The Early Cretaceous period, spanning from approximately 145 to 100.5 million years ago, was a dynamic epoch of profound global transformations in Earth’s geological and biological history. It witnessed the continued reshaping of continents, significant shifts in climate, and the widespread diversification of life forms, laying groundwork for many modern ecosystems.
Earth’s Changing Face and Climate
During the Early Cretaceous, the supercontinent Pangea continued its fragmentation. A notable event was the progressive separation of South America from Africa, which led to the opening of the South Atlantic Ocean. India and Madagascar also began to rift away from Antarctica and Australia, forming the Eastern Indian Ocean.
These continental movements generated new coastlines and ocean basins, influencing global climate patterns. The planet was considerably warmer than today, with little to no ice at the poles. Sea levels were notably higher, ranging from 100 to 200 meters above present-day levels, partly due to increased seafloor spreading. These elevated sea levels created extensive shallow inland seas, contributing to milder local climates by transporting warm equatorial waters towards the poles.
Dominant Life on Land
Terrestrial ecosystems of the Early Cretaceous were dominated by diverse dinosaur groups. Large sauropods, like Brachiosaurus and Argentinosaurus, continued to roam the lush forests, and new types of ornithischians emerged. Argentinosaurus, among the largest animals to ever walk the Earth, used its long neck to browse tall trees.
Predatory theropods also diversified, with examples including Spinosaurus and Giganotosaurus. Spinosaurus, a semi-aquatic carnivore, hunted fish and possibly other terrestrial prey. Early tyrannosauroids were present, alongside other theropods such as Deinonychus and Acrocanthosaurus. Early mammals remained relatively small and lived alongside these dinosaurs.
A significant evolutionary event was the appearance and diversification of flowering plants, known as angiosperms. Fossil evidence of angiosperms became more prominent in the Early Cretaceous, with early families like Archaefructaceae appearing around 124.6 million years ago. These plants altered terrestrial landscapes and food webs, although they did not become ecologically dominant until the early Late Cretaceous.
Life in the Ancient Oceans and Skies
The Early Cretaceous oceans teemed with diverse marine reptiles. Plesiosaurs, characterized by broad, flat bodies and four long flippers, were common. These air-breathing reptiles, some with long necks like Elasmosaurus, fed on fish and ancient squid-like molluscs. Pliosaurs, a subgroup of plesiosaurs, had shorter necks and large heads, making them apex predators that hunted fish, sharks, and other plesiosaurs.
Ichthyosaurs, resembling modern dolphins with pointed heads and conical teeth, were fast-swimming marine reptiles that thrived. Fish populations continued to flourish, providing a food source for these large predators. In the skies, pterosaurs, the first vertebrates to evolve powered flight, exhibited diverse forms. Early birds also made their appearance, coexisting with pterosaurs.
Significant Geological Activity
The Early Cretaceous was characterized by substantial geological activity, including widespread volcanism. Large igneous provinces (LIPs), massive accumulations of volcanic rock, formed during this time. These formations are often linked to mantle plumes, where magma travels through the Earth’s crust to the surface.
For instance, the Paraná-Etendeka LIP, located in what is now South America and Africa, produced an estimated 1.5 million cubic kilometers of basalts and rhyolites beginning around 133 million years ago. The release of magmatic carbon dioxide from these volcanic events is believed to have contributed to global climate warming and environmental changes, including oceanic anoxic events where deep-sea currents stagnated and oxygen levels dropped. High global sea levels also led to the deposition of extensive sedimentary layers, including the chalk formations for which the broader Cretaceous period is named.