Approximately 145 to 66 million years ago, following the Jurassic Period and preceding the Paleogene Period, the Cretaceous is the last period of the Mesozoic Era divided into two parts: the Early Cretaceous (145-100 million years ago) and the Late Cretaceous (100-66 million years ago).
Cretaceous was derived from the Latin word creta, meaning “chalk,” because of the many chalk deposits from this period.
This period was first described in 1822 by Belgian geologist Jean d’Omalius d’Halloy but was further split into two parts later that year by Conybeare and Phillips.
At the time, the supercontinent Pangaea existed but had started breaking apart into smaller land masses.
When the Atlantic Ocean widened, South America and Africa had almost completely separated.
India continued to migrate north, eventually clashing with Asia to form the Himalayan Mountains, and Australia and Antarctica remained connected.
A shallow seaway that connected the Arctic Ocean to the Gulf of Mexico spanned the western heartland of North America.
The Cretaceous Period ended with one of Earth’s most significant mass extinctions, known as the Cretaceous-Paleogene (K-Pg) extinction.
The impact caused the extinction of many dinosaur and marine reptile species and numerous other groups of animals and plants.
A lot remains unknown about the incident and the period in general; keep reading to discover more.
Timeline of the Cretaceous Period
As mentioned, the Cretaceous Period spanned from approximately 145 to 66 million years ago, and it is the final period of the Mesozoic Era.
This period divides into two sub-eras: the Early Cretaceous and the Late Cretaceous.
1. The Early Cretaceous
Like the other sub-eras under the Mesozoic Era, the Early Cretaceous marked significant changes in Earth’s climate, geography, and biodiversity.
Although connected at the beginning of the Early Cretaceous, the supercontinent Pangea was slowly breaking apart.
The separation of the continents resulted in the formation of new oceans and seas, such as the Atlantic Ocean and the Tethys Sea.
During the Early Cretaceous Period, the climate was generally warm and humid.
However, there were times when the temperature cooled and dried out, particularly during the Aptian Age (125–113 million years ago), when it was colder and drier than other times during the epoch.
At this time, sea levels were high, which caused extensive coastal flooding.
New plant and animal species, including the earliest examples of many contemporary groupings of plants and animals, emerged during the Early Cretaceous Period.
This period saw the rise of flowering plants, also known as angiosperms.
In a short period, these plants displaced many of the ferns and conifers that had flourished during the Jurassic Period as the primary plant group.
The Early Cretaceous saw the appearance of numerous new species of dinosaurs as they continued to evolve.
Amargasaurus, a sauropod dinosaur, and Eoraptor, a theropod dinosaur, are two of the most famous dinosaurs to have lived during this period.
Also, there was diverse marine life during this period. Various marine reptiles and invertebrates inhabited the oceans at the time.
Some of the most notable marine animals that lived during this time include the ichthyosaur Ophthalmosaurus and the plesiosaur Elasmosaurus.
The Early Cretaceous also brought the evolution of other animals, including marsupial mammals and the first true lizards.
2. The Late Cretaceous
The Late Cretaceous Period, which spanned from approximately 100 to 66 million years ago, was the final stage of the Cretaceous Period and the Mesozoic Era.
It was a time of significant change on Earth, with the rise of new species and the eventual extinction of the dinosaurs.
The Earth was warmer and more humid than today during the Late Cretaceous, and many coastal areas flooded due to the high sea levels.
By the end of the time, South America and Antarctica had completely parted, and India was heading straight for Asia.
Continents were still drifting apart. The Late Cretaceous saw the Western Interior Seaway‘s greatest extent, which included a large portion of interior North America.
New species emerged, and existing groupings of organisms continued to evolve during the Late Cretaceous.
This period saw the existence of some of the most well-known dinosaurs, including Triceratops, Tyrannosaurus rex, and Ankylosaurus.
By the Late Cretaceous, the diversity of other dinosaur subspecies, like the sauropods, had decreased.
Apart from dinosaurs, the Late Cretaceous was also a time of diversification for different animal groups.
