Triassic
The Triassic Period began in the wreckage of the worst catastrophe life has ever faced. About 251.902 million years ago, the Permian-Triassic extinction event tore through the biosphere of planet Earth, eliminating so many species that the world left behind was nearly empty. What came next was not simply a recovery. Over the course of 50.5 million years, from that catastrophe's aftermath to another mass extinction at 201.4 million years ago, life on Earth was remade almost entirely from scratch.
The Triassic is the first and shortest geologic period of the Mesozoic Era. It sits between two of the most dramatic mass extinctions in the fossil record. Named in 1834 by Friedrich Alberti, it takes its name from three distinct rock layers found across southern Germany. That three-part structure mirrors the period itself: a world in shock, a world rebuilding, and a world brought low once again.
Who survived the opening catastrophe? What creatures filled the void left by the dead? And how did a small, obscure group of reptiles called dinosaurs go from minor players to inheriting an entire era? The answers lie buried in the rock, the climate, and the extraordinary contingency of 50 million years of life.
Friedrich Alberti gave the Triassic its name in 1834, drawing on the Greek word for triad. He was describing three distinct rock formations visible in southern Germany: the lower Buntsandstein, a colourful sandstone; the middle Muschelkalk, a shell-bearing limestone; and the upper Keuper, a coloured clay. Each layer told a different story about the conditions that produced it, and together they formed a legible record of a vanished world.
On the geologic time scale, the period divides into three epochs: the Early, Middle, and Late Triassic. Geologists refer to the corresponding rock sequences as Lower, Middle, and Upper Triassic. This subdivision matters because the Triassic was not a stable, uniform interval. The climate swung dramatically, the continents moved, oceans opened and closed, and the populations of animals on land and sea changed repeatedly across those three broad stages.
The Triassic is also the seventh period of the Phanerozoic Eon, the vast span of time since complex animal life first appeared. Within that long sequence, the Triassic stands out for beginning and ending with major extinction events, a distinction it shares with no other period of comparable length.
Pangaea dominated the globe at the opening of the Triassic. All major continents were fused into this single supercontinent, centred on the equator and stretching in an arc from the north to south polar regions. Laurussia lay in the north and Gondwana in the south. The Paleo- and Neo-Tethys oceans curved within the arc of Pangaea's interior, while the vast Panthalassa Ocean surrounded everything else.
The sheer size of Pangaea had profound consequences for climate. The global ocean's moderating effect barely reached the continental interior, producing ferociously seasonal conditions: very hot summers, cool winters, and a massive cross-equatorial monsoon system that some researchers call the Pangean megamonsoon. The interior was mostly hot and dry, with red bed sandstones and evaporites as the characteristic deposits. The polar regions, however, were apparently moist and temperate enough to support forests and vertebrates, including reptiles.
North China and Amuria, along with South China, were initially separated from Pangaea by the Paleoasian Ocean, but this gap closed by the Late Triassic. That closure, part of the broader Indosinian orogeny, formed a single large Eastern Asian continent and maximised the total land area of the supercontinent, coinciding with a period of dramatic climate change and the development of the megamonsoon.
By the latest Triassic, in the Rhaetian, Pangaea began to rift. The supercontinent slowly pulled apart along the line of the future Central Atlantic Ocean, eventually separating into Laurasia to the north and Gondwana to the south.
The Early Triassic was the hottest portion of the entire Phanerozoic Eon. Enormous volumes of greenhouse gases had poured from the Siberian Traps volcanic province, and temperatures reflected that discharge. The period opened with the Permian-Triassic Thermal Maximum, followed by the brief Dienerian Cooling from 251 to 249 million years ago, then the Latest Smithian Thermal Maximum around 249 to 248 million years ago. During the Latest Olenekian Cooling, from 248 to 247 million years ago, temperatures dropped by about 6 degrees Celsius.
The Middle Triassic was cooler. Temperatures fell across most of the Anisian, apart from a warming spike late in that stage, and from 242 to 233 million years ago the Ladinian-Carnian Cooling continued the trend.
