The Triassic Period
251 mya - 201 mya
In 1834 Freidrich von Alberti named the Triassic Period after a particular series of distinct rock layers found throughout Germany and northwest Europe. The series consisted of red beds capped by marine limestone followed by a series of mud and sandstone called the Trias. The Triassic Period marks the beginning of the Mesozoic Era (which lasted from 250 to 65 mya), an era that saw the disintegration of Pangaea (the supercontinent formed during the Permian) and the evolution of life favoring the Reptiles over the Mammals (this is why the Mesozoic is often called ‘The Age of Reptiles’). The biggest change wrought in the Mesozoic was the rise of the dinosaurs, who were perhaps the most successful kind of animal in all of evolutionary history. Dinosaurs came from small, humble beginnings, appearing in the mid- to late-Triassic; at their genesis they played second fiddle to other creatures, and they wouldn’t begin their rise to dominance until the early Jurassic.
~ Triassic Life in the Skies ~
~ The (Stunted) Rise of Mammals ~
As reptiles dominated the ocean and the skies, so, too, they dominated on land. We would be remiss, however, not to give credit where credit is due: the shape of our world depends upon the survival of a shrew-like creature in a dry and dangerous world of tyrants big and small. The therapsids of the Permian nearly went extinct, but their early Triassic hold-outs continued on their dark journey towards mammalian-hood. The earliest true mammals evolved in the Triassic, and they were unimpressive: most were less than a few inches in length. They were mainly herbivores or insectivores, and they may have been partially arboreal and nocturnal. Most laid eggs despite having fur and suckling their young. They had three ear bones like modern mammals, but their jaws had a mixture of mammalian and reptilian characteristics.
Life wasn’t easy for the early mammals (called ‘protomammals’), and the stakes were high: the Triassic witnessed a life-and-death struggle for dominance between them and the reptiles. Had the protomammals – led by the dicynodonts and cynodonts – won out, they and their descendants would’ve likely characterized the Mesozoic; as it were, however, the reptiles won fair and square, and they came to dominate the Mesozoic Era. Though it seemed for a moment that the herbivorous protomammals would seize the laurels, the herbivorous reptiles squeezed them off-stage as they developed advanced limb muscles able to sustain a heavier size and diet. Predatory cynodonts dominated their niche in the early Triassic: with sleek profiles, elongated bodies, and slender limbs – all of which promoted rapid movement – the cynodonts were a force to be reckoned with (and their mammal-like faces and wolf-like teeth made them fierce indeed). These carnivorous cynodonts, however, were outdone by the expanding reptilian archosaurs, and by the mid-Triassic cynodonts had contracted to small and medium-sized predators and herbivores. They simply couldn’t compete with the evolving archosaurs, some of whom reached proportions similar to modern-day wolves, lions, and polar bears. The mammals would need to wait for a miraculous (or apocalyptic) intervention to retake center stage.
The Dicynodonts: A Closer Look
The Cynodonts: A Closer Look
The Mesozoic has been called ‘The Age of Reptiles’ since reptiles dominated the three major ecosystems: water, earth, and air. Our tiny mammalian ancestors scavenged for a living in a land caught by two opposing types of land reptiles: the rhynchosaurs and archosaurs. The rhynchosaurs were stocky-bodied, herbivorous reptiles sporting powerful beaks. They were abundant throughout most of the Triassic but were supplanted by the archosaurs during the mid-late Triassic. Over the course of the Triassic, certain archosaurs became erect. They were adapted to the dry, arid atmosphere of the early Triassic, evolving elevated metabolisms and avian breathing systems so that they could be continuously active in the humid, oxygen-depleted world in which they lived. The archosaurs laid the groundwork for modern crocodiles, but they're known best for the most successful archosaur of them all: the dinosaur.
The dinosaurs – or ‘terrible lizards’ – acquired an erect gait, so that the legs could be postured directly beneath the animal (like in modern elephants). This made dinosaurs efficient: it took less energy to keep its body off the ground and it avoided Carrier's Constant, since they could run and breathe at the same time. Other distinguishing characteristics have to do with muscle locations and orientations on various bones, the shape of the joints connecting the bones to the pelvis, the particularity of their ankle joints, and some details relating to the skull. The earliest dinosaurs included Coelophysis and Plateosaurus.
The dinosaurs appeared in the mid to late Triassic, and they lived in vibrant ecosystems filled with creatures much bigger (and scarier) than them. Because dinosaurs weigh heavier in popular thought than their Triassic counterparts, an image of a Late Triassic teeming with dinosaurs has become commonplace – but it is an image rooted in fantasy rather than reality. Studies of Triassic fossil beds and trackways show that while dinosaurs emerged during the mid-late Triassic, they were never dominant. They were always relegated to the sidelines. Dinosaurs first appear in southern Pangaea and began to diversify around 230-220 mya, but they remained clustered in the humid belts of Pangaea. In the arid belts closer to the equators, dinosaurs were rare if present at all (it seems that they couldn’t handle the heat). Triassic amphibians and reptiles flourished in the interior, and in the exterior they remained dominant.
Paleontologist Steve Brusatte, who has done a lot of work in Triassic studies, notes in The Rise and Fall of Dinosaurs that dinosaurs ‘didn’t just sweep across Pangaea the moment they originated, like some infectious virus. They were geographically localized, held in place not by physical barricades but by climates they couldn’t endure. For many millions of years, it looked as if they might remain provincial rubes, stuck in one zone in the south of the supercontinent, unable to break free – an aging high school football hero of faded dreams, who could have been something if only he’d been able to get out of his tiny hometown.” (Brusatte, pp. 59-60) This was the status quo for most of the Triassic, but in the last few million years, things began to change. Change didn’t happen at once but in two stages. First, in the humid regions, the dominant large plant-eaters (the rhynchosaurs and dicynodonts) began to dwindle, even disappearing in places; scientists don’t know why, but regardless of the reasons, the herbivorous dinosaurs – particularly the prosauropods – were able to fill the niches left vacant. In the humid areas of Pangaea, dinosaurs rose to comprise thirty percent of the ecosystem whereas the previous heavyweights dropped down to twenty percent. At the very end of the Triassic, at around 215 mya, dinosaurs began migrating into the drier regions; though scientists aren’t sure why, the best guess is that climate change – caused by the slow rifting of Pangaea (to be addressed shortly) – blurred the stark boundaries between the arid interior and humid exterior. The arid regions, where only the bravest (or most foolish) dinosaurs dared to tread, became manageable.
For most of the Triassic, dinosaurs were on the ecosystem’s fringes. Dinosaur diversity was stunted compared to that of their comrades. Morphological diversity – how much a type of creature shows differences between species – has been used as a barometer of an animal’s ‘fitness.’ The idea is that ‘better’ types of creatures will be able to better adapt and evolve, whereas less ‘worthy’ creatures will not. The result is that some types of animals have a lot of morphological diversity while others don’t. A 2008 study of Triassic animals found that dinosaurs had little diversity compared to their counterparts. Paleontologist Steve Brusatte (who did the project) reports that ‘All throughout the Triassic, the pseudosuchians were significantly more morphologically diverse than dinosaurs. They filled a larger spread of the map, meaning they had a greater range of anatomical features, which indicated that they were experimenting with more diets, more behaviors, more ways of making a living. Both groups were becoming more diverse as the Triassic unfolded, but the pseudosuchians were always outpacing the dinosaurs. Far from being superior warriors slaying their competitors, dinosaurs were being overshadowed by their crocodile-line rivals during the 30 million years they coexisted during the Triassic.” (pp. 80-81)
Paleontologist Brian Switeck echoes Brusatte's sentiments: "If you could travel back to [the Late Triassic], you’d see many vaguely familiar creatures, harbingers of what was to come, but they wouldn’t look exactly like anything we see around us today. The dinosaurian ancestors of birds were rare, slender creatures; the closest relatives of the first mammals were either hulking tusked holdovers from an earlier era or tiny shrew-like fuzzballs; and a profusion of bizarre crocodile relatives ruled the land, from armor-encased omnivores to aquatic ambush predators and bipedal dinosaur mimics. The Late Triassic was when reptiles began to rule, but the earliest dinosaurs only hinted at the potential of what was to come… Triassic dinosaurs were the prehistoric equivalent of Chekhov’s gun – loaded with potential, but not set to go off until much later in the world’s evolutionary story.”
