Dinosaurs of the Triassic, Jurassic, and Cretaceous Periods

The Mesozoic era, that is, the era of the dinosaurs, lasted 185 million years, beginning 251 million years ago. In that era the continents were divided and moved to their current position, with a profuse development of terrestrial and aquatic organisms. The Mesozoic is divided into three periods: the Triassic, the Jurassic, and the Cretaceous.

To have a more precise idea of ​​the location of the Mesozoic era in the Earth’s evolution, let’s remember that the first stage is the Precambrian period; This begins with the formation of the planet about 4.570 million years ago and ends 542 million years ago. The development of multicellular life marked the beginning of the Paleozoic era, which spanned the period from 542 to 250 million years ago. In turn, the Paleozoic era is divided into the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian periods. And it is at that moment of the Earth’s evolution, about 250 million years ago, that the Mesozoic era begins.

In this extremely long period of Earth’s evolution, dinosaurs, marine reptiles, fish, mammals, various flying animals including pterosaurs and birds, as well as a wide variety of plants, developed. The largest dinosaurs did not appear until the Cretaceous period, the last of the Mesozoic era, which began more than 100 million years after the start of the age of the dinosaurs.

In the following table you can see a simplified description of the characteristics of the three periods that make up the Mesozoic era.

Let’s see some characteristics of the organisms detailed in the table.

• Archosaurs were a group of reptiles that included dinosaurs and pterosaurs.
• Pterosaurs were flying reptiles that ranged in size from the size of a sparrow to the about 10-meter-long Quetzalcoatlus .
• Therapsids were also a group of reptiles that later evolved to become mammals.
• Sauropods were huge, long-necked, long-tailed vegetarian dinosaurs.
• Theropods were two-legged carnivorous dinosaurs, and included raptors , Tyrannosaurus Rex, and Giganotosaurus carolinii , found in Patagonia, one of the largest theropods that ever lived.
• Plesiosaurs were large, long-necked marine animals.
• The cycads ( Cycadidae ) are very ancient plants, whose vestiges date from before the Carboniferous period, and which were very common in the era of the dinosaurs. They are still found today; There are about 185 species and 9 genera that extend to various parts of the Earth.

the triassic period

At the beginning of the Triassic period, about 250 million years ago, the Earth was recovering from the so-called Permian-Triassic extinction, in which two thirds of land species and 95% of marine species disappeared.

As far as animal evolution is concerned, the Triassic period is notable for the remarkable diversification of archosaurs into pterosaurs, ancestors of crocodiles, and into early dinosaurs, as well as the evolution of therapsids into early mammals.

Climate and geography during the Triassic period

During the Triassic period all of the Earth’s continents were united in a vast landmass called Pangaea, surrounded by the enormous Pantalasa Ocean, which begins to divide into the continents towards the end of the period and the beginning of the Jurassic. There were no ice caps, and the climate at the equator was hot and dry, marked by violent monsoons. Some estimates put the average air temperature over most of the continent well above 38 degrees Celsius. Climatic conditions were wetter in the north, the part of Pangea corresponding to present-day Eurasia, and in the south, what would later become Australia and Antarctica.

Plants during the Triassic period

Vegetation in the Triassic period was not as lush as in the later Jurassic and Cretaceous periods, but there was an explosion of various species of land plants such as cycads, ferns, as well as Gingko biloba-like trees and various plants with seeds. There are several reasons behind the absence of large herbivorous animals during the Triassic period (such as Brachiosaurus , which is much later), but one of the most important is that there simply wasn’t enough vegetation to allow them to thrive.

Animal life of the Triassic period

terrestrial life

Life in the Permian period, prior to the Triassic, was dominated by amphibians, but the Triassic marked the rise of reptiles, particularly archosaurs and therapsids. For reasons that are still unclear, archosaurs maintained the evolutionary advantage by winning the competition for subsistence over the therapsids; they evolved in the Middle Triassic to become the first dinosaurs, such as Eoraptores and Herrerasaurus .

However some archosaurs evolved in a different direction and became the first pterosaurs, such as Eudimorphodon , as well as a wide variety of crocodilian ancestors, some of which were two-legged and vegetarian.

At the same time, therapsids gradually decreased in size and in the Late Triassic the first mammals appeared, represented by small mouse-sized creatures such as Eozostrodon and Sinoconodon .

