Ornithischians are a group of dinosaurs characterized by their bird-like hips, which distinguish them from other dinosaur groups. They lived during the Mesozoic Era and included a variety of species, such as the armored Stegosaurus and the horned Triceratops. Ornithischians were primarily herbivorous and adapted to different environments, showcasing diverse body shapes and sizes. This group is divided into several subcategories, including the Ornithopoda, known for their beak-like mouths, and the Thyreophora, which included armored dinosaurs. Ornithischians played a significant role in prehistoric ecosystems, contributing to the diversity of life during their time on Earth.