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

• 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: Llandovery, Wenlock, Ludlow, and Pridoli

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

• Llandovery Series (UK): Consists of graptolitic shales and mudstones
• Wenlock Limestone (UK): Fossiliferous limestone with abundant marine invertebrates
• Ludlow Series (UK): Includes the Ludlow Bone Bed, a famous fossil assemblage of early vertebrates
• Silurian Dolomite (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 brachiopods, trilobites, and crinoids in marine environments
• Diversification of graptolites, 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
• Reefs 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