The Mesozoic Era, from approximately 252.2 to 66 million years ago, was a period of profound geological and biological transformation. This timeframe is segmented into three periods: the Triassic, Jurassic, and Cretaceous. During this “Age of Dinosaurs,” the planet’s landmasses, climate, and dominant flora looked vastly different from the modern world.
Continents on the Move
At the onset of the Mesozoic Era, all Earth’s continents were conjoined into a single supercontinent known as Pangea. This immense landmass began its gradual fragmentation during the Late Triassic, shifting global geography. Pangea first split into two major landmasses: Laurasia in the north (future North America and Eurasia) and Gondwana in the south (future South America, Africa, Antarctica, Australia, and India).
The separation continued through the Middle Jurassic, with these landmasses further dividing and drifting apart. This continental rifting formed new ocean basins, including the widening Atlantic Ocean, and expanded the Tethys Sea, which cut between Gondwana and Laurasia. By the close of the Cretaceous, the continents had moved considerably, approaching their present-day positions.
A Warmer World
The Earth’s climate throughout the Mesozoic Era was generally much warmer than today, characterized by a pervasive greenhouse effect. A feature of this warm period was the complete absence of polar ice caps, indicating a different global heat distribution. Atmospheric carbon dioxide levels were considerably higher than pre-industrial levels, contributing to this warmer climate. For instance, the Late Jurassic saw CO2 concentrations around 1,200 parts per million (ppm), approximately four times higher than pre-industrial levels, while the Late Cretaceous registered about 750 ppm.
These elevated CO2 levels were likely maintained by substantial volcanic activity, which released vast quantities of greenhouse gases. The warm conditions resulted in less pronounced temperature differences between equatorial and polar latitudes. While generally warm, the Mesozoic climate experienced variations, with the Triassic interior often dry, the Jurassic becoming more humid, and a cooling trend emerging towards the end of the Cretaceous.
Dominant Plant Life
For most of the Mesozoic Era, the terrestrial landscape was dominated by gymnosperms, plants that produce “naked” seeds without flowers or fruits. Conifers, including ancestors of modern pines, cypresses, and redwoods, were widespread and formed extensive forests. Other gymnosperms included cycads, with their stout trunks and palm-like leaves, and ginkgoes, represented today by a single living species. These plants provided the primary food source for many herbivorous dinosaurs and shaped vast forested areas.
An evolutionary event occurred in the Early Cretaceous with the emergence and diversification of flowering plants, known as angiosperms. Initially, these early angiosperms were often small and herbaceous, radiating from tropical regions. By the Late Cretaceous, flowering plants had become increasingly widespread and diverse, beginning to replace conifers and other gymnosperms as the prevailing flora. This shift marked a fundamental change in Earth’s plant life, influencing the evolution of insects and other organisms.
Shaping the Landscape
Shallow inland seas, also known as epicontinental seas, were a prominent characteristic, particularly during the Cretaceous. A notable example is the Western Interior Seaway, which bisected North America for millions of years, creating two separate landmasses: Laramidia to the west and Appalachia to the east.
Global sea levels were considerably higher than present-day levels, due to increased seafloor spreading. This resulted in widespread marine transgressions, where seas advanced over continental interiors. The Mesozoic was also a time of volcanic activity, including the formation of igneous provinces like the Central Atlantic Magmatic Province (end of Triassic) and the Deccan Traps (end of Cretaceous). Mountain-building events also shaped the land, with the rise of major ranges such as the Rocky Mountains in western North America occurring during the Cretaceous.