In order to consume all that food, though, sauropods had to reach it. Long necks were a critical, early adaptation that allowed sauropods to attain large body sizes, according to a recent review by Martin Sander and 15 other scientists. Think of an Apatosaurus standing at the edge of a prehistoric forest. The dinosaur’s long neck would allow it to reach a wide swath of vegetation—high and low, left and right—without moving its body at all. From early on in sauropod evolution, long necks made these dinosaurs efficient feeders able to reach resources that were inaccessible to other herbivores, and even with tiny heads, big sauropods would have easily been able to vacuum up huge quantities of food.
Just how these dinosaurs converted all this green food into energy and tissue is a trickier matter. Sauropods did not have robust batteries of molars to chew their food. Many had only a few pencil- or spoon-shaped teeth to pluck food before swallowing it whole. Given sauropods’ poor table manners, scientists used to think that the dinosaurs might have swallowed stones to grind up food still in the stomach the way some birds do. Paleontologists Oliver Wings and Martin Sander have argued that this probably wasn’t the case—so-called “stomach stones” found with some sauropod fossils do not show a pattern of wear consistent with what would be expected if they were being used this way. Instead, the dinosaurs extracted as much nutrition as possible from their food by retaining it for long periods in their digestive systems.
A few details of sauropod digestion were experimentally modeled by Jürgen Hummel and colleagues in 2008. The scientists placed modern-day samples of the most abundant sauropod chow from the Mesozoic—ferns, horsetails, ginkgoes and conifers—in simple artificial stomachs. They inoculated the fake guts with microbes taken from the part of sheeps’ digestive systems where plant food is initially broken down. As the plants fermented, the scientists tracked how much nutrition they released.
Contrary to what had been assumed, many of these plants degraded relatively easily in the crude stomach environments. Horsetails and monkey puzzles were especially nutritious. Actual dinosaur stomachs might have been even better equipped at breaking down these plants, and there was certainly enough available energy in the plants of the time for sauropods to grow large. Sauropods probably did not require extraordinary gut architecture to survive.
Another major feature allowed these titans to balloon in size. It is a trait they share with birds. Birds are the direct descendants of small theropod dinosaurs related to species like Velociraptor and Anchiornis, but they are not very closely related to sauropod dinosaurs; they last shared a common ancestor more than 230 million years ago. Even so, both the theropod and sauropod lineages shared a peculiar trait that was extremely important in their evolution—a network of internal air sacs connected to the lungs.
The soft air sacs haven’t been seen directly in the fossil record, but the structures left telltale pockets where they invaded bones. Naturalists recognized the indentations more than a century ago, but modern paleontologists are only just beginning to understand their significance. As in birds, the lungs of sauropods were probably connected to a series of air sacs, and attached to these organs was a network of smaller pockets—called diverticula—that infiltrated the bones in the neck, chest and abdomen of the dinosaurs. From a structural point of view, this network of air-filled structures lowered the density of the sauropod skeleton, and allowed these dinosaurs to have a relatively lightweight construction for their size. Rather than having extra-strength bones, as had once been suggested, sauropod skeletons were made lighter by a trait they share with birds, and the network of air sacs probably had other benefits, too.
In birds, air sacs are part of a flow-through breathing arrangement that is far more efficient at extracting oxygen than is the respiratory system of mammals. We don’t yet know if sauropods breathed the same way birds did—the degree to which their skeletons were modified by air sacs varied across species—but it is likely that the air sacs of the giant dinosaurs were better equipped at delivering oxygen to their bodies than the alternative seen in giant mammals. Birds have a high metabolic rate that requires a great deal of oxygen for sustained flying; similarly, the size and active lives of sauropods would have required a great deal of oxygen, and the air sac system would have provided them with essential breathing benefits.
Not all sauropod dinosaurs were giants. Some species—such as Magyarosaurus from the strata of Romania—were small descendants of much larger species. They shrunk in size because of their isolation on islands, though the exact reason why such island dwarfs evolve is debated by scientists. Still, sauropods weighing more than 40 tons evolved independently in at least four lineages during the long tenure of this dinosaur group, all thanks to a suite of characteristics that made large body size possible.
Paleontologists are still investigating the evolutionary pressures that made such large forms advantageous. Their size gave them some protection from predators, presumably, and their long necks let them reach food that smaller creatures looked at hungrily but couldn’t reach. What other advantages giant size might have provided remain unclear. Nevertheless, sauropods were astounding creatures that could only have existed thanks to a peculiar confluence of events. They were fantastic forms unlike anything that came before or has evolved since.