252.17 million years ago, the dawn of a new era marked a major change in the course of evolution. The Mesozoic Era began with the Triassic Period, following the most devastating mass extinction event in the history of our world. However, slowly but steadily, this period would see life rebound.
Lasting some 51 million years, the Triassic Period began in the wake of the Permian mass extinction, which saw the demise of 96% of all maritime species and 70% of land vertebrates. Owing to the widespread destruction of the Earth’s biodiversity, it would take tens of millions of years for life to recover and new plant and animal niches to reach their former levels of diversity. Eventually, the world’s impoverished biosphere would give rise to the time when the first dinosaurs roamed the Earth.
The Triassic Period marks the beginning of the Mesozoic Era, an era that’s sometimes referred to as the Age of Reptiles, though botanists might prefer to call it the Age of Conifers, owing to the development of modern trees. Mesozoic means ‘Middle Life’ in Greek, and it represents the second of three eras that make up the Phanerozoic Aeon. The Triassic itself was named in 1834 from three distinct layers of rock found in Germany that define the three epochs that make up the period.
Highlights of the Triassic
- Mesozoic Era begins
- Mesozoic marine revolution
- First dinosaurs appear
- First mammals evolve
- Global climate warms and poles remain ice-free
- Major extinction event
Life Struggles for Survival in a Post-Apocalyptic World
The dawn of the Triassic marked the end of almost 290 million years of evolutionary history that made up the Palaeozoic Era. No one knows exactly what caused the Permian-Triassic mass extinction, but it was likely down to a combination of events, including rampant volcanism, reduced oceanic oxygen levels and a possible impact event like that which spelled the end of the dinosaurs’ reign. One thing that we do know for certain, however, is that our planet was left utterly devastated.
Although the Permian mass extinction was likely a drawn-out event, it would take the entirety of the Early Triassic epoch and more for Earth’s biodiversity to recover. Despite lasting only 5 million years, which is not long at all when it comes to the world of palaeontology, the Early Triassic is arguably one of the most fascinating and mysterious epochs in our history. It is divided into two geological stages – the Induan and the Oleneckian.
Pangea, a single vast supercontinent that formed during the Permian, still straddled the equator, with the vast Panthalassic ocean to its west and the Tethys and Paleo-Tethys oceans to its east. The vast landmass was barren and deserted for the most part; the once lush coal forests of the Carboniferous and Permian periods long gone. Much of the world was stiflingly hot and dry during the Early Triassic, and both terrestrial and maritime ecosystems remained severely impoverished. Further halting the progress of evolution were low atmospheric oxygen levels, which had also been responsible for the only known mass extinction of insects. Added to that was rampant volcanism pumping CO2 into the air, increasing global warming in the process.
Despite the post-apocalyptic environment, some hardy species did manage to cling to life in the harsh desert world, at least in terrestrial environments. Lystrosauruses and proterosuchids (the ancestors to crocodiles), which were among the few survivors of the Permian extinction, were the only two groups of animals to dominate terrestrial ecosystems. Plant life was equally minimal, owing to the failure of Palaeozoic spore-baring plants to adapt to the arid climate. However, pollen-producing plants, such as conifers, were much more adaptable, allowing them to survive and, eventually, thrive through the Mesozoic.
Archosaurs and Others Rise from the Ashes
It took up to 30 million years for life to fully recover from the Permian mass extinction, but things were starting to look up well before the Mid Triassic. It was, after all, during this time that one of the most successful animal groups that have ever existed evolved from other reptiles. We call these the archosaurs or ‘ruling reptiles’, a group of diapsid animals that include crocodiles, dinosaurs, birds and pterosaurs. The earliest direct descendants of the archosaurs were the proterosuchus (early crocodile) and the prolacerta (previous lizard). By the Mid Triassic, the group had already diversified greatly, giving rise to the nyasasaurus, one of the first of the dinosauriformes, the immediate relatives to the dinosaurs. Other successful groups included enormous apex predators such as rauisuchia and phytosaur, crocodile-like animals that kept the first dinosaurs hiding in the shadows for millions of years.
While it would still be millions of years before the first true dinosaurs would appear, Earth’s biotic recovery was now well underway. Life in the oceans was also recovering as coral reefs started to form the foundations of new marine ecosystems. Eventually, the oceans were finally hospitable enough again that they welcomed the return of Mid Triassic reptiles. As such, a diverse group of marine reptiles known as sauropterygia (lizard flippers) evolved. This group, which survived right up until the extinction of the dinosaurs, included the plesiosaurs and, possibly, the turtle-like placodonts of the Mid Triassic.