Marine life

Because the Permian extinction depopulated the oceans, in the Triassic period the conditions were ripe for the emergence of the first marine reptiles. These included not only unique and unclassifiable genera such as Placodus and Nothosaurus , but also early plesiosaurs and a new type of animal, lizardfish, or ichthyosaurs. Some ichthyosaurs reached exorbitant sizes; for example, the Shonisaurus was 15 meters long and weighed around 30 tons.

The vast Pantalasa ocean soon found itself repopulated with new species of prehistoric fish, as well as simple organisms such as corals, and cephalopods such as ammonites ( Ammonoidea ).

The Triassic-Jurassic extinction

Between the Triassic and Jurassic periods there was an extinction event that, although it was not as massive as the Permian-Triassic and the later Cretaceous-Tertiary, involved the disappearance of several genera of marine reptiles, as well as large amphibians and certain branches. of archosaurs.

The cause of this extinction event is not known with certainty, but it is speculated that it may have been caused by volcanic eruptions, a global cooling process, a meteorite impact, or a combination of several of these events. .

the jurassic period

In the Jurassic period, the first giant sauropod and theropod dinosaurs appeared on Earth, very different from their slender, man-sized ancestors of the Triassic period. In any case, the peak of dinosaur diversity was reached in the next period, in the Cretaceous.

Climate and geography during the Jurassic period

The Jurassic period witnessed the breakup of the supercontinent Pangea into two large landmasses: in the south, Gondwana, corresponding to present-day Africa, South America, Australia, and Antarctica; in the north, Laurasia, which today are Eurasia and North America.

At about the same time, lakes and rivers formed within the continents, opening up new evolutionary niches for aquatic and terrestrial life. The climate was hot and humid with constant rain, ideal conditions for an explosive expansion of exuberant plants.

Plants during the Jurassic period

Giant herbivorous sauropods like Barosaurus and Apatosaurus could not have evolved if they did not have a consistent food source. In fact, the land masses of the Jurassic period were covered in thick layers of vegetation that included ferns, conifers, cycads, mosses, and horsetails.

Flowering plants continued their slow and steady evolution, culminating in the explosion that helped fuel dinosaur diversity during the ensuing Cretaceous period.

Animal life during the Jurassic period

land animals

During the Jurassic period the relatives of small prosauropods, which were herbivorous quadrupeds during the Triassic period, gradually evolved into giant multi-ton sauropods such as Brachiosaurus and Diplodcus . This period also saw the evolution of medium to large theropod dinosaurs such as Allosaurus and Megalosaurus . This is related to the evolution of the first ankylosaurs and stegosaurs.

Those early mouse-sized mammals of the Jurassic period, which had evolved from their therapsid ancestors in the late Triassic, maintained a limited development, running around at night or taking refuge high in the trees to avoid being crushed underfoot. huge dinosaurs.

In some places the first feathered dinosaurs began to appear, Archeaopteryx and Epidendrosaurus being two typical cases, and extremely similar to our birds. It is possible that the first prehistoric birds evolved at the end of the Jurassic period, although there is insufficient evidence for this. Most paleontologists believe that modern birds descended from the small feathered theropods of the Cretaceous period.

flying animals

At the end of the Jurassic period, about 150 million years ago, the skies were teeming with relatively developed pterosaurs such as the Pterodactylus , the Pterandon , and the Dimorphodon . Prehistoric birds had yet to evolve, leaving the skies under the rule of these flying reptiles, with the exception of a few prehistoric insects.

Marine life

Just as dinosaurs evolved to ever greater sizes on land, the marine reptiles of the Jurassic period gradually reached the proportions of a shark, or even a whale.

Jurassic seas were inhabited by ferocious pliosaurs like Liopleurodon and Cryptoclidus , but there were also the graceful and less terrifying plesiosaurs like Elasmosaurus . Ichthyosaurs, which dominated the sea in the Triassic period, had already begun their decline.

Prehistoric fish were abundant, as were squids and the ancestors of sharks, providing a food source for all marine reptiles.

the cretaceous period

In the Cretaceous period, dinosaurs reached their greatest diversity, when the ornithischian and saurischian families branched off into a bewildering variety of meat- and plant-eating animals that had armor, claws, powerful jaws with large teeth, and long tails.

The Cretaceous was the longest period of the Mesozoic era; It was here in the Cretaceous when the Earth began to look like its present form. At that time life on Earth was not dominated by mammals, but the hegemonic species were terrestrial, marine and flying reptiles.