The fossil record first shows modern bird groupings like waterfowl and shorebirds.
Plesiosaurs remained the main oceanic predators, but a species of marine reptiles known as the mosasaurs diversified and expanded throughout the oceans.
At this time, several insects also started to diversify.
Climate and Geography of the Cretaceous Period
The Earth witnessed some of its warmest temperatures during the Cretaceous Period.
Global temperatures throughout the Cretaceous Period averaged between 18 and 22 °C (64 and 72 °F), making it generally warm and humid.
Polar regions also had higher average temperatures, from 10-15°C (50-59°F).
High atmospheric carbon dioxide concentrations, higher volcanic activity, and reduced seafloor spreading are all potential contributors to the Cretaceous’ mild temperature.
The warm climate of this period caused large, shallow oceans and the growth of tropical and subtropical forests.
Ferns, cycads, and other flowering plants dominated these forests. The temperature started to cool, and the sea levels declined throughout the Late Cretaceous.
The dissolution of Pangaea and the subsequent modifications in ocean circulation may have contributed to this cooling and a fall in atmospheric carbon dioxide levels.
At the start of the Cretaceous Period, the continents were still joined in the supercontinent Pangaea.
However, the process of continental drift had already begun, and the continents were beginning to break apart.
By the end of the Cretaceous, the continents had moved significantly, and the current configuration of the continents started taking shape.
One of the most significant geological events of the Cretaceous was the opening of the Atlantic Ocean.
A rift formed between North and South America and Africa and Europe as the continents drifted apart. This rift eventually widened and created the Atlantic Ocean.
The creation of the Western Interior Seaway, a sizable inland sea that spanned much of what is now North America, was another critical geological event of the Cretaceous.
As a result of the dissolution of Pangaea and rising sea levels, this seaway formed. Several significant mountain-building events also occurred during the Cretaceous.
At the same time, the Andes started to ascend in South America, and the Rocky Mountains formed in North America.
These mountain ranges formed as a result of tectonic plate collisions.
Major Organism Groups of the Cretaceous Period
The Cretaceous Period was a time of significant evolutionary changes and diversification in the animal kingdom.
Here are some of the major organism groups of the Cretaceous Period:
1. Dinosaurs
Dinosaurs were the dominant group of land animals during the Cretaceous Period.
While several species from the previous periods went extinct in the Cretaceous Period, these ancient reptiles continued to evolve in different sizes and shapes, from small, bird-like creatures to enormous, long-necked sauropods.
Some of the most well-known dinosaurs of the Cretaceous Period include Triceratops, Stegosaurus, and Tyrannosaurus rex.
2. Marine Reptiles
In addition to dinosaurs, the oceans of the Cretaceous Period were home to many marine reptiles, including ichthyosaurs, plesiosaurs, and mosasaurs.
These animals were adapted to life in the water and had streamlined bodies, flippers, and other adaptations that helped them swim and hunt in the ocean.
All things considered, the marine reptiles of the Cretaceous Period were an intriguing and diverse group of creatures that were crucial to the ecosystem of the oceans at the time.
Scientists can learn more about the evolution and background of life on Earth by examining their remains and understanding their biology.
3. Mammals
Although small and relatively insignificant during the Cretaceous Period, mammals were present and evolving.
Most of the mammals of the Cretaceous Period were small, shrew-like creatures, but some, like Repenomamus, were larger and more predatory.
In general, these mammals adapted significantly, occupying a wide range of ecological niches and being able to feed on more plant and animal matter.
Scientists can better understand how mammals evolved as the dominant group of animals on Earth following the extinction of the dinosaurs by analyzing the fossils of these prehistoric mammals.
4. Birds
Although birds originated from a group of tiny, feathered dinosaurs during the Jurassic Period, it was in the Cretaceous that they began to diversify and take on a wide range of sizes and shapes.
Early birds, like Archaeopteryx, had feathers and wings but also many dinosaur-like features, but later birds, such as Enantiornithes, had more contemporary bird traits.
One of the largest and most well-known birds of the Cretaceous Period was the Titanis, a flightless bird that lived in North America.
