12 Animals That Used To Be Way Bigger (And Why They're Smaller Now)

The modern day animal kingdom is full of massive and terrifying creatures, from the ferocious great white shark to the vegetarian rhinoceros. Even the largest animals alive today, however, pale in comparison to some of the prehistoric giants that used to roam the earth.

From gigantic insects with wingspans the size of birds, to towering sloths that walked on their hind legs, there are plenty of weird and wonderful ancestors to our current animal kingdom that many of us are unaware of. Whether due to the differing climate and oxygen levels or a high-quality diet, some of our modern-day animals' ancestors were truly enormous.

As the environment changed, and in some cases humans began to appear, these mega creatures were forced to adapt, and the smaller, quicker versions that we see today evolved instead. Let's take a look at 12 animals that used to be much bigger, and the likely reasons that they are smaller now.

Though modern sharks may be the biggest fish in the ocean, with the whale shark reaching up to 40 feet long, they are petite in comparison to their ancestors. As a side note, the supposedly terrifying great white shark averages at a miniscule 13 to 15 feet. The megalodon is one of the most famous prehistoric creatures, and it is no wonder. It grew to a terrifying 60 feet long and potentially weighed 94 tonnes, with a newborn megalodon pup measuring at a not-so-cute 12.8 feet.

The megalodon roamed the oceans roughly 23 million to 3.5 million years ago. Fossils of megalodons have revealed teeth as large as 7 inches in length, though the clue is in its name, as megalodon literally means "big tooth."

Sharks have been around in some form for more than 450 million years, predating even trees. They survived the mass extinction event in the Devonian that obliterated three quarters of life at the time, which may have given them the space to dominate and evolve into the creatures they have become today.  Around 3 to 4 million years ago, however, changes in climate began to affect the massive sharks of the time, including megalodon, in a negative way. As the waters cooled, they lost their tropical habitat, and since around a third of marine animals disappeared during this time, their access to plentiful food was restricted, too. Smaller, more streamlined sharks were better at adapting to these environmental changes, so natural selection meant that megalodon disappeared, and our tiny modern sharks survived.

Modern-day sloths have an image as being sleepy and cute, and their average size is between 2 to 2.5 feet in length. They spend most of their time in trees, and move at less than half a mile per hour. Their ancestors, in comparison, were up to 12 feet tall, and could reach the weight of a modern bull elephant.

The megatherium, whose name translates as "giant beast," was the biggest of all the ground sloths, and is thought to be the largest two-legged mammal to have ever existed. Predicting how fast it may have moved is extremely difficult, but recent studies analyzing its inner ear suggest it could have been far more sprightly than its modern descendants. 

The disappearance of these enormous sloths has been attributed to a combination of climate change and human intervention. As a slow-moving mammal compared to many others, giant ground sloths would have been easy targets for humans, and coupled with the threat to their habitat from a changing climate, their time came to an end around 10,000 years ago. Their smaller ancestors became adept at living in trees, avoiding land-roaming predators and enjoying life in their own ultra chilled way.

If you love to watch a dragonfly buzzing about your garden, admiring their beautiful colors and intricate wings, then you may be slightly alarmed to hear of their ancient ancestors. Prehistoric dragonflies used to be so much bigger, in some cases the size of a modern bird of prey.

Known as griffinflies, prehistoric dragonflies were around before the dinosaurs, 300 million years ago. They are thought to look extremely similar to modern dragonflies, but on a massive scale. This is due to the high oxygen levels on the earth at that time — 30% rather than the current 21% — which allowed many types of ancient insects to reach sizes we can hardly imagine. The griffinfly, also known as Meganeuropsis, is thought to be the largest ever insect, with a wingspan of up to 75 centimeters across.

The reason that insects today are nowhere near the size of primitive griffinflies is due to the major mass extinction event that killed them off. At the end of the Permian Period, around 250 million years ago, the biggest ever mass extinction occurred, which annihilated the vast majority of life on Earth, including the dragonfly ancestors. The Triassic Period began with much lower oxygen levels than those that allowed insects to reach their massive size, and the insect world since has never reclaimed the gigantic glory days of the Permian.

