You can call the sauropod dinosaurs fat and perhaps you can call them stupid, but you can’t call them failures, and for that reason they have a lot to teach us about evolution.
Those small-headed giants thrived on all continents for more than 120 million years, through the entire age of the dinosaurs. By contrast, you could say that humanlike creatures have been around for 1 million years, and our own species much less than that.
Sauropods are interesting to biologists because they were so successful and yet they represent an evolutionary experiment that was never repeated: No land animal ever got so big before or since.
Scientists estimate that the largest known sauropod’s bones belonged to animals that weighed about 80 tons. Length is harder to estimate, the scientists say, but some stretched at least 100 feet from head to tail.
University of Pennsylvania paleontologist Peter Dodson said there was only one mammal that ever rivaled it for size, a giant rhinoceros that weighed about 20 tons. But that animal died out relatively quickly and became an evolutionary dead end.
Most people don’t appreciate the sheer size of the sauropods because these creatures are so hard to reconstruct and their skeletons wouldn’t fit into most human-scale buildings, said Don Lessem, a local collector and dinosaur enthusiast who has put together a new exhibit, opening Sunday at the Franklin Institute.
Lessem said this exhibit will be based on the latest dinosaur science but will be accessible to children, dealing with such burning questions as what kind of giant cowpies, or sauro-pies?, littered the Jurassic forest floor.
Why did they grow so big? Martin Sander, a paleontologist at the University of Bonn, said the main reason that animals grow to any particular size is to fill a niche. For sauropods, there was room at the top.
By being big, sauropods could eat leaves and branches too high for other creatures to reach, said Sander, who is acting as an adviser to the exhibit. And with predators such as T. rex around, size probably helped their chances of survival.
As a general rule, big herbivorous animals grow about 10 times as massive as the largest predators. Elephants are about 10 times the mass of lions and sauropods were about 10 times the size of T. rex.
Sauropods could theoretically have grown to about 150 tons before they would collapse under their own weight, Sander said. Some of the biggest ones may have approached that size. It’s unlikely that the few samples that have been dug up represent the record holders, he said.
But even if it’s advantageous to be big, it’s not easy to get there. Sander said sauropods were gifted with an innovative, super-efficient system for eating. The sauropods had proportionally long necks and tiny heads with primitive teeth. “The head did nothing but eat,” Sander said. It simply snipped off foliage without chewing, the whole leaves and twigs getting broken down in the belly.
The favorite analogy scientists use for this system is a vacuum cleaner.
Animals that evolved to chew food had a problem that limited their size, Sander said. The bigger they got, the more food they needed, and the bigger, proportionally, their head and jaws needed to be. That’s why elephants and rhinos have relatively big heads, he said.
As a chewing animal, then, you can only get so big before your head becomes too difficult to hold up.
And herein lies an important lesson about evolution. Bigger elephants or rhinos might do better than smaller ones, said Sander, but they can’t go back and lose the teeth once they’re on that track. There’s no easy evolutionary path back to the vacuum cleaner system of the sauropods.
“Generally you think of evolution as optimization,” he said. “But it can only work with what’s there to start with and sometimes you can get into an evolutionary rut.” So we chewers are stuck with our teeth.
Another size-limiting problem for most mammals is live birth, which limits the number of offspring any given female can bear. That can make populations more vulnerable to crashing when predators increase or food decreases. Dinosaur females could lay dozens of eggs a year. The hatchlings were only about 5 pounds, and the majority perished, but a few lucky ones survived to adulthood, when they were so big even T. Rex wouldn’t mess with them, as the predator was more likely to get hurt than get dinner.
It’s not easy to tell how fast they grew, but their bones did show some imperfect annual rings that can estimate their ages, Sander said, and from that he calculated that they took about 38 years to grow up. That might sound slow in human terms, but this is fast considering they went from 5 pounds to perhaps 100,000 pounds.
There’s also some evidence in the fossils that sauropods had excellent lungs, said paleontologist Thomas Holtz of the University of Maryland. Sauropods had the same type of lungs seen in birds, which are rigid and require air sacs that allow fresh air in every time the animal inhales and exhales.
Egg laying, birdlike lungs, fast growth, and the no-chewing-required eating system all evolved first in smaller ancestors of the sauropods, Holtz said, and the combination is one that proved ideal for enabling them to become supersized. Other animals lack the cardiovascular, metabolic, or reproductive makeup to get that big.
It was a winning system that lost out to bad luck when an asteroid wiped out the dinosaurs 65 million years ago. Worldwide expeditions have refuted the idea that sauropods were in decline before the impact, which put an abrupt end to the Cretaceous Period. Now it appears they disappeared only in North America, Holtz said. “The rest of the world in the Late Cretaceous remained a sauropod world.”
If that space rock had taken a slightly different trajectory, Earth might be the planet of the sauropods still.