The Carnian brought change. Around 234 million years ago, the eruption of the Wrangellia Large Igneous Province abruptly reversed the cooling trend and triggered the Carnian Pluvial Event, an episode of widespread global humidity. This warming inaugurated the Mid-Carnian Warm Interval, which lasted from 234 to 227 million years ago. Around 212 million years ago, a 10-million-year eccentricity maximum caused a paludification of Pangaea, shrinking arid climatic zones across the supercontinent.
At the very end of the period, an extreme warming event called the End-Triassic Thermal Event drove the Triassic-Jurassic mass extinction. Carbon dioxide bubbles preserved in basaltic rocks from that boundary point to volcanic activity from the Central Atlantic Magmatic Province as a key trigger for the climate shift that closed the period.
The Permian-Triassic extinction had reduced the biosphere to a fraction of its former diversity. What grew back was a world shaped by who survived and who was missing. Three broad categories of organisms characterise the Triassic fossil record: animals that carried over from the Permian, animals that briefly flourished before disappearing, and animals that evolved and ultimately dominated the Mesozoic Era.
Among plants, lycophytes, particularly those of the order Isoetales, rose to prominence in the Early Triassic because environmental instability after the extinction gave them an opening. One notable genus, Pleuromeia, grew in a columnar fashion and sometimes reached a height of 2 metres. As conditions stabilised through the Middle Triassic, the relevance of lycophytes declined. The Carnian Pluvial Event, the best-studied and probably most intense episode of increased rainfall in the Triassic, reshaped plant communities across tropical and subtropical latitudes of the Tethys Sea and its surrounding land.
In the oceans, the shelled cephalopods called ammonites recovered from a single surviving lineage, diversifying across the Triassic seas. New types of corals appeared in the Early Triassic, though their reefs were modest compared to the great systems of the Devonian or the modern ocean. Bivalves began to rapidly diversify during the Middle Triassic, becoming highly abundant. Aquatic insects underwent a crucial diversification during the Middle Triassic as well, particularly within the clade Holometabola, which contains the majority of modern insect species.
No coal deposits are known from the start of the Triassic, a gap in the geological record sometimes called the Early Triassic coal gap. Possible explanations range from acid rain caused by the Siberian Traps eruptions, to a greenhouse climate too hot and dry for peat accumulation, to the extinction of all plants adapted to peat swamps.
Archosauromorph reptiles had already appeared in the Permian, but they exploded in diversity at the start of the Triassic as an adaptive radiation responding to all the empty ecological space the extinction had created. By the Early Triassic, several major groups had taken shape. Long-necked protorosaurs occupied one corner of this expanding world; the family Tanystropheidae pushed neck size to extremes, with the largest genus Tanystropheus carrying a neck longer than the rest of its body. The family Sharovipterygidae used elongated hindlimbs to glide.
Rhynchosaurs, barrel-gutted herbivores that sheared plants with premaxillary beaks and multiple rows of teeth along the upper jaw, thrived as the primary large herbivores in many Carnian-age ecosystems, then went extinct around 220 million years ago. Phytosaurs, long-snouted and semi-aquatic, resembled living crocodiles in appearance and probably in lifestyle, hunting fish and small reptiles at the water's edge. The resemblance is an example of convergent evolution rather than shared ancestry.
True archosaurs split early into two branches: Avemetatarsalia, the ancestors of birds, and Pseudosuchia, the ancestors of crocodilians. Pseudosuchians were far more ecologically dominant through most of the Triassic. Aetosaurs were heavily armoured reptiles common during the last 30 million years of the Late Triassic. The predatory group known as rauisuchians, formally called paracrocodylomorphs, were the keystone predators of most Triassic terrestrial ecosystems; over 25 species have been identified, ranging from giant quadrupedal hunters to sleek bipedal omnivores.
Dinosaurs evolved in the Carnian but remained a minor presence. Most were small; Coelophysis, among the more common early dinosaurs, was 1 to 2 metres long. It would take the extinction at the end of the period to clear the space the dinosaurs needed.
Therapsids had been the dominant vertebrates of the Permian, and three of their groups carried over into the Triassic: dicynodonts, therocephalians, and cynodonts. The cynodont Cynognathus served as a characteristic top predator in the Olenekian and Anisian of Gondwana. Therocephalians, which included both large predators like Moschorhinus and herbivorous forms, went extinct midway through the period.