Dinosaurs, then, were surrounded by other creatures – such as the phytosaurs, who resembled gigantic crocodiles but aren’t to be confused with the modern crocodile's ancestors (though they lived in the Triassic, they were smaller, moving with the speed and agility of a greyhound and resembling reptilian wolves). The aetosaurs were large, armored herbivorous reptiles preyed upon by the ornithosuchians, which resembled titanic crocodiles with double rows of armored plates running along their backs. The biggest and baddest Triassic carnivores were the rauisuchians, the fear and dread of the earliest dinosaurs. Like dinosaurs, rauisuchians had an erect gait with their legs oriented beneath them in a pillar-erect posture. The rauisuchians lived through most of the Triassic but died out in the Triassic-Jurassic Extinction. Their absence would give the earliest theropod dinosaurs room to grow into even larger carnivores than the rauisuchians could muster.
Rauisuchian dominance came to an end with the Triassic-Jurassic Extinction Event. The Tethys Sea was beginning to cut through Pangaea due to the shifting of tectonic plates, and the supercontinent would soon split in half between Laurasia to the north and Gondwana to the south. This great wrenching wouldn't be accomplished until the Jurassic, but the Triassic suffered the birth pangs: volcanic eruptions unlike anything the world has seen before or since (except for when the planet was boiling in its own stew) scarred the atmosphere. Oxygen depleted and carbon dioxide increased exponentially. Though nowhere near as destructive as the extinction that preceded it, the Triassic-Jurassic extinction hit the ocean especially hard: all marine reptiles except ichthyosaurs, nothosaurs, and plesiosaurs died out. Even invertebrates such as brachiopods, gastropods, and mollusks suffered. Twenty-two percent of marine families and half of marine genera went extinct. On land, the dinosaurs' predatory rivals went extinct along with some of the earlier dinosaurs. But the hardier and more adaptive dinosaurs survived into the Jurassic where they would flourish.
~ Geography and Climate ~
At the dawn of the Triassic, the supercontinent Pangaea covered nearly a quarter of the planet's surface. Pangae-like supercontinents have formed numerous times in earth’s history, and geologists speculate that the formation of these continents cycles on a timescale of about every five hundred million years. The early Triassic supercontinent was shaped like a distorted Pac-Man, and this Pac-Man's gaping mouth was the Tethys Sea, a massive bay of the vast global Panthalassa Ocean, which stretched pole-to-pole and was twice the width of the modern-day Pacific Ocean. Islands, seamounts, and volcanic archipelagos were scattered throughout Panthalassa along the Triassic equator. Pangaea would begin splitting apart in the late Triassic, and by the mid- to late-Jurassic, the Tethys bay would become a waterway cutting straight through the heart of Pangaea (the southern piece of Pangaea would be known as Gondwana, the northern as Laurasia). The changing of Pangaea came about due to plate tectonics. The earth's crust is divided into broad, rigid plates that are constantly (albeit slowly) on the move. These plates move atop firmer layers of mantle, and when the plates interact the result is earthquakes, mountain ranges, and volcanoes. As the Tethys Sea began to weasel its way through the heart of Pangaea, the result would've been a constant litany of volcanic eruptions along the shifting plates. It's speculated that this geological activity of the late Triassic is responsible for the Triassic-Jurassic Extinction Event.
Speaking of extinctions, just as the end of the Triassic would be marked by an extinction, so, too, was its beginning. The Triassic began in the wake of the Permian-Triassic Extinction. This ‘Great Dying’ was the ‘mother of mass extinctions’: 90-95% of the world's marine life and 70% of terrestrial vertebrates perished. Insects, the hardiest of the planet's creatures, suffered, too. This was the worst extinction in world history, so much so that one Permian creature –Lystrosaurus – has been called the Permian/Triassic ‘Noah’ simply because it made it through to the other side (it originated in the late Permian and is seen in early Triassic rock). The creatures that survived the extinction struggled to eek out an existence in an eerie post-apocalyptic world: the climate was unusually dry with vast areas of desert, and the temperatures flashed between extremely hot and very cold. The sea levels were low and anoxic (lacking in oxygen); water may have lacked oxygen just ten to twenty meters below the water's surface. Because the deep oceans were anoxic, the shallow waters were unaerated. The air, consequentially, was anoxic as well. Carbon dioxide in the atmosphere vaulted to record levels due to extreme volcanic activity. This volcanism resulted in early Triassic forests – vestiges of the late Permian - devastated by forest fires spawned by lava flows; even far beyond the volcanoes, acid rains mixed with ash fell in black droplets, suffocating even the hardiest plant life and making life a rather gray and bleak affair for the surviving animals. As if this weren't inhospitable enough, radiation levels were unusually high (probably caused by ill-timed cosmic forces).
It would take more than one million years before normal ecosystems, coral reefs, and large animals recovered from the extinction's fall-out. Things became pleasant (or at least readily survivable) by then: oxygen returned to about 80% of its modern level, carbon dioxide was at a comfortable setting around six times what we're used to, and the temperatures stabilized for an average warmth of about one hundred degrees Fahrenheit. Nevertheless, the Triassic remained a harsh world underscored by deserts. Temperatures were hot during the summer and cold in the winter with cyclic megamonsoons caused by Panthalassa's ocean currents. Deserts of dry, red rock devoid of vegetation stretched throughout Pangaea's interior, sand dunes scoured the beaches, and vast mountain ranges that had risen with Pangaea's creation in the Permian (such as the Appalachians in the eastern United States) began to look wilted and weathered-down. The Triassic world lacked ice caps, and its polar regions were moist and temperate (a suitable climate for plants and animals). Spiders, centipedes, millipedes, and scorpions survived from the Permian, and newer groups of beetles flourished (grasshoppers made their debut appearance in the Triassic); the Triassic also witnessed the genesis of primitive frogs, turtles, and mammals. Animal diversity was limited, since species had to cross an entire continent of deserts to find an environment favorable towards adaptation; because of the supercontinent’s vast interior deserts, most Triassic animals probably stuck to the supercontinent’s shorelines.
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| inland Pangaea of the Triassic was mostly desert |
The arid badlands of the Pangaean interior stood in stark contrast to its perimeters: near the water, richly-vegetated pockets of plant life emerged. Conifers, gingkoes, ferns, and cycads flourished. Horsetails took root in the wettest regions. Monkey-puzzle trees, tree ferns, and modern-day club mosses painted the forests green. These forests, however, lacked flowering plants or grasses (they hadn't developed yet). Moisture from the ocean, particularly from the growing Tethys Sea, resulted in massive low-pressure cells that produced titanic monsoon rains in the coastal regions; cross-equatorial ‘megamonsoons’ were further aggravated by the strong contrast between Pangaea and the global ocean. The end of the Triassic saw climates becoming seasonal with alternating warm-cool and wet-dry cycles, and Pangaea had been steadily marching southward, producing drier and warmer climates. The stabilization of the climate did much for the Panthalassa Ocean, and a variety of new reptiles evolved, some of which would become legendary for the Mesozoic Era.
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| although the Pangaean interior was mostly desert, the coastal regions were humid and were host to an array of vibrant lifeforms |
~ Triassic Marine Life ~
Shellfish, thick-scaled fish, and marine reptiles have become iconic for the Triassic ocean. Ammonites survived the Permian-Triassic extinction and thrived in the Triassic. Because fossil fish from this period are uniform, it indicates that few families survived the extinction. Stony corals emerged for the first time, and reefs sprouted along Pangaea's coasts and throughout the Tethys Sea. A variety of marine reptiles would call these coral reefs home, and perhaps the most famous of them all are the nothosaurs.
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| an artist's rendition of a Lothosaurus |
Nothosaurs averaged about ten feet in length and had a long body with a long tail. Their webbed, paddle-like feet powered them through the water. Their necks were long, the head elongated and flattened and small in relation to its body. Nothosaurs had jaws filled with sharp, outward-pointing teeth. It's speculated that nothosaurs lived a lot like modern-day seals, catching their food (likely fish and squid) in the water but spending their resting time on the beaches. Newly-discovered impressions in China indicate that nothosaurs may have dug into the seabed by paddling in rowing motions, churning up food burrowed in the seabed.
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| an artist's rendition of Triassic placodonts |
Another type of marine reptile, the placodonts, looked like barrel-bodied lizards with a superficial resemblance to modern-day marine iguanas. Triassic placodonts were relatively small, but as carnivorous marine reptiles such as nothosaurs and plesiosaurs began to develop, placodonts responded by developing bony plates on their backs to protect themselves while foraging for food. By the late Triassic, some placodonts looked less like modern-day iguanas and more like modern-day sea turtles. Weighted down with so much armor, placodonts probably lived in shallow waters rather than in the deep ocean. Their diet likely consisted of bivalves, brachiopods, and other marine invertebrates.
