Wait! Why? How Did You Get So Big?

Sauropod’s interconnected evolutionary cascades collectively provided selective advantages and feedback loops that led to their enormous sizes

Christmas 2008–2009 was quite special. I was dating a guy from my hometown, and everybody agreed we were made for each other. He was a techy computer scientist, and I was a nerdy biology undergraduate student. We were both hard workers and daydreamers.

But most important, he knew me well. He knew how much scientific research meant to me; he was starting to see my passion for paleontology grow every day.

It was a very special year for paleontology and evolution, too: Darwin’s 200th birthday and the 150th anniversary of his revolutionary best-seller “On The Origin of Species.” The Natural History Museum in London couldn’t miss the date, so they had a special exhibit that they started advertising early. I daydreamed of attending!

So, on Christmas day, my special boyfriend got us flights, accommodation, and tickets to the special event. We were traveling for my birthday, which was also days away from Darwin’s actual birthday. It couldn’t have been more perfect.

Fast-forward to February, when we were ready to visit the iconic Natural History Museum in London. I had read so much about it that I was ecstatic, and the visit didn’t disappoint.

However, I’m not going to lie. Amongst all the objects on display, nothing surprised me as much as Dippy the Diplodocus. This ginormous dinosaur took over the entire hall. It was magnificent.

And, of course, the wannabe paleontologist in me wanted to know more about this creature. How and why did it become so big?

Fortunately, over the years, I have been fortunate to explore many questions surrounding the rise and fall of the dinosaurs or the evolution of body mass in terrestrial species. Today, I’ll explore this question with you, too!

Sauropod dinosaurs were colossal giants that roamed our planet from the late Triassic to the end of the Cretaceous (from 163.5 to 66 million years ago). With their long necks, tiny heads, and massive bodies, these herbivorous behemoths include well-known species like the Natural History Museum’s Diplodocus and Brachiosaurus. Some reached lengths of up to 26 meters and weighed as much as 46,000 kilograms. In his 2013 paper, Dr. P. Martin Sander offers a comprehensive explanation through what he calls the “evolutionary cascade model” (ECM).

No worries, I’ll explain it in easy terms. Sander’s ECM breaks down the gigantism of sauropods into five interconnected evolutionary cascades: Reproduction, Feeding, Head and Neck, Avian-style Respiration, and Metabolism.

Each cascade starts with certain basal traits (traits gained by ancestors of Sauropod dinosaurs), which can be either primitive or derived and progresses through a series of evolutionary changes that provide selective advantages.

These cascades create feedback loops, where the advantages of one trait influence the development of others, leading to the enormous body sizes we associate with sauropods. Follow the below; the idea is pure genius.

Reproduction

One key trait of sauropods is their reproductive strategy. Unlike many modern large animals, sauropods laid many small eggs. Yup, you heard that right. These gigantic creatures laid very small eggs relative to their size.

This reproduction method, known as R-selection, allowed them to produce large numbers of offspring with relatively low parental investment. The number of eggs increased the chances of survival for at least some juveniles, despite predation and other environmental challenges.

However, the small size of the eggs relative to the adults also meant that young sauropods had to grow rapidly. This rapid growth rate was crucial for reaching sizes that protected them from predators. By growing quickly, juveniles could transition through vulnerable stages more swiftly and start taking advantage of the protective benefits of large size.

Feeding

Sauropods had unique feeding strategies that contributed to their gigantism. Their long necks allowed them to browse large areas without moving their massive bodies. This energy-efficient feeding strategy reduced the need for locomotion, conserving energy for growth and other vital functions. Genius!

Interestingly, sauropods did not chew their food (a strategy almost reserved to herbivore mammals). Instead, they gulped down large quantities of vegetation, which was broken down in their enormous digestive systems. This method of feeding, combined with their ability to process large amounts of plant material, supported their high metabolic needs and contributed to their rapid growth.

Head and Neck

The sauropod’s long neck is perhaps their most distinctive feature. This adaptation provided several benefits. As we’ve seen above, a long neck allowed sauropods to feed from a wide area while remaining stationary, saving energy.

Though debated, the flexibility of their necks likely varied among species, with some able to move their necks in a wide range of motion while others were more restricted.

Additionally, sauropods had proportionally small heads, which minimized the weight at the end of their long necks. This small head size required less muscle and bone support, making it easier to maintain a long neck without compromising stability or mobility. As you are provably beginning to see, lots of independent features had to evolve in the right direction.

Avian-style Respiration

This will sound like science fiction, so bear with me: One of the most fascinating aspects of sauropod biology is their avian-style respiratory system.

Unlike mammals, which have bidirectional lung airflow, sauropods (like modern birds) have a unidirectional airflow through their lungs.

This highly efficient system allows for greater oxygen exchange and supports high metabolic rates. It also helped lighten their bodies, as the air sacs involved in this type of respiration created pneumatic (air-filled) spaces in their bones, reducing overall weight.

In other words, where biology and engineering meet.

Metabolism

Yes, I know some of you may be tired of reading this word, but trust me, it’s also important here.

Sauropods likely had high basal metabolic rates (BMR), which supported their rapid growth and large body sizes. A high BMR means that an organism needs to consume more energy, and sauropods were well-equipped to meet this demand with their efficient feeding strategies and large digestive systems.

Combining a high BMR and an efficient respiratory system allowed sauropods to maintain their massive sizes and active lifestyles.

Putting It All Together

Sander’s evolutionary cascade model offers a holistic view of how sauropods may have achieved and maintained their gigantism, something that most of us, scientists and science nerds alike, may have wondered about from time to time.

Each cascade, from reproduction to metabolism, played a crucial role, and their interactions created a powerful feedback system that propelled sauropods to sizes unmatched by any other land animals we’ll talk about aquatics another day).

This model not only helps explain sauropod gigantism but also highlights the complex interplay of evolutionary adaptations that allowed these giants to dominate their ecosystems for millions of years.

As paleontologists uncover more fossils and apply new and more sophisticated technologies, our understanding of sauropod biology will only deepen, revealing even more about these incredible creatures. But for now, my undergraduate self is happy that I’ve managed to find these answers and communicate them to you.

Stay curious, my friends! (and watch the video below if you are hungry for more!)