Many people are scared of rats, and for good reason. They can easily spread disease to humans, and for a long time were thought to have caused the Black Death, a bubonic plague that killed up to 50 million people in Europe, though there is now evidence that us humans were actually to blame. Either way, rats have got a bad name, and some street rats can reach sizes of almost a foot long. Imagine, then, a prehistoric rodent that could reach up to 5 feet in length — that is the stuff of nightmares.

One unusual feature of this rodent, Neoepiblema acreensis, is the size of its brain. In spite of being the size of a small human, its brain weighed only 4 ounces, compared to the average human brain mass of around 3 pounds. This is an exceptionally small brain to body ratio, and much less than we see in modern rodents. Over time, Neoepiblema acreensis was might have been impacted by a changing climate, habitat loss, and possibly in this case, the emergence of predators with bigger brains.

As mammals go, bears are not exactly on the small side, but prehistoric bears were even more imposing, reaching heights of up to 12 feet when standing upright. The giant short faced bear, known as Arctodus simus, could weigh as much as 1,500 pounds, and in spite of its massive size, it was the fastest bear to ever exist.

Despite the fact that most modern bears are omnivores, the jaw structure of the giant short faced bear suggests a diet that consisted of primarily meat, and it had an incredibly strong bite. Its reliance on a meat-only diet, however, was likely its downfall. It ate large mammals such as bison, deer, and horses, and its survival would have been threatened as some of these creatures disappeared at the end of the ice age. The only true relative of the giant short faced bear is the spectacled bear, which measures a miniscule 5 to 6 feet, tiny in comparison to their enormous ancestors.

The king of the jungle, the majestic modern lion is a substantial mammal that terrifies most creatures that come near it, but it pales in comparison to the size of its ancient ancestors. Cave lions are thought to be the largest cats to have ever existed, measuring 11 feet long and around 800 pounds in weight, which is around 25% heavier than a modern lion.

Strangely, these enormous cats weren't found in the savannahs of Africa, but lived in prehistoric Europe and Asia, including Britain. They evolved during the ice age of around 2.5 million years ago and lived alongside the wooly mammoth and saber toothed tiger. There is growing evidence to suggest that Ice Age humans hunted this huge beast, using the skins for rugs and clothing, and eating the meat. As the Ice Age came to an end, overhunting and the change in climate meant that these enormous feline ancestors of the modern lion became extinct around 14,000 years ago.

You probably think that the rhinoceros is a pretty large animal, and considering they are the second biggest mammals on Earth — second only to elephants — that would be a reasonable assumption to make. By prehistoric rhino standards, however, they are on the petit side, weighing as little as 10% of the mass of their predecessors.

The Paraceratherium is generally considered to be the largest land mammal to have ever existed, having weighed in at around 20 tons, and with a body length of up to 26 feet. They roamed the earth around from 34 to 23 million years ago, during the Oligocene epoch, and like many prehistoric rhinos, did not appear to have horns.

The paraceratherium was a herbivore, and its distinctive long neck, a feature that has been lost during millions of years of evolution, would have allowed it to graze high in the trees of ancient Eurasia, much like the modern-day giraffe. Its feeding habits may have been a factor in its extinction: As the climate changed and habitats evolved, their lush, green vegetation would likely have disappeared. Prehistoric elephants, known as Gomphotheres also seemed to appear around the time of their extinction, suggesting that their presence may have had an impact on the survival of the paraceratherium.

If a crocodile today would give you fear (and rightly so), you would definitely not want to come face-to-face with its fearsome ancestors. The deinosuchus, which means "terrible crocodile," was a monster of a reptile that lived around 75 million years ago during the late Cretaceous period.