A contemporary of the nothosaurs and placodonts were the newly-arrived plesiosaurs. Appearing in the late Triassic and becoming dominant in the Jurassic Period, the plesiosaurs thrived until their decimation at the Cretaceous-Paleogene Extinction. Plesiosaurs have become the fiercest Mesozoic ‘monsters of the deep,’ but in their earliest days in the late Triassic, they were but weak shadows of what they would become. Plesiosaurs had broad, flat bodies and short tails. Their limbs had evolved into long flippers. These flippers made a flying motion through the water. All plesiosaurs breathed air, bore live young, and may have been warm-blooded. Some plesiosaurs had extremely long necks and small heads; these plesiosaur behemoths became the titans of the plesiosaur family. They would've been slow and fed on small sea animals, and many would fall prey to the Black Sheep of the plesiosaur families, those with short necks and massive heads that became the apex hunters of the Mesozoic Oceans.
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| plesiosaurs along the shoreline with a prowling rauisuchian |
Plesiosaurs shared the ocean with another type of marine reptile, the ichthyosaurs: literally meaning ‘fish lizards,’ these animals varied from three to nearly fifty feet in length and resembled modern whales and dolphins. Their limbs had evolved into flippers (some replete with digits and fingers) and some species had a dorsal fin. Their heads were pointed and the jaws lined with teeth. Conical-shaped teeth were designed to catch smaller prey; bladed teeth were designed to go after the bigger animals. Their eyes were humongous (useful for diving deep), and their necks were short (some later species had almost no neck at all). Ichthyosaurs (like plesiosaurs) were air-breathing, bore live young, and were probably warm-blooded. Ichthyosaurs showed up in the Triassic and excelled; but the arrival of the plesiosaurs added a new blood of competition in the ocean, and ichthyosaurs began to dwindle, being replaced by the plesiosaurs in the late Jurassic and Cretaceous Periods. And through all of this, sharks kept doing their thing.
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| ichthyosaurs frolicking in the Tethys Sea |
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| nesting Preondactylus |
As terrestrial reptiles turned to the sea, so terrestrial reptiles turned to the air. These ‘winged reptiles’ are the earliest vertebrates known to have acquired the power of flight, predating the avian dinosaurs by millions of years. The pterosaurs dominated the empty skies in the Triassic and clung to them throughout the Mesozoic; the avian dinosaurs, though late starters, survived the Cretaceous-Paleogene Extinction whereas the pterosaurs did not. Pterosaur wings were formed by a membrane of skin, muscle, and other tissues stretching from the ankles to a lengthened fourth finger. The earliest species had long, fully toothed jaws and long tails; later forms had a reduced tail and some lacked teeth. Though many paintings and artwork of pterosaurs portray them as leathery-winged lizards, we know many of them had furry coats made up of hair-like filaments called pyncofibers; many of them, like the early Preondactylus, were covered in this hair-like material.
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| the odd-looking Longisquama |
Flying reptiles dominated Triassic skies, squeezing out their Permian forebears, the avicephalians. It’s important to note than the avicephalians weren’t capable of powered flight; they were more or less gliders. They were sparse through the Permian and barely made it into the Triassic before being outdone by the pterosaurs. They had light bones, bird-like skulls, and some had toothless beaks. They had prehensile tails like modern chameleons or monkeys. Longisquama had elongated scales; as to their function, scientists don’t know. Some speculate they acted as ‘pseudo-feathers’ forming ‘wings’. That the avicephalians never attained large populations is attested by the fact that under a dozen species are known from both the Permian and Triassic.
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| a rather risque size comparison of a tantalizing model and Lisowicia, a Late Triassic herbivorous dicynodont |
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| an artist's rendition of a feisty cynodont |
The Dicynodonts: A Closer Look
The Cynodonts: A Closer Look
~ The Age of Reptiles ~
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| a spattering of Triassic animals |
The dinosaurs – or ‘terrible lizards’ – acquired an erect gait, so that the legs could be postured directly beneath the animal (like in modern elephants). This made dinosaurs efficient: it took less energy to keep its body off the ground and it avoided Carrier's Constant, since they could run and breathe at the same time. Other distinguishing characteristics have to do with muscle locations and orientations on various bones, the shape of the joints connecting the bones to the pelvis, the particularity of their ankle joints, and some details relating to the skull. The earliest dinosaurs included Coelophysis and Plateosaurus.
The dinosaurs appeared in the mid to late Triassic, and they lived in vibrant ecosystems filled with creatures much bigger (and scarier) than them. Because dinosaurs weigh heavier in popular thought than their Triassic counterparts, an image of a Late Triassic teeming with dinosaurs has become commonplace – but it is an image rooted in fantasy rather than reality. Studies of Triassic fossil beds and trackways show that while dinosaurs emerged during the mid-late Triassic, they were never dominant. They were always relegated to the sidelines. Dinosaurs first appear in southern Pangaea and began to diversify around 230-220 mya, but they remained clustered in the humid belts of Pangaea. In the arid belts closer to the equators, dinosaurs were rare if present at all (it seems that they couldn’t handle the heat). Triassic amphibians and reptiles flourished in the interior, and in the exterior they remained dominant.
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| early dinosaurs defend their kill against a rauisuchian |
Paleontologist Steve Brusatte, who has done a lot of work in Triassic studies, notes in The Rise and Fall of Dinosaurs that dinosaurs ‘didn’t just sweep across Pangaea the moment they originated, like some infectious virus. They were geographically localized, held in place not by physical barricades but by climates they couldn’t endure. For many millions of years, it looked as if they might remain provincial rubes, stuck in one zone in the south of the supercontinent, unable to break free – an aging high school football hero of faded dreams, who could have been something if only he’d been able to get out of his tiny hometown.” (Brusatte, pp. 59-60) This was the status quo for most of the Triassic, but in the last few million years, things began to change. Change didn’t happen at once but in two stages. First, in the humid regions, the dominant large plant-eaters (the rhynchosaurs and dicynodonts) began to dwindle, even disappearing in places; scientists don’t know why, but regardless of the reasons, the herbivorous dinosaurs – particularly the prosauropods – were able to fill the niches left vacant. In the humid areas of Pangaea, dinosaurs rose to comprise thirty percent of the ecosystem whereas the previous heavyweights dropped down to twenty percent. At the very end of the Triassic, at around 215 mya, dinosaurs began migrating into the drier regions; though scientists aren’t sure why, the best guess is that climate change – caused by the slow rifting of Pangaea (to be addressed shortly) – blurred the stark boundaries between the arid interior and humid exterior. The arid regions, where only the bravest (or most foolish) dinosaurs dared to tread, became manageable.
For most of the Triassic, dinosaurs were on the ecosystem’s fringes. Dinosaur diversity was stunted compared to that of their comrades. Morphological diversity – how much a type of creature shows differences between species – has been used as a barometer of an animal’s ‘fitness.’ The idea is that ‘better’ types of creatures will be able to better adapt and evolve, whereas less ‘worthy’ creatures will not. The result is that some types of animals have a lot of morphological diversity while others don’t. A 2008 study of Triassic animals found that dinosaurs had little diversity compared to their counterparts. Paleontologist Steve Brusatte (who did the project) reports that ‘All throughout the Triassic, the pseudosuchians were significantly more morphologically diverse than dinosaurs. They filled a larger spread of the map, meaning they had a greater range of anatomical features, which indicated that they were experimenting with more diets, more behaviors, more ways of making a living. Both groups were becoming more diverse as the Triassic unfolded, but the pseudosuchians were always outpacing the dinosaurs. Far from being superior warriors slaying their competitors, dinosaurs were being overshadowed by their crocodile-line rivals during the 30 million years they coexisted during the Triassic.” (pp. 80-81)
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| an early dinosaur and a Postosuchus face off |
Paleontologist Brian Switeck echoes Brusatte's sentiments: "If you could travel back to [the Late Triassic], you’d see many vaguely familiar creatures, harbingers of what was to come, but they wouldn’t look exactly like anything we see around us today. The dinosaurian ancestors of birds were rare, slender creatures; the closest relatives of the first mammals were either hulking tusked holdovers from an earlier era or tiny shrew-like fuzzballs; and a profusion of bizarre crocodile relatives ruled the land, from armor-encased omnivores to aquatic ambush predators and bipedal dinosaur mimics. The Late Triassic was when reptiles began to rule, but the earliest dinosaurs only hinted at the potential of what was to come… Triassic dinosaurs were the prehistoric equivalent of Chekhov’s gun – loaded with potential, but not set to go off until much later in the world’s evolutionary story.”
Dinosaurs, then, were surrounded by other creatures – such as the phytosaurs, who resembled gigantic crocodiles but aren’t to be confused with the modern crocodile's ancestors (though they lived in the Triassic, they were smaller, moving with the speed and agility of a greyhound and resembling reptilian wolves). The aetosaurs were large, armored herbivorous reptiles preyed upon by the ornithosuchians, which resembled titanic crocodiles with double rows of armored plates running along their backs. The biggest and baddest Triassic carnivores were the rauisuchians, the fear and dread of the earliest dinosaurs. Like dinosaurs, rauisuchians had an erect gait with their legs oriented beneath them in a pillar-erect posture. The rauisuchians lived through most of the Triassic but died out in the Triassic-Jurassic Extinction. Their absence would give the earliest theropod dinosaurs room to grow into even larger carnivores than the rauisuchians could muster.
