If you could go back in time to the Mesozoic Era, you’ll witness a planet completely dissimilar from its present condition.
All the continents were in different positions, and the climate was also different.
But arguably the most significant difference you’ll notice is the presence of dinosaurs-lots of them!
Throughout the Mesozoic Era, starting from about 245 to 66 million years ago, dinosaurs were the most dominant land animals.
While there were other creatures around, dinosaurs were the most abundant and also the most diverse, occupying various niches and ecological roles.
Their story begins in the Triassic when small dog-sized reptiles began to evolve into bigger sizes and gained adaptations that gave them a superior advantage over their ancestors and contemporaries.
The captivating tale of these ancient creatures is completely intertwined with Earth’s history, especially for the 175 million years they were around.
Therefore, understanding the dramas that unfolded during this period, the different species that emerged, and the timeline of the dinosaur’s existence has been a major fascination for scientists and enthusiasts.
It’s an intricate puzzle that paleontologists and other scientific researchers have meticulously pieced together using fossils and various scientific techniques.
In this article, we will discuss the timeline of these prehistoric creatures extensively, providing a comprehensive overview of when they lived and how life unfolded throughout their existence.
Mesozoic Era: The Age of Dinosaurs
To better understand Earth’s history, scientists have divided the planet’s existence into huge chunks of time known as eons.
The current eon we are in is known as the Phanerozoic, and it began 541 million years ago.
The eon is further divided into eras, which are then split into periods and epochs.
The Phanerozoic Eon has three eras.
Between these two eras was the Mesozoic, which aptly translates as middle life.
The era began 252 million years ago and is fondly remembered as the age of dinosaurs.
The Mesozoic is divided into the Triassic (which was when dinosaurs evolved), the Jurassic (characterized by the diversification of dinosaurs), and the Cretaceous (when dinosaurs began to decline and eventually went extinct).
Life on Earth was already in full swing when the dinosaurs came around in the Mesozoic.
Animals and plants first evolved in Earth’s aquatic ecosystems, but they transitioned to land more than 150 million years before the dinosaurs evolved.
By the Mesozoic Era, terrestrial animals were pretty advanced compared to the critters and amphibians that made the transition to land during the Devonian.
The emergence of the first dinosaurs has been dated back to about 240 million years ago.
When they first emerged, their unique anatomy quickly set them apart from their lizard-like ancestors.
They adopted an upright posture, with legs under their body instead of to the side.
Having their limbs tucked underneath their body opened the door to new possibilities regarding their overall body plan, how they could move, and the niches they could explore.
This unique anatomy set the stage for the dominant role that dinosaurs would play throughout the Mesozoic Era as they evolved to take on prominent positions across various ecological niches.
The Mesozoic Era is further divided into three periods: Triassic, Jurassic, and Cretaceous.
During these periods, the Earth’s climate, geography, and lifeforms underwent various changes, affecting how dinosaurs and other lifeforms on land and in the seas evolved.
Triassic Period: Dawn of the Dinosaurs
The Triassic is the first period of the Mesozoic Era.
The period began 252 million years ago and lasted until about 201 million years ago.
It kicks off with the catastrophic end-Permian mass extinction event, which is regarded as one of the most severe extinctions that Earth has had to go through.
Consequently, the period was characterized by a gradual rebound of life on Earth and the emergence of a new group of organisms.
Although it is fondly referred to as the dawn of dinosaurs, this group of animals was not yet the most dominant land animals at the time.
Instead, their reptilian ancestors, known as archosaurs, dominated the landscape.
Early Triassic: A World in Recovery
Periodically, life on Earth experiences a sort of biological reset characterized by a widespread die-off of various plant and animal groups.
One such extinction event occurred at the end of the Permian period.
Known as the Permian extinction or great dying, up to 70% of terrestrial vertebrates died off during this catastrophic event.
While the exact cause isn’t known, scientists think the release of toxic gasses into the atmosphere elevated global temperatures and raised the acidity of the oceans.
This led to the extinction of numerous plants and animal groups.