Titanis stood up to nine feet tall and was a top predator in its environment, using its powerful beak and sharp claws to catch prey.
In addition to these groups, many other types of birds evolved during the Cretaceous Period, including toothed birds, which had teeth in their beaks, and bird-like dinosaurs, which were small, feathered creatures closely related to birds.
5. Insects
Insects were present and diversified during the Cretaceous Period, and many modern groups of insects evolved during this time.
Some of the most significant insect groups of the Cretaceous include bees, ants, and termites, which developed complex social behaviors and colonies.
The emergence of bees, ants, and termites throughout the Cretaceous Period also had a significant influence on the ecology of terrestrial ecosystems and the evolution of flowering plants.
6. Plants
The plant kingdom saw substantial diversification and evolution during the Cretaceous Period.
Angiosperms, or flowering plants, changed throughout this period and eventually became the most common plant on Earth.
The Cretaceous saw the emergence and development of ferns, conifers, and other plant kinds in addition to flowering plants.
By studying the fossils of these ancient plants, scientists can gain insight into the history of life on Earth and better understand how these organisms evolved and interacted with each other and their environments.
The legacy of the plants of the Cretaceous Period can still be seen today, in the lush forests and diverse ecosystems that cover much of the planet.
Fossils and Their Significance in Understanding the Cretaceous Period
Fossils from the Cretaceous are found worldwide and provide a window into the past that allows scientists to learn about the plants and animals that lived during this time.
One of the most significant fossils from the Cretaceous Period is the dinosaur.
These extinct reptiles were able to adapt to various dietary and lifestyle requirements and thrived in a variety of habitats.
The anatomy, behavior, and evolutionary history of dinosaurs are understood by researching their fossil remains.
Other Cretaceous-era fossils of animals and plants are also significant. Early bird, mammal, and insect fossils showed how these species evolved throughout this period.
For instance, the finding of a preserved mammal from the Cretaceous Period in China has aided research into the development of mammals and their place in the ecology during that time.
Extinction Events of the Cretaceous Period
One of the most well-known events of the Cretaceous is the mass extinction that occurred at the end of the period, which marked the end of the age of dinosaurs.
However, other extinction events occurred during the Cretaceous. Here is a comprehensive overview of the extinction events of the Cretaceous Period:
The Early Cretaceous Extinction
The Early Cretaceous extinction event occurred approximately 120 million years ago, during the Aptian stage of the period.
This event was not as severe as the end-Cretaceous extinction, which wiped out 75% of all species on Earth but still affected the diversity of life on the planet.
This extinction affected mostly marine life, reducing their number.
For instance, a group of cephalopods with shells called ammonites, which had dominated the oceans throughout the Jurassic Period, experienced a significant decline in diversity.
Bivalves, gastropods, and belemnites were among the other groups impacted.
On land, the extinction event had a less drastic effect, but some dinosaur species, like the Apatosaurus and the rest of the sauropods, saw a drop in diversity.
There are several theories on the causes of the Early Cretaceous extinction event, albeit this knowledge is not complete.
A worldwide cooling episode that happened during the Aptian period is one idea of what led to the extinction.
In addition to lowering sea levels and altering ocean currents, this cooling event would have influenced marine life.
A second hypothesis holds that the extinction was brought on by a series of volcanic eruptions that took place in what is now known as the Paraná-Etendeka Massive Igneous Region in South America and Africa.
Other plausible causes include modifications in the sea level and ocean chemistry, global warming, ocean acidification, etc.
The Aptian Extinction
The Aptian extinction is a lesser-known mass extinction event that occurred during the Cretaceous Period, specifically during the Aptian age, which lasted from 125 to 113 million years ago.
While not as well-known as the later Cretaceous extinction that wiped out the dinosaurs, the Aptian extinction affected the evolution of life on Earth.
Many plankton species, ammonites, and benthic organisms went extinct, which affected marine ecosystems. According to estimates, up to 50% of marine species perished at this time.
