As every schoolchild knows, many dinosaurs were stupendously, spectacularly big. So big, in fact, that their size might have been their undoing, according to a new paper by the University of Chicago’s ace paleontologist, Paul Sereno.
When you live on a planet like Earth–prone to the occasional, catastrophic collision with an outsize meteor–it may help to be small in stature, Sereno wrote in an article in Friday’s issue of the research journal Science.
Sixty-five million years ago, when a meteor slammed into Earth, the strength and ferocity of the biggest, deadliest killers ever known–including the 50-foot-long, 6-ton tyrannosaurus rex–offered no protection from extinction. Nor did the size of the more placid dinosaurs that fed on plants.
It was not the big and strong that survived, but the small and adaptable–mammals no bigger than mice or rats.
The mammals may have survived because they had to scramble for 115 million years to stay out of the dinosaurs’ way. They had to find and adapt to niches in the ecosystem dinosaurs never bothered to occupy, such as underground, in the trees and in freshwater rivers and saltwater seas.
Sereno’s Science essay is not so much an announcement about a new discovery as it is a synthesis of the thinking of one of the top dinosaur scientists. In the end, he observes, evolution is not so much a plan as it is a series of accidents.
“We’re living in the golden age of paleontology right now,” he said in an interview. “Half of all the dinosaurs we know about have been found in just the last 30 years. Doing this article has been an opportunity for me to bring to a head a lot of the things I’ve been thinking about since I did my dissertation.”
Evolution has become the province of several new scientific disciplines, including molecular biology, which can genetically trace plants and animals back to their primitive ancestors.
Nonetheless, Sereno argues that the only way to see large-scale evolutionary patterns and events is to dig up hard, fossilized evidence out of the ground.
It was through the fossil record that scientists discovered the great extinction events that wiped the Earth’s slate clean of most life forms in the past, he said.
Extinction events not only wipe out some creatures, but also create opportunities for the rise of new species.
“What triggers or drives major replacements (of species) in the history of life?” Sereno asks in the Science article.
One mass extinction, about 245 million years ago, probably caused by giant meteors slamming into Earth, gave rise to dinosaurs. The other, caused by a similar collision 65 million years ago, killed the dinosaurs.
“How do novel and demanding functional capabilities, such as powered flight, first evolve?” Sereno asks in the essay.
Certainly not by any orderly progression of animals evolving as ever larger, more efficient and intelligent beings. The fossil record shows that evolutionary process is fraught with happenstance and accident, Sereno said.
For example, the first birds developed wings not for flight, but probably for sexual display, brooding their young or some other reason, and learned to use them to move through the air millions of years later.
The evolution of flight and of dinosaurs into birds is central to the lessons of evolution that dinosaurs have taught us, Sereno said.
In fact, some physiological attributes that would later be necessary for flight are present in the earliest dinosaur fossil ever found, eoraptor, which lived 225 million years ago and was discovered eight years ago by Sereno in South America. The discovery of the two-legged, 3-foot long, 25-pound carnivore put Sereno’s name on the map as a young comer in paleontology.
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Though the first dinosaurs probably appeared 5 million to 10 million years before eoraptor, paleontologists believe the earliest version were about the same size as eoraptor. That is fairly big for an animal, but dinosaurs of that size nonetheless survived an extraordinary extinction event 215 million years ago when Earth seems to have been plagued by giant meteor collisions and a worldwide eruption of volcanic activity.
At the time, dinosaurs were just one of many life forms on dry land, none of them dominant. They included amphibians of all sizes, mammal-like reptiles, lizards and armored crocodilians.
“It was a severe extinction that cleared the deck of almost all those other life forms,” Sereno said. “The dinosaurs were victors by accident. I can’t find any rhyme or reason as to why they survived. We still don’t know the precise story.”
With virtually no other land animals of consequence to compete with, the dinosaurs spread throughout the world. But they were not particularly adaptable. It took them 50 million years to evolve into the largest sizes they would ever attain. By contrast, the more adaptable mammals, which replaced dinosaurs in world dominance after the next mass extinction, reached maximum size within 2 million years.
Still, the dinosaurs had a good run. For 150 million years, every animal in the world 3 feet long or longer and living on dry land was a dinosaur.
Though mammals appeared about 50 million years after the first dinosaurs appeared, the mammals remained small in stature and number until the dinosaurs were gone.
Because they remained small and adaptable, however, the mammals survived the horrible meteor collision 65 million years ago. That also may explain how a single line of dinosaurs–birds–also survived to thrive in modern times.
“Birds are interesting,” Sereno said, “because they are the one line of dinosaurs that reversed the evolutionary process. Instead of getting bigger, they started out as fairly large, two-legged carnivores and kept getting smaller and lighter to allow them to fly, until they were down to the size of wrens.
“As they got smaller, they took up niches in the environment other dinosaurs ignored–in trees and holes in the ground. Some of them even learned how to swim and dive under water. They became adaptable as they got smaller.
“You’re more flexible as a half-pint. It’s a lot easier to change your lifestyle at a small body size than it is at a larger body size. Ecologists have long known that body size . . . influences lifestyle, reproduction, physiology, the way you look at the world, what you can possibly fit in your mouth and decide to eat. It’s just profound.”
Surprisingly, Sereno said, the potential for dinosaurs becoming birds is obvious even in the very earliest dinosaur fossil–the eoraptor he found.
“Eoraptor had hollow bones right down to its toes,” he said, “something that would be necessary for bird evolution, an anatomical design that eventually make them light enough to fly. The way their feet were formed, one toe was even lifted off the ground, which eventually would be the opposable claw that would enable them to perch in trees.
“The first birds don’t show up for many millions of years after eoraptor, but all that bird anatomy is present in eoraptor’s fossils. That still amazes me.”