By the time the Triassic Period began, terrestrial life was starting over, with a few survivors of the Permian extinction living on a hot and dry planet.
All of Earth’s continents were connected as a single landmass known as Pangea.
This meant a uniform climate and limited diversity in the planet’s flora and fauna.
The desert-like conditions allowed the dominance of a group of reptiles known as the archosaurs.
These are the reptilian ancestors of the dinosaurs, crocodiles, and pterosaurs (flying reptiles).
As the ecosystems began to recover in the wake of the Permian extinction, this group of reptiles ventured into new ecological niches and began to diversify as well.
Dinosaurs emerged to fill the vacant ecological niches created by the mass extinction.
In their early days, the differences between dinosaurs and their archosaur ancestors were negligible.
They had limbs positioned underneath their body, arms that could move in and out, and robust neck vertebrae to support stronger muscles.
Dinosaurs were also warm-blooded, which gave them an edge because they were no longer at the mercy of their environment.
The ability to regulate their internal temperature meant they could be more active and live in previously unoccupied ecosystems.
Key Species of the Early Triassic
Early Triassic Dinosaurs looked more like their lizard-like ancestors in terms of their general body plan.
Not many Early Triassic dinosaurs are known due to the limitation of fossil records from that period.
Also known as “the dawn thief” or “dawn plunderer,” the Eoraptor lived roughly 231 million years ago in a region of Pangea known as Western Gondwana.
This region is now in present-day northwestern Argentina.
Eoraptor was a small bipedal dinosaur.
The skeletal features of this dinosaur (such as multiple tooth shapes) suggest that it was an omnivore.
Being able to eat flesh and plants was an important adaptation that underscored the transitional nature of life during the Early Triassic as dinosaurs began to explore various ecological roles.
The Herrera lizard is named after the Argentine rancher who discovered its remains.
It lived between 231 and 220 million years ago and is often considered one of the earliest theropod dinosaurs.
Herrerasaurus was larger than Eoraptor.
It possessed a combination of reptilian and dinosaurian features, which indicates its position as an important transitional species.
Middle Triassic: The Rise of Dinosaurs
The breakup of the supercontinent Pangea began sometime during this period.
But continental drift is a slow process, and the continent was still relatively intact during the Middle Triassic.
Archosaurs continued their quest to dominate new ecosystems, rapidly increasing their diversity and adaptations.
Initially, the dinosaurs were not as successful as their crocodile-like relatives.
But they continued to diversify and adapt to a variety of ecological niches, setting the stage for their eventual dominance in the subsequent periods of the Triassic.
Key Species of the Middle Triassic
As the Earth continued to recover, new plant and animal groups were emerging.
For instance, the first mammals evolved sometime during the Triassic.
These small rodent-like mammalian ancestors, as well as the turtles, amphibians, and lizards present, would have served as food for the emerging carnivorous dinosaurs like the Coelophysis.
Plateosaurus lived around 214 to 204 million years ago in a region that is now part of present-day Europe.
Plateosaurus developed a range of adaptations, including elongated neck and hind limbs, allowing it to effectively access the limited plant resources on the Triassic landscape.
This 9-foot-long bipedal carnivore emerged during the Middle Triassic about 228 million years ago.
Coelophysis was likely an agile predator, well-adapted for swift movements, which made it quite an efficient hunter.
It had notable predatory adaptations, including curved blade-like teeth and sharp eyes, effective for spotting prey.
The emergence of basal theropods like the Coelophysis set the stage for the evolution of larger and more specialized predators late in the Jurassic.
Late Triassic: Paving the Way for Jurassic Giants
During the last few million years of the Triassic, the Earth experienced significant changes in its geography.
The breakup of the supercontinent into two main landforms (Gondwana and Laurasia) began during this period, paving the way for the emergence of new oceans and basins with their associated ecosystems.
Climate variability and changes in sea levels due to these geographical changes influenced the distribution of terrestrial animals, including the early dinosaurs.
Key Species of the Late Triassic
Arguably, the most significant event of this period was the Triassic-Jurassic extinction event.
This event wiped off various archosaurs and other reptilian groups.