Rauisuchian dominance came to an end with the Triassic-Jurassic Extinction Event. The Tethys Sea was beginning to cut through Pangaea due to the shifting of tectonic plates, and the supercontinent would soon split in half between Laurasia to the north and Gondwana to the south. This great wrenching wouldn't be accomplished until the Jurassic, but the Triassic suffered the birth pangs: volcanic eruptions unlike anything the world has seen before or since (except for when the planet was boiling in its own stew) scarred the atmosphere. Oxygen depleted and carbon dioxide increased exponentially. Though nowhere near as destructive as the extinction that preceded it, the Triassic-Jurassic extinction hit the ocean especially hard: all marine reptiles except ichthyosaurs, nothosaurs, and plesiosaurs died out. Even invertebrates such as brachiopods, gastropods, and mollusks suffered. Twenty-two percent of marine families and half of marine genera went extinct. On land, the dinosaurs' predatory rivals went extinct along with some of the earlier dinosaurs. But the hardier and more adaptive dinosaurs survived into the Jurassic where they would flourish.
~ The Archosaurs: A Closer Look ~
The dinosaurs were just one branch of many in a wider family tree called Archosauria, 'the ruling reptiles.' There are only two types of archosaurs alive today: birds and crocodilians. During the past 250 million years, however, there were a myriad of types, all of which died out but those two lineages that remain today. Archosaurs are ‘diapsid amniotes’: they are ‘diapsid’ because they have ‘two arches’ (or holes, called temporal fenestrae) in each side of their skulls, in front of the eyes and the jaw; and they are ‘amniotes’ because they lay eggs on land (as opposed to fishes and amphibians, which lay eggs in water) or retain the fertilized egg in the mother (as is the case with mammals). Archosaurs first evolved around 300 mya during the late Carboniferous, and they diversified like wildfire: all crocodiles, lizards, snakes, turtles, birds, non-avian dinosaurs, and pterosaurs are classified as archosaurs. Some diapsids have lost one hole in the skull (such as lizards), and snakes lost both holes; they remain archosaurs, however, because of their ancestry.
Distinguishing characteristics of archosaurs, aside from being amniotes, include (as aforementioned) two fenestrae on each side of the head, teeth set in sockets, and a prominent ridge on the femur called the fourth trochanter. Some later archosaurs (such as snakes and lizards) don’t have all these characteristics, but their descent from archosaurs keeps them ‘in the family.’ The double fenestrae on each side of the skull lightened the skull’s weight, enabling archosaurs to develop larger skulls with larger jaws. The fourth trochanter provided a site for the attachment of muscles on the femur; these stronger muscles allowed for the development of an erect gait, seen most vividly in the dinosaurs. All archosaurs fell into one of three categories when it came to posture: there were the ‘sprawlers’ (whose legs were thrown out to the sides so that they crawled, like lizards); animals with a ‘semi-improved’ posture (with upper leg bones held more vertically and placed at a decent angle to the rest of the body, as in modern crocs); and some – like dinosaurs – had ‘erect’ posture with limbs held in a column-like fashion directly beneath their bodies. Both rauisuchians and dinosaurs had erect postures, despite the arrangements of their hip bones being different while allowing a similar posture.
Archosaurs made leaps and bounds in the Permian and diversified like mad in the Triassic. They became erect, giving them a distinct edge against their rivals, the mammal-like reptilian synapsids; their dominance, however, began before they got their legs truly beneath them. They adapted to the desert landscape and arid atmosphere of the Triassic, developing elevated metabolisms and avian breathing systems so that they could be active in the hot, oxygen-deprived world in which they lived. Many scientists believe several archosaurs may have had four-chambered hearts like modern birds and crocodiles, enabling them to be more active due to better oxygenation. The archosaur clade has generally been divided into two further clades: pseudosuchia (those archosaurs closer to crocodiles than birds, and which includes modern crocodilians and their extinct relatives) and avemetatarsalia (those archosaurs closer to birds, and which includes modern birds and their extinct relatives). Archosaurs still walk – and fly! – among us today. Most archosaurs were and continue to be predators; however, some archosaurs, such as the aetosaurs, were herbivores, as were some crocodilians such as Simosuchus.
Were you to step back into the Late Permian that preceded the Triassic, you would find no inklings of an upcoming 'Age of Reptiles.' Mammal-like creatures dominated the Permian, and there was no reason to think their dominance would end. Paleontologist Brian Switeck notes that during the Permian, “landscapes the world over were filled with synapsids – the diverse array of creatures more closely related to you and me than to reptiles. Barrel-bodied, tusked dicynodonts grazed in huge herds; gorgonopsians – saber-fanged predators built like excessively muscular dogs – hunted down large prey; and the precursors to the first true mammals – the small, shuffling cynodonts – burrowed and snuffled their way around the Permian world. Then everything went to hell… [After the Permian-Triassic Extinction], complex and thriving ecosystems were replaced by degraded crisis communities populated by just a few species.” It was in this vacuum that reptiles – notably the archosaurs – were able to blossom and dominate. Black continues, “The very first archosaurs originated after the effects of the Permian mass extinction ebbed. While the precise origins of the group are as yet unclear, by 244 million years ago archosaurs were already stalking across the world.” The Age of Mammals was set to continue unabated, but the Permian-Triassic Extinction changed all that. The burgeoning Mesozoic Era turned the tables so that reptiles would have their go. Looking back down the eons from our privileged place in the Anthropocene, it's hard not to view the Age of Reptiles as a sort of interruption - after all, we are in a rejuvenated 'Age of Mammals,' standing on the shoulders of our Permian forefathers.
That reptiles came to dominate the landscape at the beginning of the Triassic is unquestionable; but questions do abound when it comes to understanding what, exactly, catered to reptile dominance. Brian Switeck, however, has a theory: reptiles simply dominated the new ecological niches first. He writes, "We don’t know why the archosaurs took over the world from our synapsids precursors. Not for certain. But a large-scale analysis of archosaur and synapsids fossils give us a clue. When researchers tracked the fates of the two lineages [of archosaurs and synapsids] from the Permian, they didn’t find many signs of direct competition between our primordial relatives and the archosaurs. It was not as if synapsids and archosaurs directly battled each other for control of habitats. Mammals and their forerunners, for the most part, remained relatively small, while the archosaurs diversified into a variety of body sizes – from creatures the size of a pigeon to the largest of the sauropod dinosaurs reaching one hundred feet in length or more. Biological constraints might have made all the difference. From what paleontologists have been able to tell from bone microstructure, the ancestors and cousins of both crocodiles and dinosaurs grew faster and began reproducing earlier in their lives than proto-mammals. This shorter generation time allowed them to proliferate and be molded by natural selection faster, and they simply outgrew their proto-mammal competition. Mammals didn’t have a chance to get big because the faster-growing archosaurs had already taken up that ecological space.”
The pseudosuchians – the first off-branching clade of the archosaurs – includes all reptiles closer to crocodiles than to birds. A distinguishing characteristic is that their feet bones resemble those of a crocodile (as opposed to the feet of the avemetatarsalians, which resemble those of birds). The pseudosuchians have been further divided into more particular clades: the phytosaurs, the aetosaurs, the ornithosuchians, the rauisuchians, and the crocodylomorphs.
Phytosaurs were large, semi-aquatic archosaurs that lived in the Late Triassic. They were long-snotted and heavily armored, resembling modern crocodiles in size, appearance, and lifestyle. Though they were deadly carnivores, their name implies that they were plant-eaters; this is because the first fossils were thought to belong to an herbivore. Paleontologists believe phytosaurs came around before the archosaurs split between crocodile- and bird-like forms.
The aetosaurs were medium- to large-sized archosaurs, heavily armored and herbivorous. They had small heads, upturned snouts, erect limbs, and bodies covered by plate-like scutes. Unlike the phytosaurs who enjoyed a good swim, aetosaurs were wholly terrestrial. It’s speculated that many species burrowed for their food, and there’s evidence that some (if not all) aetosaurs made nests.
Another type of pseudosuchian, ornithosuchians, were able to walk on their hind legs like many dinosaurs; these ‘bird-crocodiles’ would’ve spent most of their time on all fours, reserving bipedal locomotion for quick bursts of speed. Ornithosuchians, such as the name-sake Ornithosuchus, had a double row of armored plates that ran along their backs. Ornithosuchids weren't pseudosuchians, as they have lots of dissimilarities (not least of all a different ankle arrangement, called 'reverse-crocodile').
