Final answer:
The two principal saurischian clades are sauropodomorphs, including long-necked herbivores like Apatosaurus, and theropods, which were primarily bipedal predators and also the ancestors of birds.
Step-by-step explanation:
The two principal saurischian clades are the sauropodomorphs and theropods. The saurischians, or "lizard-hipped" dinosaurs, are a major division of the dinosauria and have been classified due to their distinct pelvic structure.
Within the clade Saurischia, sauropodomorphs included the large, long-necked herbivorous dinosaurs such as Apatosaurus, while the theropods were primarily bipedal predators and included the lineage that eventually gave rise to birds. This divergence and evolution signify the depth of diversity that existed within ancient reptilian lineages.