The dinosaurs that survived this extinction event would evolve to become the most dominant animals of the Jurassic Period.
Their adaptations and unique ecological behaviors set the stage for the evolutionary innovations that would characterize the Jurassic and beyond.
Some of the key species of the Late Triassic Period include:
Postosuchus was technically not a dinosaur.
It is more accurately described as an archosaur (dinosaur ancestor).
However, being one of the largest animals in the Late Triassic ecosystem, Postosuchus was an apex predator.
It was a large and robust carnivore that preyed on the herbivores present during the Late Triassic.
The extinction of large predators like this at the end of the Triassic paved the way for the dominance the dinosaurs enjoyed during the Jurassic.
By the end of the Triassic Period, large quadrupedal dinosaurs were starting to emerge.
One notable example is the Melanorosaurus which is considered a basal sauropodomorph (ancestors of the sauropod dinosaurs).
Although they were yet to attain the astronomical size of the true sauropods, they were still quite large and heavy, forcing them to adopt a quadrupedal stance to support their massive weight.
Jurassic Period: The Age of Giants
The dinosaurs really began to flourish (both in numbers and diversity) during the Jurassic Period about 201 million years ago.
Some of the largest dinosaurs emerged during this period, which is why it is often referred to as the Age of Dinosaurs.
It includes some of the most recognizable herbivorous and carnivorous dinosaurs, especially in the sauropod and theropod groups.
Two factors favored the diversification of dinosaurs during the Jurassic Period; a small mass extinction event at the end of the Triassic and the breakup of the supercontinent Pangea.
These two providential occurrences allowed the dominance and evolution of the dinosaurs throughout the Jurassic Period.
Early Jurassic: Evolutionary Progression
The Early Jurassic Period was a time of experimentation and specialization due to two major changes that began in the previous period.
The mass extinction event that occurred at the end of the Triassic was the first.
Although this was a small-scale extinction event, it wiped out several other archosaur groups, leaving the dinosaurs as the most dominant land animals.
Adaptations developed earlier in the Triassic allowed them to not only survive this extinction but also evolve into a variety of forms to take over the vacant niches.
The second factor that contributed to the abundance of dinosaurs was the breakup of the supercontinent Pangea.
The continent split into distinct landforms, with Laurasia in the North and Gondwana in the South.
Although there were still land connections between them, the climate and ecology of these two landmasses became even more distinct as the period progressed.
These geographical changes caused significant transformations in the Earth’s climate.
Temperatures dropped slightly, and lush vegetation appeared due to more abundant rainfall.
Ferns, horsetails, and other plants started to grow abundantly during the Early Jurassic Period.
These plants provided an abundant food supply for the emerging herbivorous dinosaurs.
As the population of these herbivores ballooned, carnivores evolved to prey on them, with an increase in their overall complexity and abundance.
Key dinosaur species of the Early Jurassic
During the Early Jurassic Period, diverse environments emerged, ranging from lush forests to arid plains.
This led to the evolution of new dinosaur carnivorous and herbivorous dinosaur species to occupy these new habitats.
Some of the major dinosaur species of this period include:
Scelidosaurus was an ornithischian dinosaur that lived on the supercontinent Laurasia about 191 million years ago.
It is one of the most completely known dinosaurs from that period, thanks to an abundance of fossil remains.
Scelidosaurus was also one of the earliest-known armored dinosaurs.
Its body was adorned with bony plates and spikes, which likely served as defensive adaptations against the rising number of predatory dinosaurs in its ecosystem.
Its armored body is an example of how dinosaurs evolved new anatomical features to adapt to the emerging challenges in their ecosystem.
Theropod dinosaurs like the Cryolophosaurus, characterized by their large size, powerful jaws, and sharp teeth, played a significant role in shaping the predator-prey dynamics of the Early Jurassic Ecosystem.
Cryolophosaurus was among the largest theropods of the Early Jurassic.
Fossils of this dinosaur were recovered from Antarctica, which was part of the supercontinent Gondwana during the Early Jurassic and was closer to the equator than its current position.