The erythrosuchids lived from the early to the middle Triassic Period. These reptiles technically weren't archosaurs but archosauriformes (though they had a lot of archosaur characteristics, they diverged on their evolutionary path before true archosaurs developed). These 'pseudo-archosaurs' were unusually large with outsized, deep heads that had a ‘step’ in the jaw due to the tip of the lower jaw rising above the tip of the upper jaw. These were the first archosauriformes to develop a triradiate pelvic girdle consisting of three bones: the ischium, ilium, and pubis. These were the biggest, strongest, and most dangerous predators of the early Triassic. They’ve been dubbed the ‘Crimson Crocodiles’ since being discovered in the red rock of Triassic bone-beds. Some reached up to half a ton in weight and were armed with three-feet-long, dinosaur-like heads filled with saw-edged teeth.
The rauisuchians were large Triassic archosaurs with an erect gait later seen in dinosaurs (but it was a matter of convergent evolution; the rauisuchians did not evolve from, or to, dinosaurs). Though a dinosaur’s hip socket faces outward and its femur connects to the side of the hip, a rauisuchians’ hip socket faces downward to form a shelf of bone under which the femur connects. This posture has been called a ‘pillar-erect’ posture. Rauisuchians lived through most of the Triassic but were eliminated in the Triassic-Jurassic extinction event; their demise gave theropod dinosaurs the space needed to become the dominant terrestrial predators of the Mesozoic.
Last on our list of pseudosuchians are the crocodylomorpha. These were the ancestors of modern-day crocodiles, but they looked remarkably different from what we’re used to. These were small, lightly-built, active terrestrial animals. The earliest members of this group saw action in the Triassic and lasted until the Late Jurassic. The earliest ‘crocodiles’, such as Hesperosuchus, were built like greyhounds and resembled reptilian wolves, but as the Mesozoic plodded on, the crocodiles diversified to become larger and open to new ideas: some later crocodylomorphs became semi-aquatic, like crocodiles today. The largest of these crocodylomorphs, Sarcosuchus, lived in the late Cretaceous; it was during the late Cretaceous that the earliest ‘modern’ crocodiles evolved.
The pseudosuchians continue today with the crocodiles; the other branch of archosaurs, avemetatarsalia, continues today with birds. This group includes all archosaurs more closely related to birds than crocodiles. The name of this group means ‘bird metatarsals,’ hinting at the distinguishing characteristic of these archosaurian feet, but the group also goes by other names, such as Pan-Aves, Ornithisuchia, and Ornithodira. Members of avemetatarsalia include pterosaurs, dinosauromorphs, dinosauria, and birds.
Pterosaurs, as mentioned above, were the flying reptiles of the Mesozoic. Their remains have been found from the late Triassic to the end of the Cretaceous, and they’re the earliest vertebrates known to have evolved powered flight (the avian dinosaurs, which would pave the way to birds, didn’t come until later on in the Mesozoic). Pterosaurs didn’t have feathers; their wings were formed by a membrane of skin and muscle stretching from the ankles to a dramatically lengthened fourth finger. Early pterosaurs had long, fully toothed jaws and long tails; later forms had a reduced tail and some even lacked teeth. Many had furry coats of hair-like filaments known as pyncofibers that covered their bodies and parts of their wings.
Dinosauromorpha includes dinosaurs and other closely related animals. They appeared in the Middle Triassic around 240 mya; true dinosaurs, belonging to the clade dinosauria, made their debut around 230 mya. Though dinosaurs diversified in the later Triassic, they weren’t the dominant creatures; though many later Triassic dinosaurs reached large proportions, the top-tier Triassic predators were pseudosuchians. This changed after the Triassic-Jurassic extinction event; from this point on, through the Jurassic and Cretaceous Periods, the dinosaurs diversified and dominated the landscape. Their dominance ended with the Cretaceous-Paleogene extinction event 65 mya, which wiped out up to 75% of earth’s plant and animal life. The last major clade of avemetatarsalia are the birds, which evolved from theropod dinosaurs. Due to their small size, ability to keep warm with feathery insulation, and lacking bigger predators, most were able to not only survive the Cretaceous-Paleogene extinction event but also to thrive in the Cenozoic. Thousands of bird species continue today, a testament to the ingenuity and ongoing success of the dinosaurs. One could even say, with a hint of truth, that dinosaurs still rule the world.
The Rhynchosaurs: A Closer Look
The Phytosaurs: A Closer Look
The Crocodylomorphs: A Closer Look
The Aetosaurs: A Closer Look
The Ornithosuchids: A Closer Look
The Erythrosuchids: A Closer Look
The Rauisuchians: A Closer Look
Distinguishing characteristics of archosaurs, aside from being amniotes, include (as aforementioned) two fenestrae on each side of the head, teeth set in sockets, and a prominent ridge on the femur called the fourth trochanter. Some later archosaurs (such as snakes and lizards) don’t have all these characteristics, but their descent from archosaurs keeps them ‘in the family.’ The double fenestrae on each side of the skull lightened the skull’s weight, enabling archosaurs to develop larger skulls with larger jaws. The fourth trochanter provided a site for the attachment of muscles on the femur; these stronger muscles allowed for the development of an erect gait, seen most vividly in the dinosaurs. All archosaurs fell into one of three categories when it came to posture: there were the ‘sprawlers’ (whose legs were thrown out to the sides so that they crawled, like lizards); animals with a ‘semi-improved’ posture (with upper leg bones held more vertically and placed at a decent angle to the rest of the body, as in modern crocs); and some – like dinosaurs – had ‘erect’ posture with limbs held in a column-like fashion directly beneath their bodies. Both rauisuchians and dinosaurs had erect postures, despite the arrangements of their hip bones being different while allowing a similar posture.
Archosaurs made leaps and bounds in the Permian and diversified like mad in the Triassic. They became erect, giving them a distinct edge against their rivals, the mammal-like reptilian synapsids; their dominance, however, began before they got their legs truly beneath them. They adapted to the desert landscape and arid atmosphere of the Triassic, developing elevated metabolisms and avian breathing systems so that they could be active in the hot, oxygen-deprived world in which they lived. Many scientists believe several archosaurs may have had four-chambered hearts like modern birds and crocodiles, enabling them to be more active due to better oxygenation. The archosaur clade has generally been divided into two further clades: pseudosuchia (those archosaurs closer to crocodiles than birds, and which includes modern crocodilians and their extinct relatives) and avemetatarsalia (those archosaurs closer to birds, and which includes modern birds and their extinct relatives). Archosaurs still walk – and fly! – among us today. Most archosaurs were and continue to be predators; however, some archosaurs, such as the aetosaurs, were herbivores, as were some crocodilians such as Simosuchus.
Were you to step back into the Late Permian that preceded the Triassic, you would find no inklings of an upcoming 'Age of Reptiles.' Mammal-like creatures dominated the Permian, and there was no reason to think their dominance would end. Paleontologist Brian Switeck notes that during the Permian, “landscapes the world over were filled with synapsids – the diverse array of creatures more closely related to you and me than to reptiles. Barrel-bodied, tusked dicynodonts grazed in huge herds; gorgonopsians – saber-fanged predators built like excessively muscular dogs – hunted down large prey; and the precursors to the first true mammals – the small, shuffling cynodonts – burrowed and snuffled their way around the Permian world. Then everything went to hell… [After the Permian-Triassic Extinction], complex and thriving ecosystems were replaced by degraded crisis communities populated by just a few species.” It was in this vacuum that reptiles – notably the archosaurs – were able to blossom and dominate. Black continues, “The very first archosaurs originated after the effects of the Permian mass extinction ebbed. While the precise origins of the group are as yet unclear, by 244 million years ago archosaurs were already stalking across the world.” The Age of Mammals was set to continue unabated, but the Permian-Triassic Extinction changed all that. The burgeoning Mesozoic Era turned the tables so that reptiles would have their go. Looking back down the eons from our privileged place in the Anthropocene, it's hard not to view the Age of Reptiles as a sort of interruption - after all, we are in a rejuvenated 'Age of Mammals,' standing on the shoulders of our Permian forefathers.
