4.3 Silurian period
6 min read•august 20, 2024
The Silurian period, spanning from 443.8 to 419.2 million years ago, marked significant changes in Earth's climate, geography, and life. This era saw the formation of the supercontinent Euramerica and the gradual closure of ancient oceans.
During the Silurian, marine life thrived with diverse invertebrates, while early plants began colonizing land. The period witnessed fluctuations in global temperatures, minor glaciations, and the emergence of jawed fishes, setting the stage for future evolutionary developments.
Silurian period overview
- The Silurian period is part of the Paleozoic era, spanning from approximately 443.8 to 419.2 million years ago
- Follows the Ordovician period and precedes the Devonian period
- Characterized by significant changes in Earth's climate, geography, and biosphere
Defining the Silurian
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Diorama of a Silurian seafloor - eurypterid (sea scorpion)… | Flickr View original
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Top images from around the web for Defining the Silurian
Diorama of a Silurian seafloor - eurypterid (sea scorpion)… | Flickr View original
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2012amphibians - Silurian Period View original
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Life in the Silurian | Visit my blog at ideonexus.com | Ryan Somma | Flickr View original
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Diorama of a Silurian seafloor - eurypterid (sea scorpion)… | Flickr View original
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- Named after the Celtic tribe of the Silures, who inhabited the Welsh Borderlands where Silurian rocks were first studied
- Defined by the first appearance of the graptolite Akidograptus ascensus and ends with the first appearance of the graptolite Monograptus uniformis
- Subdivided into four epochs: , , , and
Silurian timeline
- Llandovery epoch (443.8 to 433.4 million years ago)
- Wenlock epoch (433.4 to 427.4 million years ago)
- Ludlow epoch (427.4 to 423.0 million years ago)
- Pridoli epoch (423.0 to 419.2 million years ago)
Silurian paleogeography
- Significant changes in the configuration and position of continents during the Silurian
- Continued the trend of continental convergence that began in the Ordovician
Continental configurations
- Gradual closure of the Iapetus Ocean as Laurentia, Baltica, and Avalonia converged
- Formation of the supercontinent Euramerica (also known as Laurussia) through the collision of Laurentia and Baltica
- Gondwana, a large southern landmass, drifted over the South Pole
Major landmasses
- Laurentia (present-day North America, Greenland, and parts of Europe)
- Baltica (present-day Scandinavia and Baltic region)
- Avalonia (present-day British Isles, parts of New England, and Atlantic Canada)
- Gondwana (present-day South America, Africa, Australia, Antarctica, India, and Arabia)
- Siberia and Kazakhstan as separate landmasses
Silurian oceans
- Panthalassic Ocean surrounding the landmasses
- Rheic Ocean separating Gondwana from Euramerica
- Paleo-Tethys Ocean between Gondwana and Siberia/Kazakhstan
- Ural Ocean between Siberia and Baltica
Silurian climate
- Warmer global temperatures compared to the Late Ordovician
- Fluctuations in climate throughout the period
Global temperatures
- Early Silurian characterized by a warm climate following the end-Ordovician glaciation
- Cooling trend during the Wenlock epoch
- Ludlow epoch marked by a return to warmer conditions
- Late Silurian experienced another cooling phase
Glaciation events
- Remnants of the Late Ordovician glaciation in the Early Silurian
- Minor glaciation events during the Wenlock and Late Silurian cooling phases
- Glaciations primarily affected the southern hemisphere due to Gondwana's position over the South Pole
Atmospheric composition
- Oxygen levels estimated to be around 15-20% (compared to 21% today)
- Carbon dioxide levels were higher than present day, contributing to a greenhouse effect
- Gradual decline in atmospheric CO2 throughout the Silurian, possibly due to increased weathering and burial of organic matter
Silurian stratigraphy
- Silurian strata are well-represented in many parts of the world
- Characterized by a variety of sedimentary rocks, including limestones, shales, and sandstones
Lower vs Upper Silurian
- Lower Silurian includes the Llandovery and Wenlock epochs
- Upper Silurian includes the Ludlow and Pridoli epochs
- Boundary between Lower and Upper Silurian marked by changes in faunal assemblages and sedimentary facies
Key Silurian formations
- (UK): Consists of graptolitic shales and mudstones
- (UK): Fossiliferous limestone with abundant marine invertebrates
- Ludlow Series (UK): Includes the Ludlow Bone Bed, a famous fossil assemblage of early vertebrates
- (US): Dolomitic limestone in the Michigan Basin
Silurian stratotypes
- Global Boundary Stratotype Section and Point (GSSP) for the base of the Silurian located at Dob's Linn, Scotland
- GSSPs for the bases of the Wenlock, Ludlow, and Pridoli epochs located in the UK and the Czech Republic
Silurian biota
- Diversification of marine and terrestrial life during the Silurian
- Appearance of new clades and ecological innovations
Marine invertebrates
- Continued dominance of , , and crinoids in marine environments
- Diversification of , which serve as important index fossils for Silurian biostratigraphy
- Appearance of new groups such as eurypterids (sea scorpions) and cephalopods
Early land plants
- Colonization of terrestrial environments by early non-vascular plants (bryophytes)
- Appearance of the first vascular plants (tracheophytes) such as Cooksonia and Baragwanathia
- Development of simple rooting systems and conducting tissues for water and nutrient transport
Silurian reef systems
- Extensive development of reef systems during the Silurian
