How Reptiles Took Over Land and Sea – An Interview with Dr. Caleb Gordon

As mammals, we like to think of ourselves as dominant animals, and to some extent, that is true. Humans, specifically, have altered nearly every part of our planet and reshaped the climate. We’ve domesticated cattle and chickens for food and bred cats and dogs for pets. We exterminate countless other species across the tree of life. Yet, we and our mammalian relatives were not always in control, and throughout life’s history, our influence has ebbed and flowed. 

Dr. Caleb Gordon, a paleontologist in the Earth and Planetary Sciences Department at Yale University, spent several years unraveling a dynamic tale that began 300 million years ago. At that time, “the biggest animals on land were our ancestors—the sprawling, sharp-toothed cousins of mammals, and the biggest animals in the oceans were giant armored or fleshy-finned fishes,” Caleb speaks enthusiastically. “The earliest reptiles appeared around this time too,” he introduces them with a smile. “Reptiles are a big, quirky family with lizards, crocodiles, turtles, dinosaurs, and birds. They include many other species, both living and extinct, that descended from an ancient common ancestor. Reptiles were diminutive little creatures in a world dominated by bigger, scarier animals who probably ate them whenever they found them.” On land, reptiles lived in the shadows of large mammal ancestors, eating insects and other small prey.  They were not thriving in the oceans either, where various big fish were the top predators. 

Then, some 50 million years later, a sudden surge of volcanic activity and environmental upheaval transformed the landscape and the plot. “The Great Dying, we call it, triggered the worst mass extinction in the history of animal life, around 252 million years ago,” Caleb explains in a more serious tone. “Up to 95% of species were wiped out, and almost anything bigger than a modern-day deer – gone.” But reptiles were lucky. In the aftermath, they thrived, both in aquatic and terrestrial arenas. In their “glow-up era – the Age of the Reptiles,” they became the new top predators on land and in the oceans.

Back in the present day, I sit in a local New Haven coffee shop with Caleb as he draws an evolutionary tree in his notebook. He shows me when reptiles and mammals first evolved and how they have competed throughout Earth’s history. “It’s hard for many of us, at least here in the northeast U.S., to imagine encountering big reptiles today. If we hike through a national park or drive through our local neighborhood, most of the big animals we’ll see are mammals — deer, cows, bears, dogs, wolves, coyotes, and especially humans. And if we swim in the cold coastal waters, most of the big animals we’ll see are sharks, bony fish, or whales,” he says. For his dissertation, Caleb explored a different landscape. He wanted to understand how, in the aftermath of the Great Dying, reptiles managed to win both on land and in sea as the Age of Reptiles began. 

“There is an underappreciated role for the evolutionary innovations that helped reptiles win,” he explains. By evolutionary innovations Caleb means “complex features they evolved that allowed them to do new things and invade new environments after the Great Dying.” Such traits appear randomly, and on rare occasions, favoured their survival in certain circumstances. Little by little, through small changes over many years and generations, these beneficial traits became long-lasting innovations in reptiles.

To transition to water and survive the rough seas, reptiles needed special evolutionary innovations. “Marine reptiles were, over the course of millions of years, able to change the dimensions of their arms to create their own flippers. They did this by making their hands much longer while shortening their lower arm bones.” Caleb demonstrates by shrugging his shoulders close to his ear, squeezing his arms by his body, while extending and flapping his hands out. “Reptiles were able to do this quickly and easily, a bunch of times, in a bunch of different groups. But in mammals and their ancestors, flippers didn’t appear for another 200 million years.” At this broad time scale, mammals were worse at making flippers, and it rarely happened. While flippers evolved some eight to ten times in reptiles, they only evolved four times in mammals. In part because of their flippers, some aquatic reptiles evolved to become massive creatures that could now prey upon the biggest sharks.

To thrive on land, one group of reptiles managed to shape their skulls to catch big prey,” Caleb says with a smile, “imagine, a T. rex, the biggest meat-eating animal that ever walked the Earth, or its relative, the modern-day saltwater crocodile, which has one of the strongest bites among all living animals today.” To become terrifying carnivores, terrestrial reptiles needed to open their jaws wide, snap it shut quickly, and bite down hard, destroying their prey before it escaped. Caleb demonstrates this by cupping his hands open and shut like a clamshell. But nature gives and takes, and powerful jaws tend not to open as wide or bite down as fast, while nimble and lightweight jaws open wide, but usually fail to bite down as hard. Reptiles managed to overcome these limitations, Caleb explains, by “changing the architecture of their jaw muscles.”  Shortly before the Great Dying, some transformed their tiny deep jaw muscles into massive ones that could open wider while maintaining a powerful bite. These reptiles were the ancestors of dinosaurs. They ate bigger prey, became fearsome land predators, and reached a new level of carnivory.  

Reconstructing such details of a distant past is not easy. “Studying the evolution of ancient reptiles is tricky. We rely on scarce fossil records they left behind, and the vast majority of them left no trace at all.” To make these discoveries, Caleb does some of the classic paleontology work you may envision from the movies. He sifts through ancient bones at the Yale Peabody Museum and precisely records lengths, widths, heights, and depths. With these, he built the largest dataset of its kind: around 16,000 measurements from over 800 museum specimens, representing both living and extinct species, across hundreds of millions of years of evolution. 

“But these classic methods can only get you so far. To understand all of these fossils, and all this information, we need to use modern techniques and technologies.” He feeds his masses of data into a machine-learning algorithm to compare competing stories of whether extinct species occupied land, water, or both, and predict which scenario was most likely. Given the nature of the data, Caleb had to introduce a statistical method first devised by the US Navy, and almost never used to study fossils. “We also adapt 3D computer animation programs, the same software used by Pixar and Marvel Studios, to ‘repair’ and ‘un-crush’ fossils so that they look more like they would have when the animal was alive,” he explains. These approaches let us recraft ancient structures that are unseen in modern life. With these methods, Caleb can tell which past aquatic and terrestrial reptile stories are likely true. 

Caleb’s work recreates scenes from prehistoric times of life forms fighting for their existence. His PhD advisor, Dr. Bhart-Anjan Bhullar, emphasizes, “Caleb’s research represents a series of fundamental advances in our understanding of the lives of ancient organisms.” These ancient organisms, the diverse family of reptiles, flew, crawled, swam, and roamed. Through luck and remarkable evolutionary innovations, they managed to conquer two vastly distinct environments and replace our ancestors at the top of the global food chain…at least for a while. 

This work was supported by training from Carl Zimmer, and feedback from Zili Shen at the Yale University Graduate Writing Lab.

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