After the extinction, new groups of organisms emerged to fill the ecological niches left vacant by the extinct species.
For example, new species of ammonites and plankton emerged, as well as new groups of bivalves and gastropods.
The extinction also paved the way for new groups of marine predators, including the first modern sharks and the mosasaurs, a group of large marine reptiles that would later become dominant predators during the Late Cretaceous.
The Late Cenomanian Extinction
The Cenomanian-Turonian extinction event, also known as the Late Cenomanian extinction, was a catastrophic mass extinction that happened roughly 93 million years ago, during the Cretaceous Period.
This event is one of the lesser-known mass extinctions, but it affected the evolution of life on Earth.
The Late Cenomanian extinction is thought to have been caused by a combination of factors, including changes in climate, oceanic conditions, and volcanic activity.
One of the leading theories is that a series of massive volcanic eruptions in what is now the Caribbean region caused a rapid increase in atmospheric carbon dioxide, leading to global warming and ocean acidification.
With up to 50% of marine species going extinct, the Late Cenomanian extinction was one of the most catastrophic anoxic occurrences in Earth’s history.
The waters experienced a particularly severe extinction, with several species of marine reptiles, ammonites, and plankton disappearing from the fossil record.
Some species, like bivalves and gastropods, managed to survive the extinction event and flourished in the era that followed.
The Late Cenomanian extinction also affected land-dwelling species.
The emergence of flowering plants and the evolution of early animals were two of the most prominent changes during this time.
New ecological niches that enabled these groupings to thrive were probably created as a result of the extinction catastrophe.
The Cretaceous-Paleogene Extinction
The Cretaceous-Paleogene extinction event, also known as the K-Pg extinction event, is one of the most well-known mass extinction events in the history of life on Earth.
It marks the end of the Cretaceous Period and the beginning of the Paleogene Period and is estimated to have occurred approximately 66 million years ago.
Although scientists are still debating the cause of this extinction, the accepted view is that it was caused by a massive asteroid impact.
Scientists led by Luis Alvarez and his son, Walter, originally put forth this notion in the 1980s.
They found high levels of iridium, an element abundant in asteroids but rare in the Earth’s crust, in sediment layers from the extinction.
They hypothesized that this iridium was deposited by an asteroid impact, which would have caused a global catastrophe that led to the extinction of the dinosaurs and many other species.
Evidence for this theory has since been found worldwide, including a large impact crater off the coast of the Yucatan Peninsula in Mexico.
This crater, known as the Chicxulub crater, is estimated to be approximately 180 kilometers (112 miles) in diameter and was formed by an asteroid impact that occurred around the time of the extinction event.
In addition to the asteroid impact theory, other factors have also been proposed as possible causes of the K-Pg extinction event, including volcanic activity, climate change, and sea level fluctuations.
The K-Pg extinction event affected how life evolved on Earth.
According to estimates, up to 75% of all species on Earth during the time—including all non-avian dinosaurs, pterosaurs, and marine reptiles—went extinct.
New groups of organisms, such as mammals and birds, were able to emerge as a result, and they later came to rule the Earth in subsequent epochs.
The K-Pg event’s most well-known aftereffect is perhaps the extinction of the dinosaurs.
These iconic creatures had dominated the Earth for millions of years, and their sudden disappearance paved the way for the rise of mammals as the dominant group of animals.
This extinction also affected the Earth’s climate and environment.
The asteroid impact would have caused widespread fires, earthquakes, and tsunamis and released a lot of dust and debris into the atmosphere.
This would have blocked out the sun and caused a global cooling event, which would have had a significant impact on the Earth’s ecosystems.
It is also thought that the impact may have triggered volcanic activity, which would have further contributed to the environmental changes.
Sources
- https://www.nhm.ac.uk/discover/the-cretaceous-period.html
- https://ucmp.berkeley.edu/mesozoic/cretaceous/cretaceous.php
- https://samnoblemuseum.ou.edu/understanding-extinction/mass-extinctions/end-cretaceous-extinction/
- https://www.livescience.com/29231-cretaceous-period.html
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