The Early Triassic epoch, spanning from approximately 251.9 to 247.2 million years ago, represents the initial phase of the Triassic Period. This geological interval immediately followed the Permian-Triassic extinction event, known as “The Great Dying.” The epoch was a prolonged period of global recovery, with life slowly re-establishing itself across a depopulated planet.
A Harsh and Arid World
The planet’s physical environment during the Early Triassic was shaped by the supercontinent Pangea, a single massive landmass. This continental configuration contributed to extreme climatic conditions, particularly vast arid deserts in its interior regions. Geological evidence, such as widespread red-colored sedimentary rock layers known as “red beds,” indicates a hot, dry “hothouse” climate that lacked polar ice caps.
Despite the overall aridity, a monsoonal climate might have brought strong seasonal precipitation to certain areas. Atmospheric conditions were also challenging, with oxygen levels initially low following the extinction event, slowly beginning to recover over millions of years. Elevated carbon dioxide levels in the atmosphere, potentially from continued volcanic activity, further contributed to global warming and the expansion of arid zones.
Life on a Recovering Continent
Terrestrial life in the Early Triassic faced immense challenges, with ecosystems severely disrupted. This period saw the rise of “disaster taxa,” hardy species capable of thriving in the unstable, post-extinction environment. These opportunistic organisms often dominated the sparsely populated landscapes due to their ability to adapt to harsh conditions.
The most prominent example of a disaster taxon was Lystrosaurus, a pig-sized synapsid, a group that includes the ancestors of mammals. This herbivorous creature became remarkably abundant across Pangea, in some regions, constituting up to 90-95% of the vertebrate fossils found in early Triassic rocks. Its widespread distribution across continents like Africa, India, and Antarctica suggests its resilience and adaptability to varied, yet generally difficult, terrestrial habitats.
Surviving synapsids, such as Lystrosaurus and other non-mammalian cynodonts like Galesaurus and Thrinaxodon, represented the lineage leading to mammals. Alongside them, new groups of archosaurs began to emerge, including early relatives of crocodiles and dinosaurs. These early archosauriforms, such as Erythrosuchus, would eventually diversify and come to dominate later Mesozoic ecosystems.
Plant life on land was sparse, and dense forests were rare. The vegetation consisted primarily of tough, resilient species like lycophytes and ferns, which could tolerate the arid and volatile conditions. This scarcity of peat-forming forests is reflected in the “coal gap” in the geologic record, a notable interval during the Early Triassic where few to no coal deposits are found worldwide. This gap suggests a significant delay in the re-establishment of complex, widespread plant ecosystems after the Permian-Triassic devastation, possibly taking up to 10 million years for the planet’s flora to recover.
The Strangelove Ocean
The marine environment was equally devastated by the Permian-Triassic extinction, experiencing distinct and challenging conditions. Widespread anoxic, or low-oxygen, and dysoxic, very low-oxygen, conditions prevailed in the water column, extending even into shallow marine areas. This oxygen depletion was a global phenomenon, with seawater oxygen levels potentially reduced by as much as 100-fold at the extinction’s onset.
The oceans were also likely acidic and potentially toxic due to high levels of dissolved carbon dioxide and hydrogen sulfide. This combination of factors created what scientists refer to as a “Strangelove Ocean,” a term describing a marine environment with severely reduced biological productivity and disrupted carbon cycling. The recovery of marine ecosystems was slow, with oxygen levels only gradually returning to pre-extinction levels after approximately 5 million years.
Marine communities during this period were often simple, dominated by resilient survivor groups like certain bivalves and gastropods. Early rediversification occurred among groups such as ammonoids, which were shelled cephalopods, and conodonts, an extinct group of eel-like vertebrates. The prolonged anoxia inhibited the recovery of diverse benthic, or bottom-dwelling, animal life and complex marine ecosystem functions.
The Dawn of a New Era
As the Early Triassic epoch progressed, global environmental conditions slowly stabilized, paving the way for renewed biological diversification. This gradual stabilization initiated a significant faunal turnover on land. The widespread disaster taxa, particularly Lystrosaurus, became less common as more diverse and specialized forms began to populate the recovering ecosystems.
The relative decline of Lystrosaurus coincided with the increasing prominence of archosaurs. These reptiles, which include the ancestors of crocodilians, pterosaurs, and dinosaurs, proved better adapted to the evolving arid terrestrial world. Archosaurs began to radiate, filling a wider variety of ecological niches left vacant by the extinction event.
The earliest diverse archosaur assemblages are known from the Middle Triassic, implying their radiation began in the Early Triassic, less than 5 million years after the Permian-Triassic mass extinction. This “changing of the guard” between the synapsid-dominated faunas and the emerging archosaurs marked the conclusion of the post-extinction recovery period. The rise of archosaurs set the ecological stage for the Middle Triassic, eventually leading to the long reign of dinosaurs.