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Why Were Animals So Big in the Past? Here Are 6 Reasons

Evolution is a slow but sure process that in most instances takes millions of years before any serious notable changes can be noticed. One question that has puzzled many people (scientists and non-scientists alike) is, why were animals so big in the past?

Some of the modern animals today were once so gigantic that they would have been a nuisance hadn’t evolution taken its toll on them. Some like the sloth used to be bigger than elephants.

There are many theories out there that offer explanations as to why these massive animals became gigantic.

Reasons Why Were Prehistoric Animals so Big

Most of these big animals existed in the ice age period. This included mammoths, dinosaurs, mastodons, and other wild animals. Here are some of the reasons for their increased growth:

1. Self Defense

An Armadillo rolled into a ball
Image by Nathan Rupert used under CC BY-NC-ND 2.0

When faced with predators, early animals didn’t just flee or hide; they also fought back. And, according to new research, this fighting behavior may have been a key factor in the evolution of larger body size.

The study, which was published in the journal Nature Ecology & Evolution, compared the skulls of ancient animals to those of their modern counterparts. The researchers found that, over time, the skulls of predatory animals became shorter and narrower, while the skulls of their prey became longer and broader.

This change is thought to be the result of natural selection; as predators increasingly specialized in hunting their prey, the latter evolved defenses that made them better able to survive attacks. One such defense was a larger body size, which made it more difficult for predators to take down their prey. In other words, Self Defense made prehistoric creatures bigger.

Even without a lot of defensive capabilities, a huge opponent is already threatening enough. Paleontologists believe that most herbivorous animals grew so large to better increase their chances of survival in a harsh world.

Giant animals such as those belonging to the genus Shantungosaurus were almost immune to predators. It would have taken a lot of Tyrannosaurus Rex working together to be able to bring down such humongous animals.

2. Hollow Bones (Pneumatized)


Hollow bones are lighter than solid bones, so they would have made it easier for large animals to move around. In addition, hollow bones have a lot of surface area, which would have helped them to exchange oxygen and carbon dioxide more efficiently.

This would have been especially important for animals that lived in high-altitude environments, where the air is thinner and oxygen levels are lower. The hollow bone theory explains why some of the largest animals in history, such as pterodactyls and brontosaurs, were able to achieve such impressive size.

While there is still much to learn about these creatures, it seems clear that their unique skeletal structure played a key role in their gigantism.

An example of a species that had hollow bones is the Sauropods. The hollow bones were lighter which made them capable of supporting the dinosaurs without adding to their weight.

These hollow bones were also used for breathing to help sustain the dinosaurs’ active lifestyles. Dinosaurs developed these pneumatized bones around 240 million years.

Theropods are one of the main dinosaur groups represented by pneumatized bones.

See Related: Animals that Start with X

3. Environmental Conditions

In order to understand how environmental conditions may have contributed to the evolution of larger and more complex animals in prehistoric times, it is important to consider the types of habitat that would have been common during this era.

In particular, it is likely that these organisms lived in warm, moist climates, in which food was abundant and competition for resources was minimal. Given these conditions, natural selection may have favored traits such as increased body size over traits such as heightened agility or speed, as bigger animals were better equipped to survive and thrive under this type of environment.

Additionally, natural selection may also have favored a long developmental period in order to allow animals to reach larger sizes before reaching maturity. Ultimately, the combination of ideal ecological conditions and adaption towards body size allowed many ancient species to flourish in prehistoric times.

Large animals tend to produce a lot of CO², which led to an increase in vegetation which meant the availability of food for most dinosaurs. It also meant the availability of a lot of oxygen for these prehistoric animals to sustain themselves.

Abundant oxygen can be a major factor when it comes to the accelerated growth of some animals in prehistoric times.

A good example is the cockroaches of the Paleozoic era which were able to grow as big as modern-day domesticated cats! These cockroaches benefited from the excess oxygen that was available in the atmosphere.

4. Eating Habits

Cabazon Dinosaurs

A new study has revealed that the eating habits of other prehistoric animals may have played a role in their size. The study, which was published in the journal Science Advances, looked at the fossil record of an extinct species of mammal known as the Indricotherium.