Middle Jurassic: Rise of Sauropods and Theropods
During the Middle Jurassic Period, dinosaurs reached the height of their diversity, with some of them growing to astronomical sizes.
Sauropods and theropods, two of the most iconic groups of dinosaurs, diversified significantly during this period.
The Middle Jurassic was characterized by an expansion of diverse habitats as the separation of continents due to the ongoing breakup of Pangaea continued.
These changes provided opportunities for different species to evolve and specialize.
Although most groups of flowering plants emerged later in the Cretaceous, some early species, such as Nanjinganthus, have been dated back to the early-to-middle Jurassic.
The emergence of flowering plants like this favored the sauropod dinosaurs and the theropods that preyed on them.
Sauropods, known today for their colossal size, long necks, and herbivorous diet, played a crucial role as the primary consumers of the Middle Jurassic ecosystems.
Their high browsing habits allowed them to access vegetation in the upper canopy of trees.
Experts think they evolved into massive sizes as an adaptation to cope better against predators that were becoming more abundant in their ecosystem.
Armored dinosaurs such as the stegosaurs and ankylosaurs also emerged during this period.
Their armored plates were adaptations that primarily served the purpose of protecting them from predators.
Theropods, on the other hand, continued to occupy the role of carnivorous predators and evolved into larger sizes to keep up with their prey.
These bipedal dinosaurs, such as Allosaurus and other relatives, preyed on various herbivores, including smaller sauropods and other dinosaur groups.
This predator-prey dynamic created a complex web of interactions that influenced the distribution and behavior of these species.
Key Species of the Middle Jurassic Period
The aptly named Megalosaurus was one of the largest theropod dinosaurs of the Middle Jurassic.
It was discovered in the 1800s, making it one of the first dinosaurs ever discovered.
Megalosaurus had an exceptionally large head with massive jaws, which made it a formidable predator in the Middle Jurassic Period.
Stegosaurs like the Huayangosaurus were just starting to emerge during the Middle Jurassic.
It was an armored dinosaur that lived about 165 million years ago.
Huayangosaurus was a medium-sized quadrupedal herbivore.
The most notable feature of this dinosaur was the rows of armored plates on its back and tail, which provided protection against predators.
Late Jurassic: Apex Predators and Environmental Shifts
The predator-prey interactions between the carnivorous and herbivorous dinosaurs became even more intense during the Late Jurassic Period.
Apex predators emerged, solidifying their position at the top of the Jurassic food chain, thanks to a range of adaptations that made them efficient killers.
Major predator families within the theropod group, such as the allosaurids (e.g., Allosaurus) and Ceratosaurids (e.g., Ceratosaurus), evolved during this period.
Changes in Climate and Ecosystems During the Late Jurassic
During the Late Jurassic, the supercontinent Pangaea continued to fragment, forming even smaller land masses with unique ecosystems.
This division had far-reaching impacts because it created new habitats and unique climates across the different continents.
The climate of the Late Jurassic was generally warm, and shallow seas covered large areas.
The presence of these shallow seas also influenced local climates on land.
Forests dominated by conifers and cycads emerged and provided habitats for various dinosaurs.
The Late Jurassic environment also included lakes, rivers, wetlands, and other ecosystems associated with them, which fostered a diversity of plants and animal species.
Key Species of the Late Jurassic
The changing geography and climate of Late Jurassic Earth further contributed to the evolutionary diversification going on in the world of the dinosaurs.
Some of the most notable species that emerged during this period include:
Brachiosaurus was a Late Jurassic sauropod that lived in North America.
It grew to a length of up to 22 meters with a disproportionately long neck, a small skull, and a large body.
Brachiosaurus was one of the largest dinosaurs to have ever lived.
It lends its name to the family Brachiosauridae which includes other similarly-sized dinosaurs such as the Giraffatitan and Lusotitan.
Allosaurus was one of the largest predators of the Late Jurassic Period.
It was a bipedal theropod considered one of the largest carnivores to have ever lived.
In addition to being formidable predators with specialized adaptations for killing prey, Allosaurus may have hunted in groups as well.