This enormous crocodile was thought to measure up to 36 feet in length, whereas the largest crocs today tend to average 17 feet. As a member of the crocodilian family it was, of course, a carnivore, and was even thought to eat dinosaurs. Although it didn't live alongside T-Rex, the bite force of deinosuchus is thought to have been comparable in terms of danger.

Though not much is known about why this particular crocodilian became extinct, it happened before the asteroid extinction event that wiped out the dinosaurs. Competition for food is a possible reason, perhaps not helped by the fact that its enormous body made movement more difficult than other predators of the time.

Whether you love watching millipedes scuttling about their business in the garden or you can't stand to look at the creepy crawlies, as a rule, their size is not something to be concerned about. In 2018, however, a millipede fossil was discovered that would make even the biggest bug enthusiast cringe.

On a Northumbrian Beach in England, a piece of sandstone from a cliff was found that contained a fossil from a millipede-like invertebrate thought to be massive — nearly 9 feet in length. Known as Arthropleura, this prehistoric creature lived more than 300 million years ago, long before the age of the dinosaurs. The huge size of many creatures during that time is often thought to be as a result of the high levels of oxygen in the air, but this particular fossil seems to predate that time period. Instead, researchers have assumed that a nutrient-rich diet must have led to the enormous size of this prehistoric millipede.

The Arthropleura became extinct at some point during the Permian period, possibly due to climate change and competition for food. Later arthropods evolved once the high oxygen levels from the Permian period had diminished, meaning modern-day millipedes are a fraction of the size, which is good news for the gardeners amongst us.

Wombats are known for being cute, cuddly marsupials found in Australia, but their prehistoric ancestors are a little more intimidating. The diprotodon has often been referred to as the giant wombat, but is not actually related to the modern day marsupial. A recent discovery, however, has found a true giant wombat, which was at least twice the size of the cute, furry animals we know today.

Ramsayia magna existed relatively recently, compared to the rest of the animals here, roaming around what is now Australia around 80,000 years ago. The recent fossil discovery uncovered various traits of prehistoric wombats that were previously unknown. Its round head suggests that the animals did not live in burrows as their modern counterparts do. The structure of the nasal bones also suggests a fleshy nose, a little like a mini trunk.

Very little is known of the reason the Ramsayia magna became extinct, but the arrival of humans could have been a factor. 

Kangaroos are probably the most famous marsupials on the planet and are not particularly small, with the largest red kangaroo reaching around 6.5 meters in height. Their prehistoric ancestors however, were much larger, and would tower over their modern descendants.

The Protemnodon was a megafauna related to the modern-day kangaroo that reached up to double the height of the kangaroos we now know. They lived from 5 million to 40,000 years ago, and would have lived in modern-day Australia and New Guinea. Researchers have recently discovered that they traveled a surprisingly short distance from their habitat, considering their large size. The plentiful foliage around where they lived may have meant that they had no need to venture further in the way that most animals of the time would have done.

This convenient lifestyle was likely the downfall of Protemnodon: As their habitat changed and their food supply dwindled, they were unable to travel in search of quality food. They became extinct around 40,000 years ago, meaning that the smaller, more mobile version of kangaroos who were able to adapt to their environment have been able to thrive.

Cheetahs are known for being lean and agile, characteristics that help make them the fastest modern-day land animals. Their ancestors, however, were significantly more robust, likely three times the size of their modern descendants.

Acinonyx pleistocaenicus lived around 1.3 million years ago, in Eurasia, and its fossils were first discovered in China. They believe the prehistoric cat could have reached weights of up to 420 pounds, putting it on a par with modern lions and tigers. In comparison, a modern-day cheetah is more likely to weigh around 100 pounds. Instead of the nimble athletes we know, prehistoric cheetahs would have been powerhouses, with a fair bit of mass to throw around.

These enormous cheetahs began to disappear around half a million years ago, likely due to the significant change in climate. Long ice ages gave way to more warm periods with intense cold periods in between, which changed the environment drastically. As the ecosystem changed, speed became more vital for survival than size, resulting in the evolution of the modern characteristics of the athletic cheetah we now know.