tabulate corals, with contributions from other organisms such as rugose corals, bryozoans, and calcareous algae

Stromatoporoid reefs

• Stromatoporoids, a group of sponge-like organisms, were the dominant reef-builders in the Devonian
• They formed large, mound-like structures that provided habitat for a wide range of marine life
• Stromatoporoid reefs were particularly abundant in the shallow, warm seas that covered much of Laurussia during the Devonian

Coral reefs

• Tabulate and rugose corals also played a significant role in Devonian reef construction
• Tabulate corals, named for their table-like appearance, formed large colonies that contributed to reef growth
• Rugose corals, also known as horn corals, were solitary or colonial and added to the structural complexity of Devonian reefs
• The Late Devonian extinction events had a severe impact on reef communities, leading to the collapse of many reef systems

Devonian geology

• The Devonian Period is represented by a wide range of sedimentary and volcanic rocks, reflecting the diverse environments and tectonic settings of the time
• Two notable geological formations from the Devonian are the Old Red Sandstone and the Catskill Delta

Old Red Sandstone

• The Old Red Sandstone is a sequence of sedimentary rocks, primarily sandstones and conglomerates, that were deposited in non-marine environments during the Devonian
• These rocks are found in many parts of Europe and North America and are often red in color due to the presence of iron oxides
• The Old Red Sandstone preserves a rich fossil record of Devonian fish and early tetrapods, as well as trace fossils of terrestrial arthropods

Catskill Delta

• The Catskill Delta was a large, prograding delta system that formed along the eastern coast of Laurussia during the Late Devonian
• It is composed of a thick sequence of sandstones, shales, and conglomerates that were deposited in fluvial, estuarine, and shallow marine environments
• The Catskill Delta preserves a diverse fossil assemblage, including early vascular plants, freshwater fish, and some of the earliest known tetrapod remains

Economic resources

• Devonian rocks are important sources of economic resources, particularly hydrocarbons such as oil and natural gas
• These resources are often associated with organic-rich shales that were deposited in anoxic marine environments during the Devonian

Devonian oil shales

• Devonian oil shales are fine-grained sedimentary rocks that contain significant amounts of organic matter, which can be converted into oil through heating
• Major Devonian oil shale deposits are found in the United States (e.g., the Green River Formation) and Estonia
• The high organic content of these shales is attributed to the abundant marine life and anoxic conditions that prevailed in many Devonian seas

Devonian gas reserves

• Devonian shales are also important sources of natural gas, particularly in North America
• The Marcellus Shale, which underlies parts of the Appalachian Basin in the eastern United States, is one of the largest natural gas plays in the world
• Other significant Devonian gas reserves are found in the Barnett Shale of Texas and the Horn River Basin of Canada
• The development of horizontal drilling and hydraulic fracturing technologies has made the extraction of gas from these shales economically viable

Devonian in the fossil record

• The Devonian Period is well-represented in the fossil record, with a diverse array of marine and terrestrial organisms preserved in sedimentary rocks worldwide
• Some Devonian fossil sites are known for their exceptional preservation, providing unique insights into the anatomy and ecology of ancient life

Exceptional preservation

• Exceptional preservation refers to fossil sites where organisms are preserved with fine anatomical details, such as soft tissues, that are not typically fossilized
• Devonian sites with exceptional preservation include the Hunsrück Slate of Germany, which preserves pyritized soft-bodied organisms, and the Gogo Formation of Australia, known for its three-dimensionally preserved fish fossils
• These sites offer rare glimpses into the biology of Devonian organisms and help paleontologists reconstruct ancient ecosystems

Lagerstätten deposits

• Lagerstätten (singular: Lagerstätte) are fossil deposits characterized by their exceptional preservation, diversity, and/or abundance of fossils
• Notable Devonian Lagerstätten include:
  1. The Rhynie Chert of Scotland, which preserves early vascular plants and terrestrial arthropods in exquisite detail
  2. The Miguasha National Park in Quebec, Canada, renowned for its diverse assemblage of Devonian fish, including early tetrapods
  3. The Cleveland Shale of Ohio, USA, which has yielded numerous well-preserved shark and placoderm fossils
• These Lagerstätten provide invaluable windows into Devonian life and have greatly contributed to our understanding of the period's biodiversity and evolutionary history