Why did dinosaurs get so big? Is there an upper limit?

Editor’s Note:

"Why were dinosaurs so big?" is a question that scientists have been discussing for a long time, and it is also considered one of the most challenging scientific frontier issues currently.

Part 1

How big was the largest dinosaur on Earth?

The largest land animal on Earth today is the African elephant (Loxodonta africana). This giant has an average shoulder height of 3.2 meters and weighs 6 tons. When we stand in front of them, we always feel that we are so small. When the African elephant stands in front of the largest dinosaur, the African elephant will feel that it is so small.

Comparison of elephant size and human size Image source: wikipedia-Steveoc 86

The largest dinosaurs all come from the sauropod family. So who is the largest known dinosaur? It is Argentinosaurus. Because the fossils found are incomplete, there has been controversy about its size. According to an estimate by Gregory Paul in 2019, Argentinosaurus was more than 30 meters long and weighed between 65 and 75 tons [1].

Many people have no idea how huge the Argentinosaurus is. Let's take a look at its length and height. If we calculate it based on its length of 35 meters, the Argentinosaurus is longer than the main model of the civil aviation industry, the Boeing 737-300, which has a fuselage length of only 28.6 meters. When the Argentinosaurus raised its head, the height from the ground was more than 10 meters, which is equivalent to the height of a 3-story building.

Size comparison between Argentinosaurus and humans. Image source: SlateWeasel

So are there any dinosaurs that are even bigger than Argentinosaurus? Possibly, they are Barosaurus and Supersaurus.

Let's first look at Barosaurus. Barosaurus belongs to the Diplodocidae family and looks very similar to Diplodocus. The characteristic of Diplodocus is that it is slender, that is, it is large in size but light in weight. Therefore, traditional theory holds that although Barosaurus is 25 meters long, it weighs only 12 to 15 tons. Whether in terms of body length or weight, Barosaurus is not huge in the sauropod family, but in 2016, paleontologist Michael P. Taylor proposed an amazing theory, that is, Barosaurus is very big!

It turns out that Michael Taylor noticed that a vertebra identified as Supersaurus (number: BYU 9024) was likely to belong to Barosaurus. According to the size of this cervical vertebra, this Barosaurus could be up to 48 meters long and weigh about 66 tons [2]. Of course, this estimate has not been unanimously recognized by the paleontological community, so the 48-meter-long Barosaurus is just a theory.

BYU9024 and its replica dimensions Image source: svpow.com

After talking about Barosaurus, let's talk about Supersaurus. Like Barosaurus, Supersaurus belongs to the Diplodocidae family. Its fossils were very shocking when they were discovered. Just a shoulder blade (number: BYU 9025) is 2.4 meters high. When Supersaurus was first named, paleontologists believed that its body length was between 40 and 45 meters, but later it shrunk to 35 meters.

Superosaurus fossil in the museum. Visitors in the lower right corner can use it as a reference for size. Image credit: Zach Tirrell from Plymouth, USA

In November 2021, the annual meeting of the North American Society of Vertebrate Paleontology was held. At the meeting, paleontologists re-proposed their research on Supersaurus, pointing out that even with a conservative estimate, Supersaurus could reach a length of 39 meters, while a normal body length could reach 42 meters [3]. The research on Supersaurus has been recognized by the academic community, so it has become the longest dinosaur discovered so far, but not the heaviest. Because it belongs to the slender diplodocid family, even with a length of more than 40 meters, Supersaurus' weight is still limited to about 40 tons, which is almost half of that of Argentinosaurus!

Image source: Reference [4]

Many people will ask, shouldn’t the largest dinosaur be Amphicoelias fraillimus? Amphicoelias fraillimus is estimated to be over 60 meters long and weigh 120 tons. It was once a mythical super dragon! The debate over whether Amphicoelias fraillimus existed has lasted for more than a century, but in 2018, paleontologist Kenneth Carpenter proposed an astonishing theory that the fossils of Amphicoelias fraillimus actually belong to the Rebachisaurus, and renamed it Maraapunisaurus [5].

Size comparison between Diplodocus fragilis and humans, image source: Wikipedia

According to this study, Diplodocus fragilis did not exist, and Megalosaurus was between 28 and 32 meters long and weighed 62 tons, making it completely out of the running for the title of the largest dinosaur. At the same time, the myth of Diplodocus fragilis was completely shattered.

Size comparison between Diplodocus Fragile (black) and Megalosaurus (green). Image source: Reference [7]

Size comparison between Argentinosaurus (black) and Megalosaurus (red). Image source: Franoys

The above introduces some representative giant sauropod dinosaurs. The largest dinosaur known to humans is still the Argentinosaurus. With a length of 30 to 35 meters and a weight of 65 to 75 tons, it sits firmly on the throne of the largest known dinosaur. Its size represents the ceiling of dinosaur size, that is, the upper limit of dinosaur size!