This allowed them to leverage their heir numbers to take down massive sauropod dinosaurs that were abundant during the Late Cretaceous.
Jurassic Park Dinosaurs: Velociraptor and Tyrannosaurus rex
The most popular on-screen representation of dinosaurs is the Jurassic Park movies.
While appearances in movies like this have helped popularize dinosaurs with the general public, it has also caused misconceptions in some cases.
These dinosaurs (and several other dinosaurs in the movies) did not live during the Jurassic.
They were Late Cretaceous dinosaurs.
Also, many of the Jurassic Park dinosaurs did not live alongside each other or interact as presented in the movies since they existed during different periods.
Cretaceous Period: The Final Chapter
The Cretaceous Period began about 145 million years ago.
Unlike previous periods of the Mesozoic Era, where dinosaurs lived on supercontinents with similar climatic and ecological conditions across different locations, the dinosaurs of the Cretaceous lived on separate continents.
By the time the period ended, the continents were in a position similar to their present-day locations.
The breakup of the supercontinent allowed the dinosaurs to evolve independently in different locations all over the world.
They evolved into more diverse forms characterized by notable adaptations that favored their survival in the region where they lived.
But as the period progressed, changes in the Earth’s ecosystem eventually turned the tide against the dinosaurs.
Although the extinction of the dinosaurs is said to have occurred about 66 million years ago, scientists think their population and diversity started to decline long before this.
The cause of this decline has been attributed to climate change in the last few million years of the Mesozoic Era.
Early Cretaceous: Continuation of Diversity
Dinosaurs continued to diversify during the Early Cretaceous Period from 145 to 100 million years ago.
During this time, the dinosaurs lived in a wide range of changing environments.
This resulted in the emergence of new species with innovative adaptations suited to survive in the complex ecosystems where they lived.
For instance, the Nanuqsaurus, which lived in the cold region of Alaska during the Late Cretaceous, and some other large theropod dinosaurs that lived in cold regions, such as Yutyrannus, had feathers all over their body to provide insulation in the harsh climate.
This is a sharp contrast to other related theropod dinosaurs, such as the Tyrannosaurus, that either lacked feathers completely or only had them in some parts of their body.
Dinosaur species isolated on some islands also exhibited insular dwarfism.
A notable example of this is the Magyarosaurus, a sauropod that was smaller compared to its other relatives.
With limited food and the absence of large predators and competition, the Magyarosaurus did not need to grow to the massive size that characterized other sauropod dinosaurs.
Key Species of the Early Cretaceous
Iguanodon lived around 140 to 130 million years ago.
It was a bulky herbivore that lived in parts of present-day Europe back when it was still part of Eurasia.
One of the most notable features of this notable herbivorous dinosaur is its distinctive thumb spike.
This adaptation was initially considered a defensive weapon but is now believed to have been used for manipulating vegetation or foraging.
Suchomimus is a genus of spinosaurid dinosaurs that lived in parts of West Africa during the Early Cretaceous Period (about 120 million years ago).
This dinosaur’s name translates as “crocodile mimic” because it had a long and shallow skull similar to that of a crocodile.
Like other spinosaurids, Suchomimus probably preyed on fish and other small animals in its ecosystem.
This adaptation to a piscivore diet demonstrates the extent of the diversification of the Early Cretaceous dinosaur.
Mid-Cretaceous: Titans and Changing Landscapes
As the configuration of the continents continued to change during the Middle Cretaceous Period, flowering plants emerged and flourished in greater numbers.
This created new terrestrial ecosystems and also provided an abundant food source for the herbivorous dinosaurs.
The southern continents were particularly favorable to the dinosaurs as extensive forests with various kinds of vegetation characterized them.
Different groups of herbivores emerged in the extensive forests and plains of South America.
The sauropods were the most dominant dinosaurs in this region.
One group in particular—known as the titanosaurs—evolved to become the largest land animals on Earth during the Middle Cretaceous.
They lived on all continents, including Antarctica.
