If you think elephants are big, take a look at this graphic from yesterday’s New York Times comparing the size of the sauropod dinosaur Brachiosaurus with an elephant and a human (click to enlarge):
Sauropods were an infraorder of dinosaurs that included everybody’s second-favorite dinosaur, Brontosaurus (unfortunately renamed Apatosaurus). The Times article on sauropods, by John Noble Wilford, is centered on a new book, Biology of the Sauropod Dinosaurs, on an exhibit on large dinos opening this Saturday at New York’s American Museum of Natural History, and on recent published research. The interesting stuff involves their size and their diet. First, look at another figure (from Wikipedia) showing how huge they were. Wikipedia says this about the big guy in red below:
The holotype (and now lost) vertebra of Amphicoelias fragillimus may have come from an animal 58 metres (190 ft) long; its vertebral column would have been substantially longer than that of the blue whale. The longest terrestrial animal alive today, the reticulated python, only reaches lengths of 8.7 metres (29 ft).
Note that a Boeing 747 is only about 35 feet longer than this sauropod!
The main issues are two: what was the evolutionary advantage of getting so large, and how did they sustain such a big body? The article doesn’t really answer the first question (avoidance of predation is one possibility), but there are hypotheses about the second. These dinosaurs didn’t have molars, and basically couldn’t chew—they simply took big bites of the vegetation and swallowed it whole. Since their food intake was enormous (the 60-foot Mamenchisaurus, not a particularly large sauropod, ate about 1,150 pounds of vegetation per day), they might have evolved those long necks to cover more grazing area without having to expend energy moving their bodies. And since they didn’t chew, their heads could remain small.
The figure at the top shows how much grazing area a sauropod could reach if it had a neck of a given size. Because the neck is in fact a radius of consumption, the area accessible to the beast goes up roughly as the square of the neck length. Even given this efficiency, scientists estimate that it took about two weeks to digest one day’s food intake.
They also grew fast: one recent paper estimated that baby sauropods doubled their weight every five days. Their hearts probably beat at a rate of about ten per minute, compared to 28 for an elephant. One big mystery is how they got enough oxygen to fuel their metabolism: some scientists think that they had a respiratory system like birds. (Birds supplement their lungs with several air sacs producing gas exchange with the blood.) But one of my Chicago colleagues is dubious:
Paul Sereno, a dinosaur fossil hunter at the University of Chicago, said the new research “is very valuable,” but he doubted there was enough hard evidence to support the bird-lung hypothesis. Still, he said, the sauropod “is an incredible animal, one of the best land animals that’s been invented.”