Cretaceous Period

The Cretaceous Period was the final period of the Mesozoic Era, spanning approximately 145 to 66 million years ago (Ma). It succeeded the Jurassic Period and preceded the Palaeogene Period, marking one of the most dynamic intervals in Earth’s geological and biological history. The Cretaceous witnessed the peak and extinction of the dinosaurs, the breakup of supercontinents, the emergence of flowering plants (angiosperms), and profound climatic and oceanic changes that reshaped life on Earth.

Geological Timeframe and Subdivisions

The term Cretaceous derives from the Latin word creta, meaning “chalk,” referencing the extensive chalk deposits found in Western Europe, notably in England’s White Cliffs of Dover. The period was first defined by the Belgian geologist Jean Baptiste Julien d’Omalius d’Halloy in 1822.
Geologists divide the Cretaceous into two epochs:

• Early Cretaceous (145–100.5 Ma)
• Late Cretaceous (100.5–66 Ma)

These epochs are further divided into several stages, including the Berriasian, Aptian, Albian, Cenomanian, Turonian, and Maastrichtian, among others. Each stage represents distinct stratigraphic layers characterised by particular fossil assemblages and sedimentary patterns.

Continental Drift and Plate Tectonics

During the Cretaceous, the configuration of the continents underwent major transformation due to plate tectonics. The supercontinent Pangaea, which had begun fragmenting during the Jurassic, continued to separate into distinct landmasses:

• Laurasia (comprising North America, Europe, and Asia) drifted northward.
• Gondwana (including South America, Africa, Antarctica, Australia, and the Indian subcontinent) fragmented further.

These movements gave rise to new ocean basins, including the South Atlantic Ocean, as South America split from Africa. The Tethys Sea, located between the northern and southern continents, gradually narrowed as the continents shifted.
This continental rearrangement influenced global climate and ocean circulation patterns. The drifting continents also led to the formation of new mountain ranges and sedimentary basins, while high sea levels created shallow inland seas such as the Western Interior Seaway in North America.

Climate and Environment

The Cretaceous climate was warm and humid, with higher global sea levels than today. There were no polar ice caps, and tropical conditions extended into higher latitudes. Carbon dioxide concentrations were significantly elevated, promoting lush vegetation across much of the planet.
High sea levels resulted from both thermal expansion of seawater and the rapid formation of new oceanic crust. As a result, nearly one-third of the Earth’s surface was submerged under shallow epicontinental seas. These environments fostered abundant marine life and extensive chalk and limestone deposits.
Towards the end of the period, evidence suggests cooling trends and climatic fluctuations, possibly linked to volcanic activity and changes in oceanic circulation.

Marine Life

The oceans during the Cretaceous teemed with diverse life forms. Marine reptiles such as mosasaurs, plesiosaurs, and ichthyosaurs dominated the seas. Large predatory fish and invertebrates, including ammonites and belemnites, flourished in marine ecosystems.
Coral reefs expanded, and rudist bivalves replaced corals as major reef builders in some regions. Planktonic foraminifera and coccolithophores proliferated, contributing to the vast chalk deposits characteristic of the period.
Sharks and ray-finned fish diversified, while the first modern groups of teleost fish appeared. These developments marked the foundation of many marine ecosystems seen in the modern era.

Terrestrial Life and Flora

On land, the Cretaceous was a period of ecological transformation. Dinosaurs remained the dominant terrestrial vertebrates, with remarkable diversity in form and size. Notable groups included:

• Theropods, such as Tyrannosaurus rex and Velociraptor, apex predators of their ecosystems.
• Sauropods, including Argentinosaurus and Alamosaurus, among the largest land animals ever to exist.
• Ornithischians, such as Triceratops, Ankylosaurus, and Iguanodon, representing herbivorous species adapted to varied diets.

The Cretaceous also saw the evolution of birds from small theropod dinosaurs. Early birds like Archaeopteryx and Hesperornis illustrate the transition from reptilian ancestors to modern avian forms.
A defining botanical development of the period was the emergence and rapid diversification of angiosperms (flowering plants) around 125 million years ago. Fossil evidence from genera like Archaefructus and Magnolia shows that flowering plants began to dominate terrestrial ecosystems, replacing gymnosperms such as cycads and conifers in many regions. This botanical revolution transformed food chains, influencing herbivorous dinosaurs and leading to new co-evolutionary relationships with insects such as bees.

Insects and Other Terrestrial Fauna

The rise of flowering plants spurred the diversification of pollinating insects, including early bees, butterflies, and ants. Lizards, snakes, turtles, and crocodilians thrived in various environments, while mammals, though still small and nocturnal, continued to evolve. Fossil evidence from Cretaceous mammals like Repenomamus and Multituberculata indicates increasing diversity and ecological specialisation.

End of the Dinosaurs and the K–Pg Extinction

The Cretaceous Period ended abruptly about 66 million years ago with the Cretaceous–Palaeogene (K–Pg) mass extinction, one of the most catastrophic events in Earth’s history. Approximately 75% of all species disappeared, including nearly all non-avian dinosaurs, many marine reptiles, ammonites, and planktonic species.
The most widely accepted explanation is the asteroid impact hypothesis, supported by the discovery of the Chicxulub Crater in the Yucatán Peninsula, Mexico. The impact released enormous amounts of energy, triggering wildfires, global darkness, acid rain, and a dramatic drop in temperatures—a “nuclear winter” effect that disrupted photosynthesis and food chains.
Additional factors, such as massive volcanic eruptions in the Deccan Traps of India and long-term climate changes, likely exacerbated environmental stress, contributing to the mass extinction.
Despite the devastation, the extinction event paved the way for the rise of mammals, birds, and modern ecosystems in the ensuing Palaeogene Period, initiating the Cenozoic Era.

Fossil Record and Geological Evidence

Cretaceous rocks are widespread across all continents, containing rich fossil deposits that provide insights into the period’s biodiversity and environmental conditions. Notable fossil sites include:

• The Hell Creek Formation (USA) – famous for Tyrannosaurus rex and Triceratops fossils.
• The Gobi Desert (Mongolia) – yielding well-preserved dinosaur eggs and skeletons.
• The Western Interior Seaway deposits (North America) – containing marine fossils such as mosasaurs and ammonites.
• The Chalk cliffs of Dover (England) – remnants of microscopic marine organisms.

Advances in radiometric dating and palaeomagnetic studies have refined the Cretaceous timescale and enhanced understanding of global stratigraphic correlations.

Legacy and Significance

The Cretaceous Period was a time of transition, marking both the zenith of the age of reptiles and the dawn of modern biodiversity. Its legacy includes:

• The formation of present-day continental arrangements.
• The establishment of modern ecosystems, dominated by flowering plants and diverse animal groups.
• The Chicxulub impact event, a pivotal moment in Earth’s evolutionary history.