Key Species of the Middle Cretaceous
The Middle Cretaceous was the time of titans and giant predators. Some of the biggest dinosaurs ever discovered lived during this period. They include:
The Paralititan was a large titanosaurid dinosaur that lived in the coastal region of northern Africa roughly 100 million years ago.
Due to limited fossils, very little is known about this dinosaur.
But the fragmentary remains suggest that it was among the largest dinosaurs to have ever lived.
This massive theropod dinosaur lived in South America about 99 million years ago.
It is considered one of the largest known terrestrial carnivores.
It was the apex predator in South America, capable of taking down large prey with its powerful bite.
Late Cretaceous: The Last Roar and Extinction
The Late Cretaceous is known for the extinction of the dinosaurs.
But the question of how well the dinosaurs were doing before they eventually went extinct is one of the most controversial questions in the world of paleontology.
Although evolution and diversification continued in the first few million years of the Late Cretaceous Period, some experts think some dinosaur groups started to decline later in the period.
Armored herbivores like the ceratopsians and ankylosaurs emerged earlier in the Cretaceous and became more abundant towards the end of the period.
Although they were not as large as the titanosaurs, they made up for their small size with elaborate frills and horns, which gave them a more intimidating appearance.
Although this protected them from smaller predators, the apex predators of the Late Cretaceous, such as the Tyrannosaurus rex and Carnotaurus, were formidable enough to take down massive ceratopsians like the Triceratops.
These apex predators had massive jaws that closed with strong muscles, which gave them a powerful bite strong enough to kill large prey.
There’s extensive evidence that these apex predators hunted large armored herbivores in their respective ecosystems.
The dueling dinosaur fossil is a classic example of this.
The fossil preserves a fatal encounter between a Tyrannosaurus and a Triceratops.
Theories Surrounding the Mass Extinction of the Dinosaurs
Whether or not dinosaurs were already declining when the extinction event that wiped them out occurred is still open to debate.
But most experts agree that a catastrophic event at the end of the Cretaceous was the final straw that led to the extinction of this previously successful group of animals.
The most likely candidate for this is the Chicxulub asteroid impact which occurred around the same time that the dinosaurs went extinct.
This event caused a chain of catastrophes, including tsunamis, wildfires, and volcanic eruptions that killed off several dinosaurs instantly and threw up debris that blocked off the sun for several years.
This scenario would have disrupted ecosystems and led to widespread climate cooling that killed off the remnants of the dinosaurs within a few years.
Another theory suggests that extensive volcanic activity, particularly the eruptions of the Deccan Traps in present-day India, released vast amounts of gasses and particles into the atmosphere.
This led to the environmental disruptions that led to the extinction of the dinosaurs.
It is also possible that these two events occurred concurrently, with the Chicxulub triggering the Deccan Traps and other volcanic activities.
Key Species of the Late Cretaceous
Many dinosaur species have been identified as the last ones around during the last few million years of the Cretaceous.
Some of them include:
This hadrosaurid dinosaur lived in North America and Asia during the Late Cretaceous Period.
The dinosaur’s name translates as “near crested lizard,” which refers to the snorkel-like crest extending from its snout to the back of its skull.
Parasaurolophus went extinct about 73 million years ago, shortly before the end-Cretaceous extinction occurred.
Fossils of the Edmontosaurus date back to about 66 million years ago, suggesting that it was present till the very last days of the dinosaurs.
It was one of the largest hadrosaurid dinosaurs to have ever lived, which means large dinosaurs like this were still present when the catastrophic event that wiped out the dinosaurs occurred.
The dinosaurs are a unique group of reptiles that lived throughout the Mesozoic Era.
This unique group of reptiles reached the peak of their abundance and diversity during the Jurassic Period after emerging earlier in the Triassic.
Then they began to suffer a decline in the period that followed and were eventually wiped out at the end of the Cretaceous in the most dramatic way imaginable-an extinction event caused by an extraterrestrial impact.
Understanding the timeline of the dinosaurs, as highlighted in this article, helps to put prehistoric life into a better perspective.
It also aids our understanding of the key events that have shaped Earth’s history since its beginning.