This mammal was the largest land animal of its time and weighed in at around 20 tons. The study found that the Indricotherium had a diet that was high in nutrients and low in fiber. This diet allowed the animal to grow to its massive size. The findings suggest that the eating habits of prehistoric animals may have had a significant impact on their overall size.

Due to the abundance of food, these animals didn’t have to worry too much about their next meal. They were also able to swallow their food in huge chunks eliminating the process of chewing thus accelerating their rate of consumption.

This led to the already huge animals’ continuous growth, which in turn kept them safe from a lot of predators –in prehistoric times there were numerous hazards for herbivorous animals.

No chewing also meant that they had lighter heads –this contributed to the elongation of the neck to help them reach food from the treetops.

See Related: Giant Armadillo: Is This Species Endangered?

5. Cope’s Rule

Cope’s Rule is a well-known theory in evolutionary biology, first proposed by the paleontologist Edward Drinker Cope in 1877. The basic premise of Cope’s Rule is that over time, animal species tend to grow larger and larger on average.

According to this rule, for example, we would expect present-day animals like baleen whales and blue whales to be much larger than their ancestors from millions of years ago.

There are several different explanations for why Cope’s Rule seems to hold true in so many different cases. One popular theory is that as species get larger, they experience a higher rate of metabolism, making them better able to adapt and survive in changing environments.

Additionally, larger animals tend to have lower population densities and smaller home ranges, which means that they can more easily avoid threats like predators or environmental changes. Overall, Cope’s Rule offers an intriguing look at the patterns of evolution over time and helps us better understand how species have evolved over millennia.

Cope’s rule dictates that competition –in this case for survival, will always encourage the evolution of the disadvantaged.

In the wilderness, if the weak animal doesn’t evolve to better deal with threats, then they face mass extinction within a short time. This explains why most of the herbivorous prehistoric humongous animals are the huge ones; being massive ensured that no predator bothered them.

See Related: California Condor: Why Is It Endangered?

6. Bergmann’s Rule

A graph of Bergmanns rule
Image by Nmccarthy16, used under CC BY-SA 3.0

Bergmann’s Rule states that in general, larger animals tend to be found in colder climates than smaller animals. This rule is based on the principle of Bergmann’s Displacement Law, which states that animals are generally denser than water.

Because of this, larger animals have a greater volume-to-surface area ratio than smaller animals and therefore lose heat less efficiently in colder environments.

There is considerable evidence supporting Bergmann’s Rule and its underlying principle, as various animal species living in northern or alpine regions are typically much larger than their counterparts in warmer habitats.

Some scientists also believe that Bergmann’s rule may hold true for certain human populations as well, with northern Europeans tending to be taller and heavier than people from other parts of the world.

Whether or not Bergmann’s Rule applies to humans, it is certainly true for many other animal species, making it an important concept in modern biology and ecology.

See Related: Animals That Start With V

Large Prehistoric Animals

To better understand why some of these giant animals existed, you first need to familiarize yourself with several of them. The following are some of the large prehistoric animals;

1. Dinosaurs

A Dinosaur

Dinosaurs are some of the most popular prehistoric animals known today. They are widely known and elicit curiosity, amazement, and/or fear in different people.

They were able to dominate the planet for a long time and lived through the Triassic, Jurassic, and Cretaceous periods. This was when they met their sad fate around 65 million years ago.

These beautiful, humongous animals can be classified as;

  • Ornithischians (mostly herbivores) were distinguished by their bird-like hips. They were herbivores and had jaws and beaks for feeding.
  • Ceratopsia (horned faces) were herbivores and were found in Europe, Asia, and North America during the cretaceous period. Their sizes varied from 1 to 9 meters and weighed up to 9 tons! Their mouths were beak-like and they were quadrupeds. This family included the Psittacosaurus, Centrosaurus, and the famous Triceratops (large, bony frill at the back of the head, and 3 distinguishing spikes on its face.
  • Thyreophora (shield bearers) – these dinosaurs were heavily armored with rows of plates on their bodies and thick skin to boot. Many of them also had powerful tails that were covered in spikes or clubs.