That reptiles came to dominate the landscape at the beginning of the Triassic is unquestionable; but questions do abound when it comes to understanding what, exactly, catered to reptile dominance. Brian Switeck, however, has a theory: reptiles simply dominated the new ecological niches first. He writes, "We don’t know why the archosaurs took over the world from our synapsids precursors. Not for certain. But a large-scale analysis of archosaur and synapsids fossils give us a clue. When researchers tracked the fates of the two lineages [of archosaurs and synapsids] from the Permian, they didn’t find many signs of direct competition between our primordial relatives and the archosaurs. It was not as if synapsids and archosaurs directly battled each other for control of habitats. Mammals and their forerunners, for the most part, remained relatively small, while the archosaurs diversified into a variety of body sizes – from creatures the size of a pigeon to the largest of the sauropod dinosaurs reaching one hundred feet in length or more. Biological constraints might have made all the difference. From what paleontologists have been able to tell from bone microstructure, the ancestors and cousins of both crocodiles and dinosaurs grew faster and began reproducing earlier in their lives than proto-mammals. This shorter generation time allowed them to proliferate and be molded by natural selection faster, and they simply outgrew their proto-mammal competition. Mammals didn’t have a chance to get big because the faster-growing archosaurs had already taken up that ecological space.”
The pseudosuchians – the first off-branching clade of the archosaurs – includes all reptiles closer to crocodiles than to birds. A distinguishing characteristic is that their feet bones resemble those of a crocodile (as opposed to the feet of the avemetatarsalians, which resemble those of birds). The pseudosuchians have been further divided into more particular clades: the phytosaurs, the aetosaurs, the ornithosuchians, the rauisuchians, and the crocodylomorphs.
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| a phytosaur sunbathing beside a stream |
Phytosaurs were large, semi-aquatic archosaurs that lived in the Late Triassic. They were long-snotted and heavily armored, resembling modern crocodiles in size, appearance, and lifestyle. Though they were deadly carnivores, their name implies that they were plant-eaters; this is because the first fossils were thought to belong to an herbivore. Paleontologists believe phytosaurs came around before the archosaurs split between crocodile- and bird-like forms.
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| an artist's rendition of an aetosaur |
Another type of pseudosuchian, ornithosuchians, were able to walk on their hind legs like many dinosaurs; these ‘bird-crocodiles’ would’ve spent most of their time on all fours, reserving bipedal locomotion for quick bursts of speed. Ornithosuchians, such as the name-sake Ornithosuchus, had a double row of armored plates that ran along their backs. Ornithosuchids weren't pseudosuchians, as they have lots of dissimilarities (not least of all a different ankle arrangement, called 'reverse-crocodile').
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| an Ornithosuchus in bipedal stance |
The erythrosuchids lived from the early to the middle Triassic Period. These reptiles technically weren't archosaurs but archosauriformes (though they had a lot of archosaur characteristics, they diverged on their evolutionary path before true archosaurs developed). These 'pseudo-archosaurs' were unusually large with outsized, deep heads that had a ‘step’ in the jaw due to the tip of the lower jaw rising above the tip of the upper jaw. These were the first archosauriformes to develop a triradiate pelvic girdle consisting of three bones: the ischium, ilium, and pubis. These were the biggest, strongest, and most dangerous predators of the early Triassic. They’ve been dubbed the ‘Crimson Crocodiles’ since being discovered in the red rock of Triassic bone-beds. Some reached up to half a ton in weight and were armed with three-feet-long, dinosaur-like heads filled with saw-edged teeth.
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| a pair of erythrosuchids sparring over a kill |
The rauisuchians were large Triassic archosaurs with an erect gait later seen in dinosaurs (but it was a matter of convergent evolution; the rauisuchians did not evolve from, or to, dinosaurs). Though a dinosaur’s hip socket faces outward and its femur connects to the side of the hip, a rauisuchians’ hip socket faces downward to form a shelf of bone under which the femur connects. This posture has been called a ‘pillar-erect’ posture. Rauisuchians lived through most of the Triassic but were eliminated in the Triassic-Jurassic extinction event; their demise gave theropod dinosaurs the space needed to become the dominant terrestrial predators of the Mesozoic.
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| a quartet of rauisuchians |
Last on our list of pseudosuchians are the crocodylomorpha. These were the ancestors of modern-day crocodiles, but they looked remarkably different from what we’re used to. These were small, lightly-built, active terrestrial animals. The earliest members of this group saw action in the Triassic and lasted until the Late Jurassic. The earliest ‘crocodiles’, such as Hesperosuchus, were built like greyhounds and resembled reptilian wolves, but as the Mesozoic plodded on, the crocodiles diversified to become larger and open to new ideas: some later crocodylomorphs became semi-aquatic, like crocodiles today. The largest of these crocodylomorphs, Sarcosuchus, lived in the late Cretaceous; it was during the late Cretaceous that the earliest ‘modern’ crocodiles evolved.
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| a Sarcosuchus has a theropod dinosaur for a snack |
The pseudosuchians continue today with the crocodiles; the other branch of archosaurs, avemetatarsalia, continues today with birds. This group includes all archosaurs more closely related to birds than crocodiles. The name of this group means ‘bird metatarsals,’ hinting at the distinguishing characteristic of these archosaurian feet, but the group also goes by other names, such as Pan-Aves, Ornithisuchia, and Ornithodira. Members of avemetatarsalia include pterosaurs, dinosauromorphs, dinosauria, and birds.
Pterosaurs, as mentioned above, were the flying reptiles of the Mesozoic. Their remains have been found from the late Triassic to the end of the Cretaceous, and they’re the earliest vertebrates known to have evolved powered flight (the avian dinosaurs, which would pave the way to birds, didn’t come until later on in the Mesozoic). Pterosaurs didn’t have feathers; their wings were formed by a membrane of skin and muscle stretching from the ankles to a dramatically lengthened fourth finger. Early pterosaurs had long, fully toothed jaws and long tails; later forms had a reduced tail and some even lacked teeth. Many had furry coats of hair-like filaments known as pyncofibers that covered their bodies and parts of their wings.
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| a group of Coelophysis at a watering hole |
The Rhynchosaurs: A Closer Look
The Phytosaurs: A Closer Look
The Crocodylomorphs: A Closer Look
The Aetosaurs: A Closer Look
The Ornithosuchids: A Closer Look
The Erythrosuchids: A Closer Look
The Rauisuchians: A Closer Look
~ Triassic Plant Life ~
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| Triassic Colorado, ca 225 million years ago |
The northern half of Pangaea was dominated by ginkgo and tree-fern forests, and the forest floor was carpeted by ferns. Heading south towards the equator, where it was drier and the deserts began, these lush northern forests disintegrated into patchier and thinner forests built mostly of conifers and cycads. Rivers and coastlines sported highly fertile areas, much as the modern Euphrates waters the deserts of the Middle East. Near these waterways, dense vegetation grew up with ferns and horsetails sprouting along the ground and interspersed with groves of cycads, reed-ferns, ginkgos, and conifers. This general sketch is derived from fossilized plant life from the Triassic, but it’s unfortunate that floral fossils are rare. Nevertheless, paleontologists have been able to give us a glimpse into Triassic plant life.
Triassic plant life generally fell into two categories: spore-bearing plants and seed-bearing ones. Spore-bearing pteridophytes rely on the wind for spreading their spores; when windblown spores land in moist settings, they’re able to germinate; when sperm is reproduced, the sperm ‘swims’ in search of eggs formed in nearby germinated plants. Because this method of reproduction is inefficient (not to mention laborious!), most spore-bearing plants sprout new branches from a single underground stem so that one reproductive event can generate an entire field of plants. Spore-bearing plants are dependent upon moist conditions for reproduction; thus spore-bearing plants couldn’t meander far from bodies of water – quite a disadvantage in the arid ecosystems of the Triassic. Seed-bearing plants (called gymnosperms) freed plants from the restrictions imposed by spore reproduction. Gymnosperms were able to radiate away from water sources and colonize drier terrain; and because the spore-bearing plants couldn’t compete, gymnosperms were able to multiply as if on steroids.
The four major groups of gymnosperms alive today are cycads, ginkgos, conifers, and angiosperms; of these four, only one – the ‘flowering plants’, or angiosperms – were absent in the Triassic. Cycads were evergreen plants with short, stout trunks and long, pinnate leaves that look like those of modern palms; ginkgo trees have characteristic, fan-shaped leaves, and only one species remains alive today; and conifers are woody plants with seeds born in cones and leaves that usually take the form of needles (modern conifers include redwoods, pines, junipers, firs, cypresses, and cedars). We will examine some spore-bearing plants of the Triassic (such as ferns and horsetails) before looking at the gymnosperm newcomers. However, we would be remiss if we failed to mention the lycopods, one of the most primitive types of plants with descendants still around today.
The spore-bearing lycopods are the oldest living vascular plant division, having appeared on the scene 410 mya in the Silurian Period. They flourished in the Carboniferous Period, reaching proportions hard to imagine today. It’s difficult to picture modern lycopods – such as club mosses – reaching to the heavens, but Lepidodendron grew up to one hundred feet tall and could have a trunk three feet in diameter. Unlike modern trees, leaves grew out of the entire surface of the trunk and branches, falling off as the plant grew, eventually leaving only a small cluster of leaves at the top. These lycopod trees comprised vast forests during the Carboniferous, but they began to get squeezed out as more advanced plants (such as ferns, horsetails, and conifers) developed in the Permian. Lycopods were able to regain a foothold in the wake of the Permian-Triassic extinction event, spreading like kudzu in the early Triassic. Their resurgence, however, was brief, and the Mesozoic landscape was soon dominated by the rising gymnosperms.
