The Carnian Age represents a dramatic chapter in Earth’s history, marked by profound global instability that fundamentally reshaped life on land and in the oceans. This ten-million-year span was a time of immense environmental upheaval, demonstrating how sudden climate change can serve as a powerful engine for evolution. It was within this era that the world experienced a massive biological turnover. The unique conditions of the Carnian Age set the ecological and climatic parameters that allowed the dominant reptiles of the Mesozoic, the dinosaurs, to take their first steps toward global supremacy.
Placing the Carnian Age in Geologic Time
The Carnian Age marks the beginning of the Late Triassic Epoch, following the Middle Triassic and preceding the Norian Age. It began approximately 237 million years ago and concluded around 227.3 million years ago, spanning a significant portion of the early Mesozoic Era. During this time, all of Earth’s major landmasses were still fused into the single supercontinent known as Pangaea.
This colossal landmass dictated a harsh global climate, characterized by vast, arid interiors far removed from oceanic moisture. The world was a “hothouse,” lacking polar ice caps, with a single global ocean, Panthalassa, wrapping around the supercontinent. The extreme distribution of land and sea fostered a system of “mega-monsoons” that controlled moisture distribution. The arid conditions meant that much of the terrestrial fossil record consists of red beds, which are sedimentary rocks stained by iron oxides that form in dry, oxygen-rich environments.
The Carnian Pluvial Episode
The period of relative stability was shattered by a massive, global-scale climate event known as the Carnian Pluvial Episode (CPE), or the “Carnian rainy period.” This episode of intense and prolonged global rainfall occurred around 234 to 232 million years ago and lasted for approximately one to two million years. The CPE represents a dramatic shift from the prevailing semi-arid conditions of Pangaea’s interior to a much warmer and highly humid climate.
The leading hypothesis for this climate disruption points to massive volcanic activity from the Wrangellia Large Igneous Province (LIP). This enormous volcanic system, now preserved in parts of western North America, erupted an estimated volume of over one million cubic kilometers of basalt onto the ocean floor. This sustained volcanism released vast quantities of greenhouse gases, particularly carbon dioxide and sulfur dioxide, into the atmosphere.
The injection of these gases triggered rapid global warming, which intensified the planet’s hydrological cycle. Warmer air holds more moisture, causing the mega-monsoon system of Pangaea to intensify dramatically, resulting in the prolonged, heavy rainfall seen globally. Evidence of this shift is found in the sedimentary record, where arid-adapted red beds are suddenly replaced by widespread deposits indicative of standing water, such as lake sediments and coal seams.
Geochemical evidence further supports this volcanic cause, including sharp negative shifts in carbon isotope ratios (delta-13-C) found in both marine and terrestrial sediments. These isotopic perturbations are consistent with a massive, rapid release of light carbon into the atmosphere and ocean system, a classic fingerprint of LIP-driven climate change. Mercury spikes found in the same sediment layers, derived from volcanic fallout, provide additional corroboration for the link between the Wrangellia volcanism and the global climate shift.
Biological Turnover and New Life Forms
The radical environmental changes of the Carnian Pluvial Episode acted as a global evolutionary reset button, triggering a significant biological turnover. The intense climate disruption caused a substantial extinction event, particularly affecting marine life. In the oceans, nearly one-third of all marine genera disappeared, including many species of ammonoids and conodonts.
This sudden ecological emptying created numerous vacant niches, allowing new life forms to diversify rapidly in the wake of the CPE. In the marine realm, the crisis spurred the rise of new groups that form the basis of modern ocean ecosystems. The first scleractinian corals appeared, establishing the first modern-style reefs, along with the earliest rock-forming calcareous nannofossils.
On land, the newly humid and warm conditions fostered the expansion of several groups of terrestrial plants and animals. Modern conifer forests began to diversify significantly, replacing some of the older flora. Among the terrestrial vertebrates, the turnover led to the appearance of several groups that would become prominent throughout the Mesozoic.
These included the first crocodiles, known as crocodylomorphs, and early turtles. Pterosaurs, the first vertebrates to achieve powered flight, also made their debut, alongside the diversification of early mammal relatives, or mammaliaforms.
The Earliest Dinosaurs Emerge
The ecological vacancies created by the Carnian Pluvial Episode provided the specific opportunity for the earliest definitive dinosaurs to emerge and begin their ascent. The oldest unequivocal body fossils of dinosaurs are found in rocks dated to approximately 230 million years ago, immediately following the major climatic disruption of the CPE. These initial finds are geographically concentrated in the southwestern portion of Pangaea, specifically in the Ischigualasto Formation of Argentina and the Santa Maria Formation of Brazil.
Among the earliest known species are small, agile forms like Eoraptor and Herrerasaurus from Argentina, and Staurikosaurus from Brazil. These pioneers were small to medium-sized reptiles, often overshadowed by other large archosaurs of the time, such as phytosaurs and rauisuchians. Yet, they possessed specialized traits suited for the fluctuating post-CPE world.
A key evolutionary innovation was the development of a fully erect posture, where their legs were positioned directly beneath their bodies, unlike the sprawling gait of most other reptiles. This anatomical arrangement, often coupled with bipedal locomotion, allowed for greater stamina and more efficient breathing during movement. In the rapidly changing, humid, and ecologically stressed environment following the CPE, this superior mobility provided a distinct advantage over their competitors.
The initial diversification of dinosaurs was characterized by the emergence of basal ornithischians, such as Pisanosaurus, and the ancestors of the saurischian lineage. The saurischian lineage includes the later long-necked sauropodomorphs and the predatory theropods. While initially a minor component of the fauna, their numbers and variety increased significantly in the subsequent Norian Age.