Questions about the gigantism of ancient animals inevitably arise for anyone interested in paleontology. Dinosaurs undoubtedly lead the way in popularity, but other animals—mammoths, dragonflies, sharks, sloths, crocodiles, pterosaurs, and even some species of primates—also capture the imagination. In fact, any ancient creature that exceeds a certain stereotypical size norm often prompts people to ask: Why were animals so much bigger in the past? Before answering this question, let’s explore three popular myths associated with this topic.

The initial premise seems logical. After all, the size of organisms is indeed influenced by the planet’s gravity. However, gravity is directly related to the planet’s mass. Simply put, to significantly alter gravity, you would need either to remove a substantial chunk of Earth or add one. Given Earth’s mass (5.97×10^24 kg), to change gravity by even 1%, the mass removed or added would need to weigh about as much as Jupiter’s moon Europa (4.8×10^22 kg). Such an event would leave bacteria as the planet’s only inhabitants—and they might even be larger than usual. A similar catastrophe occurred 4.5 billion years ago when Earth collided with the protoplanet Theia, resulting in the formation of the Moon. Since then, Earth’s gravity has remained constant, and we can consider it a fixed value.

This myth is somewhat similar to the one about gravity. Every creature, even a party member, is subjected to atmospheric pressure equivalent to a 214-kilogram column. If this pressure were reduced, the creature would grow into a giant. In reality, the first issue a potential giant would face is hypoxia, caused by low partial pressure of oxygen. But let’s assume we could bypass this problem and eliminate the force pressing down on us.

Archimedes’ principle, which could help us, depends on the density of the medium: F = Vpg. To compensate for the organism’s weight, you would need to increase, not decrease, the density of the medium (and atmospheric pressure). Under current conditions, Archimedes’ force in air is 800 times lower than gravity. To compensate for even 10% of the weight, air density would need to increase 80 times, resulting in 80 atmospheres of pressure—equivalent to a depth of 790 meters in the ocean. To survive such conditions on land, animals would require a vastly different anatomy than what we know today.

Alternatively, you could decrease your body’s density by increasing its volume. To gain that same 10% weight advantage, you would need to reduce your body density by 80 times. This would mean that a 100-kilogram human would occupy a volume of 8 cubic meters. Unfortunately, animal anatomy does not support the emergence of balloons and blimps.

Like many myths, this one contains a grain of truth. High oxygen levels in the atmosphere, combined with high pressure, can indeed affect animal size—but only for certain species of arthropods, specifically insects, centipedes, and many spiders. These animals breathe through a system of tubes, or tracheae, that run through their bodies. The tracheal system’s ventilation is achieved through body movements. This method of respiration, along with their exoskeletons, severely limits the size of insects and their relatives. An increase in oxygen concentration could allow for larger sizes.

This situation only occurred once on our planet, during the Carboniferous period. Oxygen levels reached up to 35%, enabling giant dragonflies (Meganeura) with a wingspan of one meter and 2.5-meter-long millipedes (Arthropleura) to thrive.

Where did all this oxygen come from? Vast forests of horsetails, clubmosses, and ferns covered large areas, absorbing carbon dioxide from the atmosphere and releasing oxygen. After their death, these plants were buried in swampy soils, gradually turning into coal deposits. Instead of returning to the atmosphere as carbon dioxide, carbon accumulated in the lithosphere. This was possible because mold fungi and cellulose-decomposing microorganisms were not yet present in significant numbers. With their appearance, the oxygen percentage dropped to 15%, only reaching modern levels of 20-21% by the end of the Triassic, where it has remained stable.

Having addressed the main misconceptions, it’s clear that these myths share a common desire to offer a single, simple explanation for the complex phenomenon of gigantism. But what do we mean by gigantism and large sizes?

In biology, there is a concept called “megafauna,” which refers to a group of animal species with a body mass greater than 40–45 kg. Strictly speaking, this makes us giants too. Megafauna is a relative concept; for example, in soil communities, moles and shrews would be considered megafauna. And importantly, megafauna always exists. Anomalocaris, with its modest 60 cm length, might not seem impressive, but in the Cambrian period, it was also considered megafauna.