Ferns, such as we have today, existed in the Triassic, and they reproduce by spores rather than seeds. Ferns have stems and leaves, and most have fiddleheads, furled fronds that unroll into a new frond. Ferns appeared in the late Devonian Period, but their diversification skyrocketed in the early Cretaceous, and in the latter part of that same period, the ‘Great Fern Radiation’ resulted in the emergences of many modern fern families. Another spore-bearing plant of the Triassic was the horsetail, also known as snake grass or scouring-rush. Modern horsetails are considered ‘living fossils’ because they’ve undergone few changes since their genesis in the Devonian or Carboniferous Period. Triassic horsetails grew along the waterways, and scientists speculate they could have reached up to one hundred feet in height. Though horsetails would’ve been a decent source of nutrients for Triassic herbivores, they have an adaptation that dissuades consumption: special cells ‘drink up’ silica from the soil, and the silica ‘armors’ the horsetail stem with row upon row of glass-hard micro-lumps. An herbivore would quickly learn that horsetails erode their teeth down to the gum-line, but the less intelligent (or more laissez-faire) may not have cared. Horsetails have a backup protection system: because they grow from roots buried underground, the stem could be eaten to the ground and the horsetail would continue re-growing from underground.
Now we turn to the seed-bearing gymnosperms of the Triassic, paying special attention to seed ferns (not to be confused with their spore-bearing counterparts), cycads, ginkgos, conifers, and the infamous monkey puzzle trees. Seed ferns evolved in the late Devonian and flourished during the Carboniferous and Permian. They declined during the Mesozoic and had all but disappeared by the end of the Cretaceous (a few species carried on into the early Cenozoic). Seed ferns carried their seeds in their fronds with distinctive glandular hairs. Some seed ferns are called ‘tree ferns’ because they had a trunk elevating the fronds above ground level. There were at least three major species of seed ferns that have been readily fossilized from the Triassic. The first, Glossopteris, had tongue-shaped leaves and grew to about twelve feet tall, tapering upwards like a Christmas tree. Its limbs were probably widely-spaced to take advantage of the low-angle sunlight at high latitudes. They had large, broad leaves fell to the ground at the end of summer; Glossopteris leaves are often found in dense accumulations from autumnal leaf banks. This deciduous tree went extinct with the Triassic-Jurassic extinction event (but some paleobotanists argue that it all but disappeared at the end of the Permian). Glossopteris, so prevalent in the Permian, was outdone by another seed fern called Dicroidium. This seed fern spread like a plague over Triassic Pangaea. Dicroidium’s leaves resembled those of modern ferns, except they were forked, giving the appearance of two leaves joined at the base. Like Glossopteris they were deciduous trees, leaving their leaves seasonally. Dicroidium flourished in all Triassic environments, from heath and woodlands to broad-leafed forest. In some locales it’s the only plant present in the Triassic fossil-beds. A third seed fern, Psaronius, lacked Dicroidium’s success but is notable in that it could grow up to thirty feet tall.
The four major groups of gymnosperms alive today are cycads, ginkgos, conifers, and angiosperms; of these four, only one – the ‘flowering plants’, or angiosperms – were absent in the Triassic. Cycads were evergreen plants with short, stout trunks and long, pinnate leaves that look like those of modern palms; ginkgo trees have characteristic, fan-shaped leaves, and only one species remains alive today; and conifers are woody plants with seeds born in cones and leaves that usually take the form of needles (modern conifers include redwoods, pines, junipers, firs, cypresses, and cedars). We will examine some spore-bearing plants of the Triassic (such as ferns and horsetails) before looking at the gymnosperm newcomers. However, we would be remiss if we failed to mention the lycopods, one of the most primitive types of plants with descendants still around today.
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| an artist's rendition of Lepidodendron in various cycles of its life; from Carboniferous Period |
Ferns, such as we have today, existed in the Triassic, and they reproduce by spores rather than seeds. Ferns have stems and leaves, and most have fiddleheads, furled fronds that unroll into a new frond. Ferns appeared in the late Devonian Period, but their diversification skyrocketed in the early Cretaceous, and in the latter part of that same period, the ‘Great Fern Radiation’ resulted in the emergences of many modern fern families. Another spore-bearing plant of the Triassic was the horsetail, also known as snake grass or scouring-rush. Modern horsetails are considered ‘living fossils’ because they’ve undergone few changes since their genesis in the Devonian or Carboniferous Period. Triassic horsetails grew along the waterways, and scientists speculate they could have reached up to one hundred feet in height. Though horsetails would’ve been a decent source of nutrients for Triassic herbivores, they have an adaptation that dissuades consumption: special cells ‘drink up’ silica from the soil, and the silica ‘armors’ the horsetail stem with row upon row of glass-hard micro-lumps. An herbivore would quickly learn that horsetails erode their teeth down to the gum-line, but the less intelligent (or more laissez-faire) may not have cared. Horsetails have a backup protection system: because they grow from roots buried underground, the stem could be eaten to the ground and the horsetail would continue re-growing from underground.
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| Glossopteris grow above a bed of ferns in the early Triassic |
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| a rauisuchian wanders among a grove of cycads |
Another type of Triassic gymnosperm was the cycads, seed plants that emerged in the Carboniferous Period. They’re still around today, but only sparsely compared to their prevalence in the Mesozoic. Most cycads have a stout and woody trunk with a crown of large, hard, stiff, evergreen leaves. Their leaves are usually pinnate (arranged in a feather-like pattern) and grow in a rosette form from the top of the trunk. Cycads have a cylindrical trunk; as the trunk grows, the leaves – which are generally the same size or larger than the trunk – sprout only from the top, so that cycads are marked by a crown of leaves at their top. Some cycads’ trunks are buried underground, giving the cycad an appearance like a forest-hugging fern. Cycads reproduce by a cone sprouting from the top of the trunk in the middle of the leaves. Cycads are slow growers and can live up to a thousand years. They are only distantly related to palms and ferns, despite their obvious similarities. During the Triassic and Jurassic Periods, cycads comprised twenty percent of the earth’s plant life (paleobotanists sometimes refer to the Triassic-Jurassic as the Age of the Cycads). Cycads formed the ground cover of the Mesozoic: these primitive cycads were stumpy, with round or barrel-like stems, or they had tall, slender trunks topped by a tuft of palm-like leaves. Though cycads never reached the height of Triassic conifers, some could rise up to five to ten feet in height. Common Triassic cycads include Leptocycas, Cycas, Zamia, Dioon, Bowenia, Stangeria, and Microcyas. Similar in appearance to cycads – with thick woody trunks and tough, palm-like leaves – were the Bennettitaleans; these plants resembled cycads but differed in the arrangement of their stoma. Bennettitaleans appeared in the Triassic, but they could never compete with the cycads, and they dwindled to extinction at the end of the Cretaceous with the rise of the flowering plants.
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| a modern 'maidenhair' ginkgo tree, a 'living fossil' dating back to the Triassic |
Another type of seed-bearing plant, ginkgos, appeared in the Permian Period. They flourished in the Jurassic but waned in the Triassic; they went all but extinct in the Pliocene. We say ‘almost’ because there is an exception: Ginkgo biloba, the ‘maidenhair tree,’ considered a ‘living fossil’ since its undergone very little change from its emergence in the Permian. This tree grows in the wild only in China, and it can reach 65 to 115 feet tall (some have even reached over 160 feet in height). The maidenhair has an angular crown and long, erratic branches; it’s deep-rooted and resistant to wind and snow damage. Young maidenhairs are tall and slender and sparsely branched; the crown becomes broader as the tree ages. The maidenhair leaves are fan-shaped with veins radiating out into the leaf blade; every autumn the leaves turn a bright yellow before falling. One can envisage a Triassic dinosaur standing under the branches of a maidenhair and thus capture an accurate image from the past.