    Their best-known groups included the Ankylosauria and the Stegosauria;

    The Stegosauria group included dinosaurs such as the Kentrosaurus, Hesperosaurus, Stegosaurus, and the Wuerhosaurus. Their bodies were covered in protective plates located along their backs.

    Ankylosaurus lived through the Mesozoic period and had protective thick skin and a strong tail with huge clubs used for defensive purposes.
  • Saurischians (mainly carnivores) could be distinguished by their lizard-hipped hips. They are divided into two main groups which are sauropods and theropods.
  • Theropods were vicious meat-eaters who were bipeds. They were around from the late Triassic era up to their extinction during the Cretaceous period. They were a large family that was mainly made up of:

    Coelurosaurs (hollow-tailed lizards) were a lot more like birds and they included the Tyrannosaurus Rex.
  • Maniraptora is the dinosaurs that evolved into modern-day birds. They also include the raptor family (Dromaeosauridae). This family was of medium-sized dinosaurs that had feathers such as the Velociraptor and Microraptor.
  • Abelisauridae – this species of theropods lived during the cretaceous period in Asia, African South America. Some of its examples are the Carnotaurus and Abelisaurus.
  • Tyrannosauridae (tyrant lizard) – these theropods had huge skulls, short arms, and powerful jaws. The family consists of the T-Rex, Albertosaurus, Tarbosaurus, and Gorgosaurus.
  • Spinosauride– this family consisted of large bipeds that had long, thin skulls that looked like that of a crocodile. They included Spinosaurus, Irritator, Baryonyx, and the Suchomimus.
  • Carnosauria – this group is made up of two families (Allosauridae and the Carcharodontosauridae). The Allosauridae existed in the late Jurassic and early cretaceous periods. They were predatory and at the top of the food chain at the time. Some examples of allosaurids include the Allosaurus and the Saurophaganax.
  • The Carcharodontosauridae (shark-toothed lizards) are some of the largest land carnivores to have ever walked the earth. Examples of these dinosaurs include the Tyrannotitan, Giganotosaurus, and the Carcharodontosaurus.
  • Sauropods- this group contained dinosaurs that were quadrupeds, with large round bodies, small heads, long necks, and a long powerful tail.
  • Some of the largest prehistoric animals to ever grace the earth such as the Diplodocus (Supersaurus, Diplodocus, Brontosaurus, and Apatosaurus), and Titanosaurus (Argentinosaurus and Saltasaurus) belonged to this group.

Though they had lizard-like features, these animals eventually evolved to become birds millions of years later.

See Related: Most Endangered Amphibian in the World

2. Indricotherium (Paraceratherium)

Image by ABelov2014, used under CC BY-SA 3.0

The Indricotherium was a mammal that lived in the Oligocene epoch about 23 million to 33 million years ago. They are some of the largest mammals to ever walk the earth.

They are the ancestors of the modern-day rhinos and they could grow to weigh 20 tons. However, unlike the rhinos, they had no horns. They were herbivorous and therefore had plenty of food to support their enormous appetites.

These great appetites are what contributed to their extinction due to dwindling sources of food. When the Asian forests that they used as their source of food became replaced by grasslands, they had no chance of long-term survival.

Both their necks and feet were long, and they had three toes on their feet.

See Related: Best Books on Endangered Species

3. Blue Whale (Balaenoptera musculus)

Blue Whale

The blue whale is another humongous mammal from the prehistoric era that has managed to survive to date. They experienced a near-extinction event, but they are still around and slowly growing their numbers.

Though they have no natural enemies courtesy of their huge size, they nearly went extinct as a result of hunting and ship accidents. The hard and flexible part of their jaws known as baleen was used for making corsets, hats, and other clothing items back in the day.

These carnivorous animals can live for as many as 80 – 90 years of age. During this time, they could grow as long as 105 feet and weigh up to 200 tons.

They feed on animals called krill that look more like tiny shrimps. They feed by swallowing a lot of water with krill and then sieving out the water and swallowing the krill.

See Related: War and the Effect on Wildlife

4. Andrewsarchus (mongoliensis)


In the middle of the Eocene epoch, Andrewsarchus roamed the earth in search of something to sink its teeth in. This was somewhere between 45 and 35 million years ago.