Naturally, besides megafauna, every ecosystem includes animals of medium and small sizes. These creatures are of little interest to the general public, especially when it comes to extinct species. Who cares about the hundreds of Mesozoic mammals the size of a mouse when there’s a majestic Tyrannosaurus rex nearby? Yet, in any ecosystem, the majority of animal biomass consists of such small creatures, with megafauna being the proverbial cherry on top. However, it is this cherry that captures attention and creates the false impression that all animals in the past were larger than their modern counterparts. Nevertheless, there is a trend toward increasing size in the evolution of animals. This observation, known as Cope’s Law, reflects general statistical patterns. Several factors contribute to increasing size, and below we will attempt to characterize the main ones.

1. Aquatic Habitat This one is simple. Thanks to Archimedes, we know that buoyant force in water significantly compensates for the weight of a submerged body. It’s easier to be a large aquatic animal than a terrestrial one.
2. Internal Skeleton Having a framework to support organs and tissues is a significant evolutionary advantage. There are two options for such a framework: internal or external. Both allow animals to transition into the megafauna category. Structurally, the internal skeleton is superior, and its bearers demonstrate record sizes among animals.
3. Lung Respiration Animals have diverse ways of breathing—gills, tracheae, body surface, and finally, lungs. As body size increases, the surface area grows much more slowly than volume, leading to inevitable problems with gas exchange. Neither tracheal nor skin respiration can supply sufficient oxygen to a truly large animal. Gills are more efficient in this regard, but they too have a limit, determined by surface area. Moreover, gills can only be used in aquatic environments. In contrast, lungs grow proportionally with body volume and can be used both in water and on land. If you want to be a giant, breathe with lungs.
4. Low Metabolism Indeed, the lower the metabolism, the less food and oxygen an animal needs, allowing it to achieve larger sizes on the same resource base than more energetic neighbors. Low metabolism can significantly compensate for gill respiration, allowing some fish to reach up to 40 tons. However, it also results in reduced activity, which is less of an issue in water but becomes critical on land.
5. High Metabolism High metabolism creates numerous challenges, such as increased consumption of oxygen and food and the need to maintain a constantly high body temperature. In return, it offers the potential to grow into a true giant. As size increases, relative muscle strength decreases, and a significant portion of energy is spent just maintaining an upright posture. The energy expenditure of a sauropod just to stand or walk slowly far exceeds that of a jerboa. High metabolism helps maintain the necessary level of energy expenditure for large sizes.
6. Heat Retention It is harder for small animals to retain heat because their body surface area is relatively larger than that of larger animals. Heat loss can slow down metabolic processes and ultimately lead to death, a situation that worsens in cold environments. Increasing size can reduce heat loss, especially in polar regions or at great depths.
7. Arms Race Large size is an effective way to protect against predators. If you are a predator yourself, increasing your size broadens your prey base, allowing you to choose larger prey and spend less energy hunting for food. However, overly large prey can be dangerous, so for potential prey, increasing size is a good way to defend against predators. This cycle can continue for quite some time, leading to a sequential increase in the size of both predators and their prey. In the long run, we get gigantic herbivores and filter feeders that have no enemies in adulthood, except for equally gigantic super-predators.

Having explored the factors, let’s move on to their application and try to understand why the megafauna of the Mesozoic was larger than that of the Cenozoic.

The Mesozoic era is characterized by marine reptiles, primarily ichthyosaurs, plesiosaurs, and mosasaurs. The size range of large species was between 10 and 20 meters, with Shonisaurus leading at 21 meters. The Cenozoic era belongs to mammals, and here we encounter cetaceans. The size range of large cetaceans is similar, 10-20 meters, but some baleen whales easily surpass this limit. The largest, the blue whale, can reach 33 meters and weigh 150 tons, making it the absolute record-holder for size among animals. Among fish, we can mention the Jurassic Leedsichthys, the Pliocene Megalodon, and modern whale and giant sharks. Realistic estimates put their average size at 10-15 meters.

