Two hundred and forty million years ago, Earth was in the midst of the Middle Triassic Period, a time of profound transformation. This era followed the Permian-Triassic extinction event, often called “The Great Dying,” which was the most severe mass extinction in the planet’s history. Earth was slowly re-establishing complex ecosystems after the catastrophic collapse of the late Paleozoic. This period was characterized by a unified landmass, a harsh global climate, and the rapid evolution of new life forms, particularly the archosaurs, which would soon dominate the terrestrial landscape.
The Supercontinent Pangaea
At 240 million years ago, all of Earth’s major continental crust was fused into a single, colossal landmass known as Pangaea. This immense supercontinent stretched from the planet’s northern to its southern pole, giving it a distinct, elongated, C-shaped configuration. The eastern curve of this gigantic landmass cradled a significant body of water, the Tethys Ocean, which indented Pangaea’s eastern margin.
Pangaea’s vast size and north-south orientation created a profound geographical isolation for its interior regions. It was surrounded by a single global ocean called Panthalassa. The deep interior of Pangaea was situated thousands of kilometers from any coastline, cutting it off from the moderating influence of the ocean.
Pangaea was composed of the northern part, Laurasia (including North America and Eurasia), connected to the southern part, Gondwana (including South America, Africa, India, Antarctica, and Australia). By the Middle Triassic, rifting—the process that would eventually break the supercontinent apart—was beginning, though the landmass remained essentially intact. The Neo-Tethys Ocean was actively opening between the Cimmerian terranes and Gondwana, signaling the future fragmentation of the continent.
Global Climate and Atmospheric Conditions
The unified geography of Pangaea was the primary driver of a globally warm and extreme climate system during the Middle Triassic. With nearly all land connected, there were no polar ice caps, contributing to an overall “greenhouse” world. Global temperatures were significantly higher than today, and the extensive landmass severely inhibited the distribution of moisture across the globe.
The interior was characterized by extreme aridity and pronounced seasonality. While coastal regions may have experienced some moisture, the center of the continent was a vast desert, with huge temperature fluctuations between seasons. This landscape was likely dominated by massive monsoonal weather systems, where seasonal shifts in air pressure over the enormous landmass drove intense but highly localized rainfall.
Following the massive volcanic eruptions that caused the end-Permian extinction, atmospheric carbon dioxide (\(\text{CO}_2\)) levels remained high. Estimates for \(\text{CO}_2\) concentration during the Early to Middle Triassic suggest levels were significantly elevated, up to three times higher than present-day concentrations. This high concentration of greenhouse gases amplified the global warming trend. The combination of high \(\text{CO}_2\) and the continental configuration resulted in a sustained, hot climate that lasted for millions of years.
Life on Land and Sea
The biological world at 240 Ma was defined by recovery and the rapid diversification of survivors from the mass extinction. Marine ecosystems, which had suffered catastrophic losses, were slowly re-establishing themselves in the warming oceans. New groups of marine reptiles began to emerge, including the dolphin-like Ichthyosaurs. The oceans also saw the diversification of modern-type corals, known as hexacorals, which began the slow process of rebuilding the great reef systems that had been destroyed.
On land, the flora was dominated by hardy, extinction-resistant plant groups. The landscape was largely covered by gymnosperms, or “naked seed” plants, which were well-suited to the arid conditions. Conifers, seed ferns (like Dicroidium), ginkgoes, and cycads were the major components of the forests and scrublands. The lack of extensive coal deposits, a phenomenon known as the “coal gap” in the Early Triassic, was finally ending, signaling that plant life was once again abundant enough to form peat layers.
The terrestrial vertebrate fauna was undergoing an evolutionary shift. The Synapsids, or “mammal-like reptiles,” which had dominated the preceding Permian Period, were in decline. While some Cynodonts, a group close to the ancestry of mammals, were present, they were small creatures. The rising group was the Archosaurs, a lineage that includes modern crocodiles and birds.
Early members of the Archosaur group, such as the ancestors of crocodilians (Pseudosuchians) and the earliest Dinosauromorphs, were diversifying rapidly. The first true dinosaurs, such as small, bipedal forms, appeared in the fossil record around this time, though they were not yet the giants of later periods.