Part 2

Why did dinosaurs grow so large?

As a terrestrial vertebrate, the largest extant terrestrial species among our mammals is the aforementioned African elephant. Even if we look at all the known terrestrial mammals, the largest one, Paraceratherium Linxiaense, was 8 meters long, 5 meters tall at the shoulder, and weighed 24 tons, which is only the level of a medium-sized sauropod in the dinosaur family [6].

Height comparison between the giant rhinoceros and the author himself

So why were dinosaurs so big?

Here we need to mention the famous Cope's rule, which was observed by the famous American paleontologist Edward Drinker Cope (but not formally proposed by him). Based on the information in the fossil record, he believed that the size of species in the population lineage generally became larger and larger, that is, organisms have a tendency to become larger. However, according to today's research, Cope's rule does not apply to the evolution of all species, but only conforms to the trend of some biological groups.

The traditional view is that the reason why dinosaurs could grow so huge is inseparable from the atmospheric composition, temperature, food composition and other factors of the era in which they lived. To put it bluntly, it was the environment that made dinosaurs so huge, which is also the generally accepted explanation.

In the view of paleontologists, an important issue that limits the growth of animals is energy consumption. How did giant sauropod dinosaurs keep their energy intake and consumption equal? ​​Paleontologists have found that there were abundant C3 plants in the Mesozoic Era, which were the staple food of sauropods. The energy value of such plants is similar to that of today's herbs.

Digestive biology: Today's herbivores, such as the cattle, horses, sheep and other animals we are familiar with, have developed chewing ability and digestive system, which indeed improves the efficiency of food utilization, but limits the efficiency of eating, so it also limits the continued growth in size. Sauropod dinosaurs are different. They can hardly chew food (this is also the reason why sauropods have small heads), so they spend more time swallowing. A large amount of food will be slowly digested in their huge stomach and intestines, and the huge digestive system requires a huge body to carry it.

Because of the huge amount of food they eat, even if sauropods stop eating, their digestive systems continue to work and obtain energy, so these big guys can continue to move in a low-energy state.

Another advantage of sauropods in eating is their extremely long necks, which ensure that they can eat over a larger area without expending more energy on movement, thus saving a lot of energy during long periods of eating.

Respiratory system: Sauropods have air sac structures like today's birds. In the respiratory system of sauropods, there are not only lungs, but also a large number of air sacs throughout the neck and body. The presence of a large number of air sacs can greatly improve the efficiency of breathing and increase the absorption of oxygen. Therefore, in the Mesozoic Era, when the oxygen content in the atmosphere was not much higher than today, sauropods were able to obtain more oxygen with their efficient respiratory system to meet their own survival needs.

In addition to improving breathing efficiency, the presence of air sacs can also help the body dissipate heat, because body temperature is also an important issue for large animals. Excessive body temperature can cause functional disorders or even death of the organism. Relying on the air sac structure, sauropod dinosaurs can effectively control body temperature and avoid overheating.

Cardiovascular system: The huge size of sauropods necessitates a strong heart to supply blood to the body, and their powerful four-chambered hearts are fully capable of this task. At the same time, during the evolution process, the blood vessel walls of sauropods became thicker, the permeability of capillaries decreased, the connective tissue continued to develop, the muscle venous pump was strengthened, etc., all of which ensured the circulation of blood in the huge body.

Growth strategy: Despite the huge size of sauropods, they were very small when they just hatched from their eggs, about 1 meter long and weighing only 1 kilogram. In order to grow up faster, young sauropods must grow faster, and they also have a high basal metabolic rate that is different from that of adult dinosaurs. So from birth, young sauropods maintain an astonishing growth rate, and their annual weight gain is calculated in tons. While maintaining a high metabolic rate and a high growth rate, sauropods also delay their maturity age. Studies have shown that Diplodocus will become an adult after the age of 20, which is longer than us humans. This gives sauropods a longer period of rapid development, ensuring that they have more time to grow rapidly.

When sauropods reach adulthood, their high basal metabolic rate turns into a low basal metabolic rate, and they enter a low energy consumption state. Adult sauropods do not stop growing completely, their size will continue to grow slowly until they die, so the older the individual, the larger the size!

A study of sauropod fossils shows that their bone cells had a larger surface area, which also facilitated the rapid growth of sauropod dinosaurs.

Reproduction strategy: We have always thought that dinosaurs were animals that gave birth but did not care about their offspring, but more and more fossil evidence shows that many dinosaurs not only guarded their nests, but also raised their newly born baby dinosaurs. However, the giant sauropod dinosaurs obviously did not have this behavior, so their reproduction costs were lower.

