All About the Triassic Period: The Age of Reptiles

Triassic Period
Photo: CoreyFord via Getty Images

The Triassic Period was a time of significant transformation.

This era was marked by enormous upheavals in the diversity and dominance of life on Earth, ushering in the emergence of numerous well-known animal groupings that would dominate the world for tens of millions of years.

The Great Dying

The Permian-Triassic extinction event is still poorly understood. Several ideas have been postulated, including an unknown asteroid impact, enormous volcanic eruptions in what is now Siberia, methane release from the ocean depths, sea level change, rising aridity, or a combo of many of these.

Whatever caused the extinction catastrophe, the outcome was disastrous for vast swaths of life.

According to some estimates, the Great Dying triggered the extinction of up to 90% of all species. It wiped out several insect families, enormous numbers of mammal-like reptiles, and all trilobites, a group of animals that had lived in the waters for about 300 million years.

During the Great Dying, however, it was not only the animals who suffered greatly. The absence of coals throughout the first few million years of the Triassic Period is notable. This is assumed to be related to the mass extinction, and the time it took for plants to recover.

According to some estimations, the world may have taken up to 10 million years to recover from the destruction inflicted by the Great Dying.

The Revival Of Life

The Triassic is defined mostly by extinctions, although the arrangement of the continents also distinguishes it during the time. There was just one massive landmass: Pangaea.

Conifers first appeared during the early Triassic period. Before the evolution of flowering plants and grasses, conifers formed enormous forests with trees as tall as 30 meters.

Where the climate became drier, enormous fern prairies replaced the trees.

As animal life began to rebound, what appeared on land resembled what had come before. Lystrosaurus, a small herbivorous synapsid or mammal-like reptile, was the most frequent vertebrate on the land.

While these inhabited the land, amphibians dominated freshwater. Around 250 million years ago, the Lissamphibians (frogs, salamanders, and caecilians) were most likely just beginning to emerge.

The rivers were instead inhabited by a unique type of amphibian known as Temnospondyls. These were substantially larger beasts, with some reaching four meters in length. They looked like current crocodiles and most likely played a similar ecological role.

The first ichthyosaurs, a group that would later dominate the waters, appeared between 250 and 246 million years ago. The origin of this successful group of aquatic reptiles remains unknown.

A Power Shift

The first sphenodonts appeared when ichthyosaurs went extinct. Only the tuatara represents this sister group to lizards and snakes, resembling lizards but representing a separate lineage. Triassic lizards were more diversified than their forebears and played similar ecological roles.

Early in the Triassic, mammal-like reptiles ruled, but archosaurians became important. By the end of the Middle Triassic, synapsids declined, and archosaurs arose.  Archosaurs belong to the diapsid lineage, which includes dinosaurs, birds, pterosaurs, crocodilians, and turtles.

By the Late Triassic, mammal-like reptiles lost dominance to archosaurs. Competition in a warming, drier climate, or evolutionary stalemate are the possible causes. Archosaurs were able to replace niches left by synapsid extinctions. The 

The Triassic period was rich with weird archosaur groups like the aetosaurs. These massive armored animals were similar to later armored dinosaurs. Others included the long-necked Tanystropheus and chameleon-like Drepanosaurs with clawed tails.

Phytosaurs were also significant. These animals resemble crocodiles but are archosaurians. The Triassic ancestors of crocodiles were significantly smaller and slender.

A Quick Rundown Of The Creatures Of The Triassic

Here's a quick summary of the creatures that would have roamed Pangaea during the Triassic Period. 

The Triassic Dinosaurs

As early as 240–230 million years ago, in the Middle Triassic period, dinosaurs began to emerge — little, bipedal, carnivorous creatures.

Dinosaur legs, in contrast to the legs of most other reptiles, sprang straight down from the base of the body. This gave dinosaurs an advantage over their reptile competitors, allowing them to grow to gigantic sizes while still using less energy to move about.

Herrerasaurus and Eoraptor, two of the earliest dinosaurs discovered, both lived in what is now Argentina. Eoraptor was the smallest of the two, reaching roughly 1 m (3 ft. 3 in) in length. The maximum length of a Herrerasaurus was roughly 6 meters (20 feet).

Procompsognathus, a predatory dinosaur measuring just 1 m (3.3 ft) in length, existed in what is now Germany during the Late Triassic, alongside Coelophysis, another little bipedal meat-eater.

There are specimens of Coelophysis in both North America and southern Africa. Many of its fossils have been discovered, making it one of the most abundant dinosaurs.

The Triassic period saw the appearance of the herbivorous saurischian dinosaurs known as sauropodomorphs. Giant dinosaurs like Brachiosaurus and Argentinosaurus would descend from them.


The herd-dwelling, plant-eating Plateosaurus stood at least 8 meters (26 feet) in length and inhabited what is now Europe.

Therapsids & Triassic Mammals

The therapsids, which included the progenitors of mammals, took a heavy hit during the Permian extinction event and ultimately went extinct.

But not all therapsids died out, and the ones that did eventually become the preeminent land vertebrates of the Early Triassic.

The therapsid dicynodonts were massive, intermediate-to-large animals that had two tusks. These herbivores were the prehistoric equivalents of domesticated cattle and were widely dispersed over Pangaea.

The cynodonts are yet another group of therapsids that made it through the Great Dying. They may have been warm-blooded and hairy despite having produced eggs. Maybe they even had whiskers. The earliest known mammals belong to this category.

Tiny, nocturnal, burrowing creatures came first among mammals. Nocturnal habits were probably the norm for them. Being warm-blooded let them keep working even when it was dark outside, giving them an advantage over their reptile rivals.

The Eozostrodon is an example of an early mammal-like creature. It was about the size of a big shrew, about 10 centimeters (3.9 inches) long. The area that is today known as England is where it was discovered. The Morganucodon was a related British creature that some experts consider to be the same species.

Non-Dinosaur Triassic Reptiles

The archosaurs, a group of reptiles that separated into two families in the Early Triassic (Pseudosuchia, early crocodile-like animals, and Avemetatarsalia, the group that eventually became dinosaurs), replaced the synapsids as the dominating land vertebrates.

Most pseudosuchians resembled crocodiles in shape and size, with their lengthy bodies, thick necks, and massive heads. A few walked with their legs spread wide apart like lizards, while others' legs protruded beneath their body, allowing them to bulk out.

Large pseudosuchians called saurischians had their legs pointed downward. Some reached heights of 6 meters in length (20 ft.). This crocodile-like, fish-eating rauisuchian was called Ticinosuchus, and it lived in Switzerland.

Some pseudosuchians ate just plants, contrary to popular belief. The aetosaurs were one group that ate just plants. Massive, armored dinosaurs from the Mesozoic era were widespread over Pangaea.

The first turtles appeared during the Triassic period. Odontochelys semitestacea fossils, dating to 220 million years, were discovered in China.

Megachirella is the oldest known squamate (a member of the order Squamata, which includes lizards and snakes). It was a lizard-like creature that existed during the Middle Triassic and was 15 cm (5.9 in) in length. The earliest squamates probably appeared in the Early Triassic.


Pterosaurs, the first true flying reptiles, didn't develop until the Late Triassic, even though gliding reptiles existed in the Permian.

Early pterosaurs, like the eudimorphodon, hung out in rocky outcrops and subsisted on fish. Peteinosaurus, on the other hand, may have thrived on insect prey.



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