It seems that for both cetaceans and marine reptiles, size increase became the primary means of protection from predators throughout their evolution. For mosasaurs, short-necked plesiosaurs (pliosaurs), and Megalodon ancestors, increasing size was the result of specialization in hunting large prey.

The large sizes of baleen whales, giant and whale sharks, and Leedsichthys (and possibly some ichthyosaurs) allowed them to effectively filter large volumes of water, sifting plankton and small fish.

Two common factors facilitated the achievement of such sizes: living in an aquatic environment and having an internal skeleton. Gills and low metabolism worked against fish, preventing them from becoming the largest creatures. High metabolism and lung respiration gave whales and marine reptiles an equal chance of victory. Recent data suggests that all described groups of reptiles (except possibly some plesiosaurs) had high metabolism and body temperatures of 28-39°C. However, another factor may have played in favor of whales—heat retention in a cold environment. Throughout the Mesozoic and much of the Cenozoic, the climate was warm. Serious cooling began in the Pliocene, leading to a series of Pleistocene glaciations. The concentration of plankton in the cold waters of high latitudes contributed to the gigantism of cetaceans.

In the history of the planet, only two groups of animals managed to master true flight and achieve significant sizes: pterosaurs and birds.

Pterosaurs include the largest creatures ever to take to the air. The family Azhdarchidae boasts giants like Hatzegopteryx and Quetzalcoatlus, with wingspans of 10-12 meters. Pteranodons had wingspans of 7-10 meters. Among flying birds, gigantism is rare, occurring in different groups and geological periods. The modern record belongs to the royal albatross, whose 3-meter wingspan seems modest compared to pterosaurs. The largest flying birds known from fossils include Pelagornis and Argentavis, with wingspans of around 7 meters.

Active flight requires a specific body structure and high energy expenditure. Pterosaurs, like birds, were warm-blooded, with a high metabolism. Pycnofibers, hair-like structures homologous to bird feathers, helped them retain heat. Large lungs with air sacs supplied the body with oxygen. A lightweight skeleton, a keel for powerful pectoral muscles, and a developed brain—these traits were acquired independently by both birds and pterosaurs.

However, the physics of pterosaur flight differed significantly from that of birds. Differences in wing structure, body proportions, and the ratio of body mass to wing area make direct aerodynamic comparisons between birds and pterosaurs impossible. Compared to birds, pterosaurs achieved maximum skeletal lightness. It seems that this, combined with more efficient aerodynamics, allowed pterosaurs to surpass birds in size.

Increasing size for flying animals is a challenging task. Weight increases faster than body area, muscle strength decreases, heart rate slows, and metabolism drops. The range of possibilities in this direction is narrow, with the only significant advantage being a transition to soaring flight, which conserves energy and allows for long-distance travel in search of food.

The cause of gigantism in pteranodons, pelagornis, and albatrosses could be high interspecies competition in coastal communities. In such cases, large size is an advantage, allowing them to fly farther and farther from shore in search of food. Azhdarchid ancestors led similar lifestyles, but as they grew larger, they transitioned to terrestrial ecosystems. Quetzalcoatlus and similar pterosaurs spent most of their time on the ground, moving like giant stork-marbou hybrids. However, when necessary, they could take to the air, soaring above fern savannas. Argentavis, unlike these pterosaurs, was associated solely with steppe ecosystems. Perhaps its large size gave it an advantage in competition for carrion and helped it fend off predators from their prey.

Mesozoic terrestrial megafauna is the largest in Earth’s history. Sauropods reached lengths of 20-30 meters and weights of 40-60 tons. Some species, like Amphicoelias, are estimated to have weighed 70-80 tons. Large sizes were also characteristic of other non-avian dinosaurs, including theropod predators, which weighed up to 7-13 tons. Of course, there were many species of medium and even small size. Small theropods weighed around 100-150 grams. Given this, the average weight of dinosaurs is estimated to have been around 50-100 kg, much larger than the average weight of Cenozoic mammals, which is 2-5 kg.