In sharp contrast to the low breeding cost is the higher reproductive capacity of sauropod dinosaurs. Like turtles, they lay many eggs at a time and then rely on numbers to quickly restore the population. This reproductive strategy is called: R strategy.

Being able to lay more eggs at a time requires the dinosaur to be larger, and not having to expend energy on building nests and raising young offsets possible limitations on size. After all, it is impossible for a 30-meter dinosaur to build nests and raise young.

Some external factors: In addition to the structure and behavior of sauropods themselves, pressure from theropod predators, competition between species with similar ecological niches, and intraspecific competition may all be the reasons that led to the gigantic growth of sauropods.

Among other environmental factors, some sudden, large-scale events also contributed to the rise of large, advanced sauropods, such as the extinction event that occurred 183 million years ago:

183 million years ago, during the Prynthian-Toarcian stage of the Early Jurassic, a series of large-scale volcanic eruptions were caused by the breakup of the Gondwana continent. The volcanic eruptions seriously disturbed the Earth's climate at that time, causing rising temperatures, a dry climate, and a dramatic change in vegetation composition. Common low and soft plants such as seed ferns and cycads disappeared, and tall gymnosperms began to occupy the land. The change in vegetation structure was not good news for the primitive sauropod dinosaurs, because they could not bite the hard needles and their gastrointestinal tract was difficult to digest. However, the true sauropods, which had stronger jaws and harder teeth, perfectly adapted to the world occupied by gymnosperms and became the new elites in the new world!

This extinction event that occurred 183 million years ago is called the Early Jurassic extinction event. Although the scale of the extinction was not large, it changed the composition of the terrestrial ecology and created an opportunity for the rise of true sauropods. The extinction event that occurred in the Early Jurassic also overturned our understanding of extinction events. The traditional view that large animals are more likely to be impacted and disappear in extinction events does not apply here. Instead, large animals adapted to the changed environment in the extinction event and became the biggest winners on the edge of extinction, opening the curtain for giant sauropod dinosaurs to roam the earth.

There is obviously more than one reason why dinosaurs were able to grow so large. Many reasons both promoted and restricted the enlargement of dinosaurs. When sauropod dinosaurs broke through these restrictions, they would step by step impact the pinnacle of the largest land animals and eventually win the crown!

In the future, paleontologists will continue to explore why dinosaurs were so large. Eventually, a comprehensive model integrating paleontology and paleoenvironment will be established, and this model will reveal the mystery of the large-scale growth of the largest terrestrial animals in prehistoric times!

References:

1. Paul, GS (2019). "Determining the largest known land animal: A critical comparison of differing methods for restoring the volume and mass ofextinct animals" (PDF). Annals of the Carnegie Museum. 85 (4): 335–358.doi:10.2992/007.085.0403. S2CID 210840060.

2. "The size of the BYU 9024 animal". June 16, 2019.

3.Curtice, Brian (2021). "New Dry Mesa Dinosaur QuarrySupersaurus vivianae (Jensen 1985) axial elements provide additional insight into its phylogenetic relationships and size, suggesting an animal thatexceeded 39 meters in length

4. Lovelace, David & Hartman, Scott & Wahl, William. (2008). Morphology of a specimen of Supersaurus (Dinosauria, Sauropoda) from theMorrison Formation of Wyoming, and a re-evaluation of diplodocid phylogeny. Arquivos do Museu Nacional, Rio de Janeiro. 65.

5. Carpenter, Kenneth (2018). "Maraapunisaurus fragillimus, NG(formerly Amphicoelias fragillimus), a basal Rebbachisaurid from the MorrisonFormation (Upper Jurassic) of Colorado". Geology of the IntermountainWest. 5: 227–244.

6. Deng T, Lu X, Wang S, et al. An Oligocene giant rhino providesinsights into Paraceratherium evolution[J]. Communications Biology, 2021, 4(1):1-10.

7. Carpenter, K. (2018). "Maraapunisaurus fragillimus, NG(formerly Amphicoelias fragillimus), a basal Rebbachisaurid from the MorrisonFormation (Upper Jurassic) of Colorado". Geology of the IntermountainWest. 5: 227–244. doi:10.31711/giw.v5i0.28. Archived from the original on October 22, 2018. Retrieved October 21, 2018.

Produced by: China Science Expo×Zhihu

Author: Jiang Hong, an excellent answerer of paleontology on Zhihu

The article only represents the author's views and does not represent the position of China Science Expo

This article was first published in China Science Expo (kepubolan)

Please indicate the source of the public account when reprinting

China Science Expo

Please indicate the source of the reprint. Reprinting without authorization is prohibited.

For reprint authorization, cooperation, and submission matters, please contact [email protected]