This section is taken from the Dinosaur Field Guide.
Massospondylus is a long, sleek plant eating dinosaur with a huge claw on each hand. Theropods during the Early Jurassic had not yet reached "super size" (like Allosaurus), so a claw of this type would make an excellent weapon. Plants take a lot more energy to digest than meat, so Massospondylus has a wider pelvis and a larger set of guts in order to process them properly. The neck is longer to get at more food higher up in the trees. Dinosaurs were the first four-legged ground dwellers to be able to feed higher off the ground than sprawling animals. Being able to get at this new food source allowed prosauropods (such as Massospondylus), and then sauropods, to become the dominant plant-eaters for the next 50 million years.
For a long time, prosauropods were considered to be the ancestors of true sauropods because they look like smaller versions of the true sauropods. But there are some problems with this theory. Their food anatomy is different enough from that of true sauropods to suggest that they are just a side branch that died out.
Massospondylus doesn't appear in the original novels by Michael Crichton, nor in the movies.