The available evidence of this magnificent beast is a single skull that was discovered in 1923 in the Gobi desert, Mongolia. However, the shape and appearance of the rest of its body remain a mystery to date.

It weighed over 2000 pounds and could grow up to a length of 13 feet – its discovered skull is 3 feet long.

The large jaws of this mammal made it capable of feeding on prey and even protecting itself from other hostile animals.

See Related: Animals That Start With I

5. Pterosaurs (Quetzalcoatlus)


Before there were birds in the skies, the Pterosaurs (winged lizards) were the first to do so millions of years ago. These vertebrates flew the skies of the earth during the cretaceous period, and by the time they went extinct they had spread all across the planet.

The largest type of the pterosaur species is the Quetzalcoatlus northropi which is the size of a small airplane at 36 feet across. They are reptilian and first appeared in the scene about 250 million years ago.

There have been around 130 species of Pterosaurs that have been identified by paleontologists. Some of these types of large prehistoric animals include Nyctosaurus, Tapejara imperator (late cretaceous), pterodaustro (early cretaceous), and the Pteranodon (which had a wingspan of 22 feet.)

Their most distinctive feature was their head crests that were made of soft tissue fussed with hornlike materials and supported by bones.

They could have served any purpose from attracting mates, regulating heat, and maybe even for defense; paleontologists haven’t been able to figure out the specific of these beautiful head crests.

See Related: Three-Lettered Animals You’ve Probably Never Heard Of

6. Gigantopithecus (black)

The Gigantopithecus blacki
Gigantopithecus blacki” by Sam_Wise is marked with CC BY-NC-SA 2.0.

Even about a hundred thousand years ago, some animals were still massive compared to their modern-day cousins. Animals such as the Gigantopithecus, a not so distant relative of the Gorillas that lived during the Early to Middle Pleistocene

These ancient primates measured up to 10 feet and weighed over 1 ton. They lived in the part of Asia that’s now China. Due to their massive sizes, they required a huge amount of food to keep them going, and that might have been their undoing.

However, some believe that Gigantopithecus blacki is still alive and kicking –just hiding from people. Although this isn’t proven, stories of people running into a big-foot or a Yeti are associated with Gigantopithecus by some.

Just imagine if this huge primate had been around in prehistoric times when food was in abundance, he would have given some dinosaurs a run for their money.

See Related: Most Comfortable Animals in the World

7. Josephoartigasi (monesi)

A replica of Josephoartigasi monesi
Image by Nobu Tamura used under CC BY 3.0

This was a rodent from the Dinomyidae family, a species of the caviomorph rodent that became extinct about 2 million years ago -its main habitat was in South America

It’s the largest rodent ever known and is believed to have weighed up to 1000 pounds. It was present between the Pliocene to the beginning of the Pleistocene era.

Its closest living relative is the pacarana –Dinomys branickii, and it only has one other species in its genus -the J Magna.

It had an estimated body length of 10 feet, a height of 5 feet, and weighed anywhere from 770 pounds to 3370 pounds. This rodent was bigger than some –if not most, of the modern-day bulls.

Josephoartigasi monesi was first introduced in a study that was conducted by Andres Rinkerknecht, a paleontologist at the National Museum of Natural History and Anthropology in Montevideo, Uruguay.

Though some of the reasons offered to explain why prehistoric animals grew so big are data-backed, the rest are just estimations and extrapolations.

Recently, natural selection seems to be going for smaller animals and plants, unlike the prehistoric giants.

See Related: Endangered vs Threatened vs Extinct Species

8. Arthropleura

A replica of the Arthropleura
Arthropleura, Parco Natura Viva” by spencer77 is marked with CC BY 2.0.

Arthropleura was one of the giant insects in prehistoric times. This giant millipede lived during the Carboniferous period, about 315 million years ago. Measuring up to 8 feet in length, it was one of the largest land animals of its time. Arthropleura had a segmented body with two pairs of legs per segment.

Its exoskeleton was covered in tough plates, which provided protection from predators. The creature is thought to have been a herbivore, and it likely fed on soft vegetation. Arthropleura probably spent most of its time burrowing underground, where it could avoid being eaten by predators.