- primarily built by tabulate and rugose corals, stromatoporoids, and bryozoans
- Provided habitats for diverse marine invertebrate communities
Emergence of jawed fishes
- First appearance of jawed fishes (gnathostomes) in the Silurian
- Early jawed fishes include acanthodians and placoderms
- Jawed fishes would go on to dominate vertebrate evolution in the Devonian and beyond
Silurian extinctions
- Silurian period bookended by two extinction events
- Extinctions shaped the evolution and diversity of Silurian biota
End-Ordovician mass extinction
- One of the "Big Five" mass extinctions in Earth's history
- Occurred at the Ordovician-Silurian boundary (443.8 million years ago)
- Caused by a combination of factors, including global cooling, glaciation, and changes in sea level and ocean chemistry
- Resulted in the loss of approximately 85% of marine species
Late Silurian extinctions
- Series of extinction events in the Late Silurian, particularly affecting marine invertebrates
- Lau event (420 million years ago) associated with global cooling and a drop in sea level
- Kozlowskii event (424 million years ago) impacted graptolites and conodonts
Causes of Silurian extinctions
- Climate change, particularly cooling events and glaciations
- Changes in sea level and ocean chemistry
- Possible disruptions in the global carbon cycle
- Competition and ecological restructuring following the end-Ordovician mass extinction
Economic resources of Silurian
- Silurian rocks contain various resources of economic importance
- Hydrocarbons and mineral deposits are the most significant
Silurian hydrocarbon deposits
- Source rocks for many petroleum systems, particularly in North Africa and the Middle East
- Silurian shales are important source rocks due to their high organic content
- Examples include the Tanezzuft Formation in Libya and the Qusaiba Member in Saudi Arabia
Silurian mineral resources
- Silurian rocks host various mineral deposits, including base metals and industrial minerals
- Examples include lead-zinc deposits in Ireland (Irish Midlands) and Australia (Broken Hill)
- Silurian limestones and dolomites are used as building stones and in the production of cement
Silurian vs Devonian periods
- The Silurian is followed by the Devonian period in the Paleozoic era
- Significant changes in Earth's systems and biota between the two periods
Changes in biota
- Devonian marked by the diversification of jawed fishes and the appearance of tetrapods (four-limbed vertebrates)
- Expansion of terrestrial ecosystems in the Devonian, with the development of more complex vascular plants and the appearance of the first forests
- Decline of certain Silurian groups, such as graptolites and trilobites
Shifts in paleogeography
- Continued convergence of landmasses in the Devonian, leading to the formation of larger continents
- Closure of the Iapetus Ocean and widening of the Rheic Ocean
- Uplift of mountain ranges, such as the Caledonian Mountains, due to continental collisions
Climatic differences
- Devonian characterized by generally warm and stable climatic conditions
- Absence of major glaciations in the Devonian, in contrast to the Silurian
- Devonian atmosphere had higher oxygen levels compared to the Silurian, possibly due to the expansion of terrestrial plant life
Key Terms to Review (21)
Brachiopods: Brachiopods are marine animals with hard shells on the upper and lower surfaces, resembling clams but belonging to a different phylum. They were incredibly diverse and abundant in ancient seas, especially during early geological periods, and their fossils provide crucial insights into past marine environments and the evolutionary history of life on Earth.
Charles Lapworth: Charles Lapworth was a British geologist and paleontologist who is best known for his work in stratigraphy and the study of the Silurian period. He played a crucial role in establishing the geological timescale and is credited with defining the Silurian system, which helped to better understand Earth's history during this period. His research laid the groundwork for future paleontological studies and significantly advanced the field of geology.
Colonization of land: The colonization of land refers to the process by which organisms, particularly plants and animals, establish themselves and adapt to terrestrial environments. This critical event marked a significant shift in the evolution of life on Earth, as species transitioned from aquatic habitats to land, leading to new ecosystems and diverse biological innovations.
Coral fossils: Coral fossils are the preserved remains of ancient corals, primarily composed of calcium carbonate, that provide valuable insights into past marine environments and biodiversity. These fossils are significant as they can reveal information about the conditions of the oceans during the time they were formed, including climate changes and sea-level fluctuations, especially during the Silurian period, when corals experienced significant evolutionary developments.
Fish evolution: Fish evolution refers to the gradual process through which fish species have developed and diversified from their earliest ancestors over millions of years. This evolution is marked by significant changes in anatomy, physiology, and behavior, which allowed fish to adapt to various aquatic environments. The study of fish evolution helps to illuminate the origins of vertebrates and the complexities of aquatic ecosystems.
Graptolites: Graptolites are an extinct group of colonial marine animals that thrived during the Paleozoic era, particularly prominent from the Cambrian to the Devonian periods. They are characterized by their distinctive, often branching structures, which are composed of chitin and resemble small, tree-like forms. Graptolites serve as important index fossils for dating and correlating the age of sedimentary rock layers, especially during the Silurian period and in relation to the end-Ordovician extinction.