In the Cenozoic, the largest land mammals were indricotheres. These hornless rhinos could raise their heads to a height of 7 meters and weigh up to 17-20 tons. Elephants, slightly smaller in size, also demonstrated impressive dimensions. The steppe mammoth, also known as the Trogontherium elephant, stood 4.7 meters tall at the shoulders, weighed 10 tons, and had 5-meter-long tusks. Giant sloths could reach elephant-like sizes: Megatherium was 6 meters long and weighed 4 tons. Among predators, Andrewsarchus was a hoofed animal weighing a ton and with a body length of 4 meters without the tail.

There is a clear connection between the transition to a herbivorous diet and an increase in size among terrestrial animals. The vegetative parts of plants have low caloric value, and their effective digestion is impossible without the help of gut microflora. More food is required, and it takes longer to digest than meat or fruit. The digestive system becomes more complex and occupies more space in the body. This makes increasing size advantageous for herbivores. Another driving force is predation pressure, with predators increasing in size as their prey grows larger.

The next step toward extreme size is metabolism. Having low metabolism and gaining mass in the tens of tons is convenient in water; you don’t have to worry about supporting body weight and only expend energy on bodily functions and movement. On land, the situation changes drastically. The square-cube law becomes your worst enemy as size increases. The relative cross-sectional area of muscles decreases, making them weaker, which increases the load on the body. A large heart beats more slowly, reducing the speed of blood circulation and oxygen delivery to tissues. As a result, energy expenditure increases while metabolism decreases, closing the door to extreme sizes for cold-blooded animals.

However, there is a caveat: size helps conserve heat. Could dinosaurs have compensated for their cold-bloodedness with large size, accumulating heat and living in the consistently tropical climate of the Mesozoic? It’s not that simple. Indirect evidence suggests that a four-chambered heart and a high metabolism associated with it are basic characteristics for all archosaurs. In the last decade, studies of tooth enamel, eggshells, blood vessel density, and tissue growth rates have been conducted for various dinosaur groups, including sauropods. All indicate high body temperatures in the range of 26 to 40°C. There is a correlation between size and high temperature, but it doesn’t explain other things, like the rapid growth of sauropod juveniles or the high activity of smaller dinosaurs, especially feathered ones. Moreover, the climate in high latitudes during the Cretaceous was cool, even featuring snowy winters, yet local dinosaurs thrived.

The advantage of dinosaurs over mammals lies in their anatomy. Dinosaurs were long and lightweight. A savanna elephant, 7 meters long and nearly 4 meters tall, can weigh 7 tons—comparable to a large, massive Saurolophus or Tyrannosaurus rex, which had an average weight of 6-7 tons, with a length of 12 meters and a hip height of 4 meters. Sauropod leg bones were strong and thick, while their vertebrae and skull bones had cavities that significantly reduced their weight. An air sac system from the neck to the sacrum reduced the animal’s weight and allowed more air intake, maintaining high oxygen levels in the blood. These anatomical features were present in other dinosaurs on a smaller scale, but in sauropods, they reached their maximum development, allowing them to rival cetaceans in size.

It’s time to wrap up our exploration and return from the distant past to the present. The Holocene, the epoch we live in, is a provisional warm period in a series of ice ages over the last 2 million years. Essentially, we are witnessing the decline of terrestrial Pleistocene ecosystems, with the gradual diminishment and extinction of megafauna. Furthermore, we have been actively contributing to this crisis since the Stone Age. However, this crisis only affects terrestrial ecosystems. Marine megafauna, on the other hand, is thriving, though not without anthropogenic impacts.

Does this mean that animal gigantism has come to an end, and species larger than those we see today will never again appear on our planet? Not at all.

The Cenozoic has only lasted 65 million years. Yes, mammals and birds appeared back in the Jurassic period, but their true story began after the Cretaceous-Paleogene extinction. If we were to travel back to the Triassic period, who could have predicted that the descendants of small archosaurs or cynodonts would one day weigh tens of tons? There is no reason to believe that mammals and birds have exhausted their potential. Extreme size is one of the ways biological species specialize, leading to significant changes in anatomy and physiology. There is no single reason why some species grow to extreme sizes; in each case, it’s a complex interplay of factors, acting in different sequences and intensities. But these factors continue to operate, and so, in the future, our planet may once again be shaken by the footsteps of giants.