Size and Weight:
True giants like Brachiosaurus and Apatosaurus may get all the press, but pound for pound, the most common sauropod (giant herbivorous dinosaur) of late Jurassic North America was Camarasaurus. This medium-sized dinosaur, which weighed "only" about 20 tons, is believed to have roamed the midwestern plains in herds, and was probably a prime source of food for the hungry theropods of its day (the most likely antagonist being Allosaurus).
Paleontologists believe that Camarasaurus subsisted on more challenging fare than its larger cousins, since its teeth were adapted to shredding especially tough vegetation. Like other herbivorous dinosaurs, Camarasaurus may have swallowed small stones--called "gastroliths"--to help grind down its meals in its massive gut. (By the way, this dinosaur's name, Greek for "chambered lizard," refers to Camarasaurus' head, which contained numerous large openings that probably evolved for cooling purposes.)
Does the prevalence of Camarasaurus specimens (especially in the stretch of the Morrison Formation spanning Colorado, Wyoming and Utah) mean that this sauropod vastly outnumbered its more famous competition? Not necessarily: for one thing, just because a given dinosaur happens to persist in the fossil record speaks more about the vagaries of the preservation process than the size of its population. On the other hand, it only makes sense that the western U.S. could support a larger population of medium-sized sauropods, compared to smaller herds of 50- and 75-ton behemoths, so Camarasaurus may well have outnumbered Apatosaurus and Diplodocus.