Llandovery: Llandovery is an age within the Silurian period, dating from approximately 443 to 433 million years ago. It marks a significant time in Earth's history, characterized by the diversification of marine life and the appearance of early terrestrial plants. The Llandovery age is essential for understanding the evolution of organisms during this era, as well as changes in climate and sea levels.
Llandovery Series: The Llandovery Series is the first series of the Silurian period, spanning from about 443.8 to 433.4 million years ago. It is known for significant evolutionary developments in marine life, including the appearance of the first jawed vertebrates and diverse coral reefs. The Llandovery marks a crucial transition in Earth's history as it follows the mass extinction at the end of the Ordovician period, leading to a recovery in biodiversity.
Ludlow: Ludlow refers to a specific time interval within the Silurian period, approximately 427 to 423 million years ago. This age is notable for significant geological and biological developments, including changes in marine biodiversity and the emergence of new species. The Ludlow interval serves as a critical point for understanding the evolution of early life on Earth, particularly in relation to the diverse ecosystems that flourished during this time.
Niagara Limestone: Niagara Limestone is a sedimentary rock formation that dates back to the Middle Silurian period, approximately 430 million years ago. It is primarily composed of limestone, which formed from the accumulation of marine organisms' remains, including coral and other carbonate materials. This formation is significant for understanding the geological history and paleoenvironments of North America during the Silurian period.
Pridoli: Pridoli is the uppermost age of the Silurian period, spanning from approximately 423 to 419 million years ago. This geological time frame is marked by significant developments in both terrestrial and marine ecosystems, including the diversification of early land plants and the proliferation of various fish species in the oceans. It serves as a transitional phase leading into the Devonian period, known for further advancements in life forms and ecological complexity.
Reefs: Reefs are underwater structures made from calcium carbonate, primarily formed by the skeletal remains of corals and other marine organisms. They serve as crucial ecosystems, providing habitat and protection for diverse marine life, while also playing a significant role in the health of marine environments and coastal protection against erosion.
Regression: Regression refers to a geological process where sea levels fall, causing the land to emerge relative to the sea. This process can significantly impact sedimentation patterns, marine environments, and the distribution of organisms. In geological history, regression is often marked by changes in fossil records and sedimentary layers that indicate shifting habitats from marine to terrestrial settings.
Shallow marine: Shallow marine refers to the areas of the ocean that are relatively close to shore and typically characterized by shallow waters, often less than 200 meters deep. These environments are crucial for various ecological processes and play a significant role in sediment deposition, biodiversity, and fossil formation, especially during specific geological periods.
Silurian Dolomite: Silurian Dolomite refers to a specific type of sedimentary rock primarily composed of dolomite, which was formed during the Silurian period, around 443 to 419 million years ago. This geological formation is significant because it reflects the environmental conditions of the time, characterized by warm shallow seas and diverse marine life. It also plays a crucial role in understanding the paleoecology and sedimentary processes that took place during the Silurian period.
Silurian-Devonian extinction event: The Silurian-Devonian extinction event refers to a significant biological crisis that occurred around 419 million years ago, marking a period of substantial loss of biodiversity between the Silurian and Devonian periods. This extinction event resulted in the decline of various marine organisms, including some groups of trilobites and reef builders, while paving the way for new life forms to thrive in the subsequent Devonian period, often called the 'Age of Fishes'.
Transgression: Transgression refers to the geological process where sea levels rise, causing marine waters to advance over land areas, leading to the flooding of previously exposed regions. This process can significantly alter the sedimentary record, often resulting in a sequence of deposits that reflects changing environmental conditions. It is a crucial concept in understanding the dynamics of sedimentation and the evolution of ancient marine environments.
Trilobites: Trilobites were marine arthropods that thrived during the Paleozoic era, known for their three-lobed body structure and diversity of forms. They are often considered one of the earliest complex life forms and play a crucial role in understanding evolutionary history, particularly during major geological events and periods.
Wenlock: Wenlock is a stage of the Silurian period that is characterized by specific geological and paleontological features, occurring approximately 433 to 427 million years ago. This time frame saw significant developments in marine life, particularly the rise of diverse coral reefs and various types of fish, marking an important phase in the evolution of early vertebrates. The Wenlock stage also played a critical role in shaping the geological landscape that influences many areas today.
Wenlock Limestone: Wenlock Limestone is a geological formation from the Silurian period, primarily composed of marine limestone and rich in fossils. This rock formation provides crucial insights into the marine environments and biodiversity of the time, showcasing a variety of life forms that thrived in ancient seas. Its significance extends to understanding sedimentary processes and paleoenvironments during this pivotal period in Earth's history.
William Smith: William Smith was an English geologist and civil engineer, often called the 'Father of English Geology' for his pioneering work in stratigraphy and the development of the principle of faunal succession. His groundbreaking geological maps laid the foundation for relative dating and biostratigraphy, significantly advancing the understanding of Earth's history and the distribution of fossils throughout different geological periods, including the Silurian.