They were the most gigantic animals ever to walk the earth. Sauropod dinosaurs—“thin at one end; much, much thicker in the middle; and then thin again at the far end,” as comedian John Cleese described them—were titans that thrived for more than 130 million years. The largest known species, such as Argentinosaurus and Futalognkosaurus from prehistoric South America, stretched more than 100 feet long and weighed in excess of 70 tons. Bones found in the 1870s (and since somehow lost) hint that an enigmatic species dubbed Amphicoelias may have been even bigger still.
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No land mammal has ever come close to the size of these gargantuan dinosaurs. The prehistoric hornless rhino Paraceratherium—the largest land mammal ever—was a mere 40 feet long and weighed a paltry 17 tons, and today’s African bush elephants, at 5 tons, would look dainty next to the largest sauropod dinosaurs. (Blue whales, at 100 feet and 200 tons, are a bit more massive than sauropods, but it’s easier, physiologically, to be large in an aquatic environment.)
What was it about these dinosaurs that allowed them to become the biggest terrestrial animals of all time? Paleontologists have been puzzling over the question for more than a century. Even relatively modest-sized giants such as Apatosaurus and Diplodocus, early naturalists believed, were so huge that they must have been confined to rivers and lakes deep enough to support the dinosaurs’ bulk. On land, the argument went, these dinosaurs would collapse under their own weight. By the 1970s, skeletal evidence and preserved footprints in trackways confirmed that sauropods were land-dwellers. But it has only been recently that paleontologists have been able to start unlocking the secrets of how these seemingly improbable animals developed over their lifetimes and how they evolved in the first place.
Understanding the natural history of sauropods has been crucial to figuring out how they got so big. Though some of the earliest members of the sauropod lineage—such as the 230 million-year-old Panphagia from Argentina—were less than five feet long, even they possessed a unique combination of traits that eventually allowed the group to attain huge sizes.
The way sauropods reproduced may have been a key to their ability to grow to such prodigious sizes. Mother sauropods laid about 10 eggs at a time in small nests; scores of fossilized egg clutches have been found, as have thousands of eggs from sites all over the world. (Some even preserved embryos inside, allowing paleontologists to definitively identify sauropod eggs by their shape.) That means these dinosaurs grew outside of their mother’s bodies. According to Christine Janis of Brown University and Matthew Carrano of Smithsonian’s National Museum of Natural History, laying eggs opened up evolutionary possibilities for these dinosaurs.
For large mammals, carrying a fetus is a major investment. Developing African bush elephants gestate inside their mothers for a staggering 22 months, for example, and the larger mammal species get, the longer their offspring have to develop before birth. A lot can go wrong during a long gestation, including miscarriage, and nourishing such a large embryo for so long is a huge energy drain on an expectant mother (to say nothing of nursing the baby and providing care after birth). As mammals get larger, the risks and costs of carrying offspring increase, and so there might be some kind of size threshold that land mammals can’t cross.
Mother sauropods, on the other hand, did not have to carry their developing babies for nearly two years, and they could lay numerous eggs at relatively short intervals. Some species may have provided parental care after hatching: rare trackways show that some herds likely included sauropods of different ages. But fans of the animated movie The Land Before Time may be disappointed to know that others probably didn’t care for their young. Paleontologists have also found bone beds that contain only young sauropods of species such as Alamosaurus, indicating that these dinosaurs were on their own after leaving the nest.
Regardless of whether juvenile sauropods hung out in big herds or in smaller groups of dinosaurs their own age, though, the young dinosaurs were probably picky eaters. They had to be if they were to grow to adult size. Diplodocus is one of the most iconic sauropod dinosaurs, and adults of this Jurassic herbivore had broad, squared-off muzzles indicative of an indiscriminate diet. In addition to energy-rich ginkgo trees and conifers called monkey puzzles, they could have also survived on lower-quality food like cycads and the tough parts of conifers. The skull of a juvenile, described by John Whitlock, Jeffrey Wilson and Matthew Lamanna last year, hints that young Diplodocus had different tastes.
Paleontologists have recognized that the differences in menu choice between grazing and browsing herbivores can generally be seen in skull shape. While grazers have broad muzzles to scarf up a wide variety of food, selective browsers have narrower and rounded snouts that make it possible for them to pick specific plants or plant parts. (Some fanciful reconstructions gave Diplodocus and other sauropods elephant-like trunks with which to pluck food, but this idea has been thoroughly debunked.) Since the juvenile Diplodocus skull had a more rounded shape, Whitlock and colleagues proposed that it selected the juiciest browse – juvenile Diplodocus may have focused on foods like horsetails, ferns and high-energy evergreens, instead of sucking down whatever was available, as adults did.
From an energy perspective, it made sense for young sauropods to be choosy. Small dinosaurs required the most bang for their buck in terms of food; they were specialized to pick high-energy plants to fuel their rapid growth. Adults, which were already large and merely had to maintain—rather than grow—large bodies, could afford to hork down large amounts of lower-quality fuel. While they consumed more food in absolute terms, adult sauropods could eat lower-quality foods, whereas smaller sauropods required high-quality food. (This is a common pattern seen among animals even today: a tiny shrew has to eat nutritious insects almost constantly, but African elephants can subsist on a diet of lower-quality grass and other plant food.) The dietary difference may have allowed young and mature Diplodocus to live in the same area through a phenomenon ecologists call “niche partitioning.” The specialization of the juveniles and the more generalist diet of the adults kept them out of constant competition for food, meaning that the young and old Diplodocus fed almost as if they were two different species.