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| a modern 'monkey puzzle tree' rises above the forest canopy in Brazil |
Conifers, which dominate much of our present world, emerged during the early Triassic and crawled to dominance through the Mesozoic. The eight conifer families that exist today are represented in Triassic-era fossils. Conifers are typified by a pyramidal growth in many species, along with needle- or scale-like and often waxy leaves. Conifers have a pollen tube that delivers sperm directly to the eggs, making them more reproductively advanced than other Triassic seed-bearing plants. Conifers excelled in the Triassic’s drier climate, especially in the arid interior. Examples of Triassic conifers include Mataia and Rissikia, though the most common was Araucarioxylon. This last conifer could reach up to hundred feet in height and measured more than two feet in diameter. Their fossilized bark is often scoured by boreholes made by ancient beetles. Araucarioxylon was the dominant tree in the northern hemisphere, where its trunks reached high above the cycads and ginkgoes; in the south, Araucarioxylon were common, but the warmer, wetter climate forced them to compete for space with other plants. An honorable mention is the Araucaria, or monkey puzzle tree. These evergreen conifers emerged during the Triassic and have lasted to the present day. Modern monkey puzzle trees are considered ‘living fossils.’ They can reach between 100-250 feet in height. Their horizontal, spreading branches grow in whorls and are covered with leathery- or needle-like leaves. In some species the leaves are narrow and awl-shaped, barely overlapping; but in others they are broad and flat, overlapping broadly. Some paleontologists believe that the long necks of sauropod dinosaurs evolved specifically to browse the foliage of the tallest money puzzle trees; their global distribution in vast forests during the Jurassic makes it likely that they were the major high energy food source for adult sauropods.
~ The Triassic-Jurassic Extinction Event ~
The ‘dividing line’ between the Late Triassic and the Early Jurassic is an extinction event caused by the supercontinent Pangaea ripping apart. Pangaea was created during the Permian as earth’s tectonic plates crunched together (the Appalachian Mountains were created by this ancient event). One of earth’s most recent mountain ranges, the Himalayas, were created long after the Mesozoic by the collision of India with the southern edge of the Eurasian plate; scientists speculate that the rising Himalayas disrupted global air circulation, creating monsoons and helping to nudge the earth into a prolonged phase of cooling and drying that culminated in the Cenozoic Ice Ages. The earth’s crust is divided into broad, rigid, and constantly moving plates that float on firmer layers of mantle. When the plates interact, the result is geological activity (think earthquakes, growing mountain ranges, and volcanoes). The drifting and interacting of tectonic plates is called ‘continental drift.’ On the ocean floor are mid-oceanic ridges where new sea floor is being created; where sea floor is being destroyed, the result is ocean trenches (as an aside, the oldest known ocean floor dates back to the Jurassic Period). The pattern of these ridges and trenches reveal a pattern of gargantuan ‘plates’ that behave like lumbering, slow-moving conveyor belts carrying the continents around the earth’s surface. The process is driven by convection currents in the mantle that are created by the intense heat running through our planet’s core. Earth’s tectonic plates move at a snail’s pace of about four centimeters (1.6 inches) each year, about the same rate as the growth of human fingernails. Over the vast spans of geological time, the plates move about twenty-five miles every million years.
The plates were moving in the Late Triassic just as they’re moving now; but as the Permian saw the formation of the supercontinent Pangaea, the Late Triassic saw the genesis of its breakup. As Pangaea began to split (the upper portion would become ‘Laurasia’ and the southern portion ‘Gondwana’), it stretched at the seams, creating ‘rift basins.’ The shallow bowls of stretched earth were often filled with water, creating lakes and rivers; many would seasonally dry out, resulting in fantastic ‘bone beds’ for future paleontologists. As the stretching intensified, the ripping began. Pangaea ruptured along the modern-day U.S. eastern seaboard (the Palisades in New York are remnants of this split), and the rift basins were inundated by volcanic lava flows. There would be four cycles of mass eruptions before Pangaea finally split down the middle, and the process took about half a million years. These volcanic ‘pulses’ weren’t meager little things: they sent tsunamis of lava surging across the Late Triassic landscape. This is no exaggeration, as some of the lava flows reached up to three thousand feet in thickness. These lava tsunamis could’ve buried the Empire State Building twice over.
Three million square miles of central Pangaea were buried by lava, and the effect on climate was catastrophic. Survival wasn’t just a matter of hoofing it away from the split; the split itself initiated a domino-effect of rising greenhouse gases that makes today’s climate change scare look like amateur hour. The climate change would wipe out thirty percent of earth’s species and help the emergent dinosaurs break out of their slump and diversify into the dominant creatures of the Mesozoic. The intense volcanism spewed carbon dioxide, a ‘greenhouse gas’ (so-called because it warms the planet), into the atmosphere. As the oceans warmed due to the increased carbon dioxide, clathrates melted; clathrates are chunks of ice buried in the seafloor in which methane is trapped, and when clathrates melt, the trapped methane – another greenhouse gas – is released. The unchained methane piggybacked on the carbon dioxide, and the greenhouse gases in the atmosphere tripled within a few tens of thousands of years. The planet’s temperature warmed at a nauseating pace, and life struggled to keep tempo. The ocean became more acidic, resulting in the collapse of marine food webs; shellfish were particularly hard-hit. The climate became more variable: periods of intense heat were followed by rapid cooling, megamonsoons became far more severe, and coastal regions became wetter while interior regions became drier. The hotter temperatures led to the death of ninety-five percent of plant life; herbivores suffered from the lack of food, and the carnivores that preyed upon them suffered in turn. Most reptiles, amphibians, and early mammals died out.
Late Triassic trackways (in which we catch glimpses of the footprints of ancient creatures) tell the grim story. Before the volcanic deluge, dinosaurs made up twenty percent of the trackways; afterwards, they’re pretty much the only tracks left – and they diversify like mad. Historian Steve Brusatte notes, “After some of the largest volcanic eruptions in Earth history desecrated ecosystems, dinosaurs became more diverse, more abundant, and larger. Completely new dinosaur species were evolving and spreading into new environments, while other groups of animals went extinct. As the world was going to hell, dinosaurs were thriving, somehow taking advantage of the chaos around them.” (97) He continues, “Somehow dinosaurs were the victors. They endured the Pangaean split, the volcanism, and the wild climate swings and fires that vanquished their rivals.” (98) No one knows why this was the case, but the Jurassic was theirs for the taking. “Was there something special about dinosaurs that gave them an edge over the pseudosuchians and other animals that went extinct? Did they grow faster, reproduce quicker, have a higher metabolism, or move more efficiently? Did they have better ways of breathing, hiding, or insulating their bodies during extreme heat and cold snaps?” (98) The answers aren’t forthcoming, but what is known is that the Jurassic would play host to a menagerie of creatures wholly different from the period that preceded it. The Jurassic was a warmer planet than it had been in the Triassic. Storms were more intense, and wildfires lighted with ease. New types of ferns and ginkgoes replaced the broadleaf conifers of the Late Triassic. And most of the life-forms that called the Triassic home weren’t around to see it.
The supersalamander amphibians? They were gone.
The piglike dicynodonts? Gone.
The beaked rhynchosaurs? Gone.
The long-snouted phytosaurs, the tank-like aetosaurs, and the apex-predator rauisuchians were gone, gone, GONE.
And the pseudosuchians? They were almost wiped out. The only traces of them were a few types of primitive creatures that would eventually evolve into modern alligators and crocodiles, but they would never again enjoy the success they’d claimed in the Triassic.
Explore Further: Dinosaurs of the Triassic
PREVIOUS: THE PERMIAN PERIOD NEXT: THE JURASSIC PERIOD
The plates were moving in the Late Triassic just as they’re moving now; but as the Permian saw the formation of the supercontinent Pangaea, the Late Triassic saw the genesis of its breakup. As Pangaea began to split (the upper portion would become ‘Laurasia’ and the southern portion ‘Gondwana’), it stretched at the seams, creating ‘rift basins.’ The shallow bowls of stretched earth were often filled with water, creating lakes and rivers; many would seasonally dry out, resulting in fantastic ‘bone beds’ for future paleontologists. As the stretching intensified, the ripping began. Pangaea ruptured along the modern-day U.S. eastern seaboard (the Palisades in New York are remnants of this split), and the rift basins were inundated by volcanic lava flows. There would be four cycles of mass eruptions before Pangaea finally split down the middle, and the process took about half a million years. These volcanic ‘pulses’ weren’t meager little things: they sent tsunamis of lava surging across the Late Triassic landscape. This is no exaggeration, as some of the lava flows reached up to three thousand feet in thickness. These lava tsunamis could’ve buried the Empire State Building twice over.
The supersalamander amphibians? They were gone.
The piglike dicynodonts? Gone.
The beaked rhynchosaurs? Gone.
The long-snouted phytosaurs, the tank-like aetosaurs, and the apex-predator rauisuchians were gone, gone, GONE.
And the pseudosuchians? They were almost wiped out. The only traces of them were a few types of primitive creatures that would eventually evolve into modern alligators and crocodiles, but they would never again enjoy the success they’d claimed in the Triassic.
Explore Further: Dinosaurs of the Triassic
PREVIOUS: THE PERMIAN PERIOD NEXT: THE JURASSIC PERIOD
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