Although the global climate remained much warmer than it is today throughout the Triassic, and with no ice at the poles, steadily increasing oxygen levels managed to lift the brakes on evolution. While the vast swathes of desert in inland regions of Pangea remained throughout the period, biological diversity had largely recovered by the dawn of the Late Triassic.
Dinosaurs Find a Niche in a Changing Climate
Some 230 million years ago, in the early years of the Late Triassic, the global climate started to change, causing large-scale extinctions, particularly among marine organisms. However, the ongoing biotic recovery following the Permian continued to outshine the so-called Carnian Pluvial Event, namely with the appearance of the very first dinosaurs. Among these was the eoraptor, one of the earliest true dinosaurs. A small and slender bipedal predator, it wasn’t much larger than a domestic cat. Although it’s practically impossible to pinpoint the exactly when and which animal was the first true dinosaur, eoraptor is widely regarded as the closest thing to it.
Throughout the Late Triassic, the archosaurs formed new clades and groups, radiating splendidly into what would become some of the most diverse and successful animals to ever roam the Earth. While the earliest dinosaurs may have appeared small and insignificant; speedy little bird-like carnivores, they quickly found their way into almost every terrestrial ecosystem in the world, building the foundations for their future global empire. At the same time, the ancestors to crocodiles, a clade known as suchia, grew and diversified, becoming ever more uniquely set apart from their former close relatives, the dinosaurs.
From the beginning of their reign, the clade of dinosauria formed two taxonomic orders, defined by their hip structures. One of them, saurischia, survives to this day in the form of birds, while the other, ornithischia, disappeared with in the Cretaceous extinction event 66 million years ago. Both orders diversified enormously to the extent that many families ended up having little in common with each other, at least in terms of appearance.
The Late Triassic also saw the size variations of both dinosauric orders start to vary enormously, with the largest saurischians, such as the plateosaurus, reaching lengths of up to 33 feet (10 metres). Belonging to a suborder known as sauropodamorpha, plateosaurus was a direct ancestor to all the iconic long-necked beasts, such as diplodocus and brontosaurus, which were still some 80 million years into the future. Another highly successful family of saurischians at the time were the herrerasaurids, fierce carnivores that stood higher than an adult human and grew up to 20 feet (6 metres) in length.
The largely herbivorous ornithischians, which would come to include triceratopses and ankylosaurs, remained relatively small and primitive throughout the Late Triassic. Only two known ornithischian dinosaur families are known from the time; heterodontosaurus and pisanosaurus. Both were only about the size of a fox. Interestingly, however, recent evidence suggests that heterodontosaurus may have sported a coat of proto-feathers.
Mammals Are Born in the Shadow of the Dinosaurs
Early mammaliaforms were cynodonts, which evolved from other synapsids some 225 million years ago. In fact, according to Oxford University’s T. S. Kemp, these animals may also be defined as the first true mammals. Most of them were small, shrew-like creatures cornered into specific biological niches by the then far more successful archosaurs. This situation persisted, with a few exceptions, right up until the extinction of the non-avian dinosaurs, an event that finally gave mammals the chance to dominate the Earth instead. One of the most iconic of the Late Triassic mammaliaforms is megazostrodon, a 5-inch-long (12 cm) furry creature that evolved in the very end of the period.
Evidence suggests that among the earliest true mammals were monotremes, although they remain sparse in the fossil record. Of course, monotremes exist to this day, indigenous only to Australia and Papua New Guinea, partly owing to hundreds of millions of years of largely separate evolutionary development. Characterised by the fact they lay eggs, unlike any other mammal, monotremes are the only extant animals belonging to a subclass called prototheria which, suitably, means ‘first wild beasts’ in Greek.
By the end of the Triassic, the Age of the Dinosaurs was well underway, and the reptilians continued to diversify. Meanwhile, the ichthyosaurs and plesiosaurs ruled the oceans, now once again abundant with oxygen and perfect for the enrichment of life. The earliest mammals had also managed to establish a foothold, but it would be hundreds of millions of years before they would have a chance to break free from the all-dominant dinosaurs. Next week, we’ll be exploring the Jurassic Period, the real Land of Giants, the time when apartment-block sized sauropods roamed the Earth.