During the Carnian, some advanced cynodonts gave rise to the first mammals. This transition happened under pressure. As archosaurs grew more ecologically dominant, the surviving therapsids and their mammaliaform successors were pushed into a narrower niche: small, mainly nocturnal insectivores. Researchers refer to this process as the Triassic Takeover. Nocturnal life may have driven mammaliaforms to develop fur and a higher metabolic rate, traits that would later prove essential.
The first stem-group mammals, technically classified as mammaliamorphs and themselves a specialised subgroup of cynodonts, appeared during the Triassic and survived the extinction event at its close. That survival allowed them to diversify during the Jurassic. The cynodont lineage thus threaded a path from top predator to marginal survivor to the ancestor of every mammal alive today, including the one listening to this documentary.
The Triassic ended as it began: with a mass extinction. The Triassic-Jurassic extinction event was particularly severe in the oceans. The conodonts, an ancient group of jawless vertebrates, disappeared entirely. All marine reptiles except ichthyosaurs and plesiosaurs vanished. Invertebrates including brachiopods and molluscs were severely affected. In the oceans, 22% of marine families went missing, along with possibly about half of all marine genera.
On land, the damage was uneven. Pseudosuchians, which had dominated Triassic terrestrial ecosystems, were nearly wiped out; aetosaurs and phytosaurs disappeared entirely. Most large labyrinthodont amphibians died out, along with groups of small reptiles and most synapsids. Some early primitive dinosaurs also went extinct, but more adaptable lineages survived.
The cause remains contested. The Central Atlantic Magmatic Province, one of the largest known inland volcanic events since the planet stabilised, erupted as Pangaea broke apart, covering a region of about 10 million square kilometres across North America, northeastern South America, northwestern Africa, and southwestern Europe. Individual dykes reached up to 800 kilometres in length. The magmatism injected large quantities of carbon and sulphur into the atmosphere, first triggering volcanic winters and then longer-term climate warming and ocean acidification.
A bolide impact has also been proposed, with the Manicouagan Reservoir in Quebec, Canada, identified as a candidate crater. However, the Manicouagan impact has been dated to 214 plus or minus 1 million years ago, which precedes the end of the Triassic by approximately 10 plus or minus 2 million years. The evidence suggests it could not have been the immediate cause. What is clear is that the extinction opened the door the dinosaurs walked through, allowing them to assume dominance in the Jurassic and hold it for the next 150 million years.
Common questions
How long did the Triassic Period last?
The Triassic Period spanned 50.5 million years, from 251.902 million years ago to 201.4 million years ago. It is the first and shortest geologic period of the Mesozoic Era.
Why is the Triassic Period called the Triassic?
Friedrich Alberti named the Triassic in 1834, drawing on the Greek word for triad. He was describing three distinct rock layers widespread in southern Germany: the Buntsandstein (colourful sandstone), the Muschelkalk (shell-bearing limestone), and the Keuper (coloured clay).
What animals dominated the Triassic Period on land?
Reptiles, especially archosaurs, were the chief terrestrial vertebrates during the Triassic. Pseudosuchians, the relatives and ancestors of modern crocodilians, were more ecologically dominant than dinosaurs through most of the period. Dinosaurs first appeared in the Late Triassic but did not become dominant until the Jurassic.
What caused the Triassic-Jurassic mass extinction?
Volcanic eruptions from the Central Atlantic Magmatic Province, one of the largest known inland volcanic events, are the primary candidate. The CAMP covered about 10 million square kilometres and injected carbon and sulphur into the atmosphere, causing volcanic winters followed by climate warming and ocean acidification. A bolide impact at Manicouagan has been proposed but predates the extinction by approximately 10 million years.
When did the first mammals appear, and what was the Triassic Period?
The first stem-group mammals, called mammaliamorphs, appeared during the Triassic Period, with some advanced cynodonts giving rise to them during the Carnian stage of the Late Triassic. They survived the end-Triassic extinction event and went on to diversify during the Jurassic Period.
What was the climate like during the Triassic Period?
The Triassic climate was generally hot and dry in continental interiors, producing red bed sandstones and evaporites as characteristic deposits. The Early Triassic was the hottest portion of the entire Phanerozoic Eon. The polar regions were moist and temperate, and the Carnian Pluvial Event brought a widespread episode of increased rainfall around 234 million years ago.
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