If there's one rule of evolution, it's that all mighty creatures have smaller, less overwhelming ancestors lurking somewhere back in their family trees--and nowhere is this rule more evident than in the relationship between the giant sauropods of the late Jurassic period and the smaller prosauropods that preceded them by tens of millions of years. Prosauropods (Greek for "before the sauropods") weren't simply scaled-down versions of Brachiosaurus or Apatosaurus; many of them walked on two legs, and there's some evidence that they may have pursued an omnivorous, rather than strictly herbivorous, diet. (See a gallery of prosauropod dinosaur pictures and profiles.)
You might assume from their name that prosauropods eventually evolved into sauropods; this was once thought to be the case, but paleontologists now believe that most prosauropods were actually second cousins, once removed, of the sauropods (not a technical description, but you get the idea!) Rather, it appears that prosauropods evolved in parallel with the true ancestors of sauropods, which have yet to be definitively identified (though there are a number of likely candidates).
Prosauropod Physiology and Evolution
One of the reason prosauropods are fairly obscure--at least compared to raptors, tyrannosaurs and sauropods--is that they didn't look all that distinctive, by dinosaur standards. As a general rule, prosauropods had long (but not very long) necks, long (but not very long) tails, and only attained median sizes of between 20 and 30 feet and a few tons, max (with the exception of odd genera like the giant Melanorosaurus). Like their distant cousins, the hadrosaurs, most prosauropods were capable of walking on two or four feet, and reconstructions tend to show them in a relatively clumsy, ungainly posture.
The prosauropod family tree stretches back to the late Triassic period, about 220 million years ago, when the first dinosaurs were just beginning to establish their worldwide dominance. The earliest genera, like Efraasia and Camelotia, are wrapped in mystery, since their "plain vanilla" appearance and anatomy meant their ancestors could have evolved in any number of directions. Another early genus was the 20-pound Technosaurus, named after Texas Tech University, which many experts believe to have been an archosaur rather than a true dinosaur, much less a prosauropod.
Other early prosauropods, like Plateosaurus and Sellosaurus (which may have been the same dinosaur), are much better established on the dinosaur evolutionary tree thanks to their numerous fossil remains; in fact, Plateosaurus appears to have been one of the most common dinosaurs of late Triassic Europe, and may have roamed the grasslands in giant herds like modern bison. A third famous prosauropod of this period was the hundred-pound Thecodontosaurus, which was named for its distinctive, monitor-lizard-type teeth. Massospondylus is the best-known of the early Jurassic prosauropods; this dinosaur did in fact look like a scaled-down sauropod, but it probably ran on two legs rather than four!
What Did Prosauropods Eat?
Over and above their evolutionary relationship (or lack of relationship) to the giant sauropods, the most controversial aspect of prosauropods concerns what they ate for lunch and dinner. Based on an analysis of the teeth and relatively lightweight skulls of certain prosauropod genera, some paleontologists have concluded that these dinosaurs weren't very well equipped for digesting the tough vegetable matter of the late Triassic period, though there are no direct proof that they ate meat (in the form of fish, insects or smaller dinosaurs). On the whole, the preponderance of the evidence is that prosauropods were strictly herbivorous, though that "what if" still lingers in the minds of some experts.