While it is now extinct, this massive millipede serves as a reminder of the strange and wonderful creatures that once roamed our planet.

9. Prionosuchus

The Prionosuchus
Image by Nobu Tamura, used under CC BY 3.0

Prionosuchus was a fearsome prehistoric creature, renowned for its massive size and powerful jaws. This giant crocodilian lived during the late Triassic period, occupying swamps and rivers across what is now modern-day Europe and North Africa.

Despite their enormous size and predatory nature, little is known about Prionosuchus or how they hunted their prey. Some scholars suggest that these beasts may have used stealthy ambush tactics to take down their victims, while others believe they may have been more active predators who engaged in open combat with other large animals.

Whatever the case, it is clear that Prionosuchus was one of the most impressive creatures to ever walk the earth. With its immense size and formidable jaws, this ancient animal continues to capture our imagination over 200 million years later.

See Related: Cheetah: Is This Animal Endangered?

10. Megalodon

A replica of Megalodon in a museum
Megalodon Jaws” by Eligius4917 is marked with CC BY-SA 2.0.

Megalodon was a massive and predatory sea creature that lived millions of years ago. With its large teeth, powerful jaws, and crushing bite force, it hunted and preyed on other marine animals with great ferocity.

Indeed, this fearsome predator was one of the largest sharks to ever exist, reaching sizes up to 20m in length and weighing 40,000kg. Despite its tremendous size, however, Megalodon suffered a dramatic extinction at the end of the Pliocene epoch.

Scientists have speculated on a variety of possible causes for this extinction event, including competition with other predators or climatic changes, but no definitive explanation has yet been established.

Nevertheless, Megalodon remains an iconic emblem of prehistoric marine life and a marvel of evolution.

Over millennia since its disappearance from our planet’s oceans, its massive fossils continue to capture the imaginations of many who long to encounter this ancient behemoth once again.

See Related: Is Hunting Good or Bad for the Environment?

Modern Giant Animals

In a world of modern humans, it can seem like the truly giant animals have gone extinct. After all, how could creatures as large as dinosaurs or mammoths survive in a world of skyscrapers and motor vehicles?

Yet recent research suggests that some so-called “modern giants” may still roam the earth.

For example, some biologists believe that the elusive Sasquatch – or Bigfoot – might actually be an undiscovered species of the giant ape. In addition, scientists have recently discovered several unknown species of massive cloud rats in remote parts of the tropics, suggesting that other giant animals may still exist in places that humans haven’t yet explored.

Whether they’re hidden deep in the rainforests or living under our very noses, these modern giants are sure to continue capturing our imaginations for years to come.

The largest animals today include elephants, hippopotamuses, and rhinoceroses. These creatures are all massive, weighing in at several thousand pounds apiece.

While they may not be as large as some of the prehistoric giants on this list, they are still impressive creatures that deserve our respect. Each large animal plays an important role in their respective ecosystems, and their loss would be a great tragedy.

So the next time you see one of these animals, take a moment to appreciate its size and power – and remember that it is only one member of a long and glorious lineage of giant creatures that have walked our planet.


What is the biggest animal?

The biggest animal today is the African elephant.

Can dinosaurs survive today?

There is no definitive answer to this question, as there is still much we don’t know about dinosaurs. Some scientists believe that they may still exist in remote parts of the planet, while others think that they went extinct millions of years ago.

Nevertheless, dinosaurs are a fascinating and iconic part of our planet’s history, and they continue to capture the imaginations of people all over the world.

Has Dinosaur DNA been found?

There is no definitive answer to this question, as scientists have not been able to find any complete dinosaur DNA. However, they have found fragments of DNA that may be from dinosaurs, and this has led to some interesting theories about what these creatures were like.

What caused the extinction of most prehistoric animals?

The most likely cause of the extinction of most prehistoric animals is that they were killed by climate change. A warming Earth would have made their habitats uninhabitable and led to food shortages.

Changes in the Earth’s climate could also have caused dramatic shifts in weather patterns, which would have made it difficult for creatures to adapt. Other potential causes of animal extinctions include disease, asteroid impacts, and human hunting and poaching.

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