Climate and geography during the Cretaceous period

During the early Cretaceous period, the division of the Pangaean supercontinent continued, and the outlines of modern North and South America, Europe, Asia, and Africa began to be drawn. North America was bisected by the Western Interior Sea, which has produced countless fossils of marine reptiles, and India was a gigantic floating island in the Tethys Ocean. The prevailing climatic conditions were as hot and humid as in the earlier Jurassic period, albeit with intervals of cooling. In that period there was a rise in sea level and the expansion of endless swamps, another ecological niche in which dinosaurs, like other prehistoric animals, could develop.

Plants during the Cretaceous period

As far as plants are concerned, the most important evolutionary change of the Cretaceous period was the rapid diversification of flowering plants. These plants are spread across the drifting continents, along with forests and other varieties of dense, tangled vegetation. All this vegetation not only supported the feeding of the dinosaurs, but also allowed the evolution of a wide variety of insects, especially beetles.

Animal life during the Cretaceous period

land animals

The Cretaceous period was the heyday of the dinosaurs. In the course of its 80 million years, thousands of carnivorous genera roamed the slowly separating continents. These carnivorous dinosaurs included raptors , tyrannosaurs, and other varieties of theropods such as ornithomimids, as well as the strange feathered therizinosaurs and an uncounted profusion of small feathered dinosaurs, including the extraordinarily intelligent Troodon .

The Giganotosaurus carolinii found in Patagonia, in South America, is one of the largest theropod dinosaurs ever identified. Researchers have associated this species with a larger group that includes the Asian synraptorids. The close affinity of Giganotosaurus with the North African Carcharodontosaurus supports the hypothesis that intercontinental connections were maintained until the mid-Cretaceous period.

Herbivorous sauropods, characteristic of the Jurassic period, had practically become extinct; but their descendants, the lightly armored titanosaurs, spread across all continents and reached even greater sizes.

Ceratopsians (horned dinosaurs) such as Styracosaurus and Triceratops became abundant, as did hadrosaurs (duck-billed dinosaurs), which were especially common at the time and roamed the plains of North America and Eurasia in large numbers. herds. Among the latter, at the time of the Cretaceous-Tertiary extinction were herbivorous ankylosaurs and pachycephalosaurs.

In May 2014, the largest animal ever to have lived on Earth was found in Patagonia: the Patagotitan mayorum , or Patagonian Titan, the only known species of titanosaur sauropod dinosaur to have lived in South America during the Cretaceous period. It is estimated that this animal was 37 meters long and weighed 69 tons.

For most of the Mesozoic era, including the Cretaceous period, mammals were intimidated by their dinosaur cousins ​​and therefore spent most of their time high up in trees or cowering in underground burrows. Even so, some mammals were able to develop and increase in size, such as the Repenomamus , a carnivorous animal that weighed up to 10 kilos.

Marine life

Shortly after the beginning of the Cretaceous period, ichthyosaurs disappeared. Their place was taken by ferocious mosasaurs, giant pliosaurs like Kronosaurus , and smaller plesiosaurs like Elasmosaurus .

A new form of fish, called teleosts, roamed the seas in huge schools. A wide variety of shark ancestors developed, benefiting from the extinction of their antagonists, the marine reptiles.

flying animals

By the end of the Cretaceous period, pterosaurs had reached enormous sizes; the most spectacular example was the 10 meter wingspan Quetzalcoatlus . However, pterosaurs did not last long, gradually displaced by prehistoric birds. These birds evolved from land-dwelling feathered dinosaurs, and were better adapted to changing climatic conditions.

The Cretaceous-Tertiary extinction

At the end of the Cretaceous period, about 65 million years ago, the impact of a meteorite on the Yucatan Peninsula in Mexico raised huge clouds of dust, blocking the sun and causing the extinction of most of the vegetation. Conditions may have been aggravated by the collision of India and Asia which triggered intense volcanic activity.

The herbivorous dinosaurs that fed on these plants went extinct, as did the carnivorous dinosaurs that fed on the herbivorous dinosaurs. The way was now clear for the evolution and adaptation of the dinosaurs’ successors, the mammals, during the next period of Earth’s evolution: the Tertiary period.

Sources

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• Coria, R.A., Currie, P.J. The Braincase of Giganotosaurus carolinii (Dinosauria: Theropoda) from the Upper Cretaceous of Argentina . Journal of Vertebrate Paleontology. Vol. 22, No. 4, p. 802-811, 2003.
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