Note: The new discovery of Chilesaurus and some statistical studies of skeletal characteristics have cast some doubt on the validity of the traditional Saurischia / Ornithischia split in favor of a new Saurischia / Ornithoscelida structure. The reason this difference is important is that it’s a useful hallmark for identifying remains, and it’s almost always consistent within each group. This makes room for the larger gut required to effectively digest plants (meat, lacking tough cell walls, is much easier to digest). The hip distinction is a little obscure at first sight, but this is what it’s about: of the three main hip bones, saurischians have one on top, one pointing forward, and one pointing backward, while ornithischians have one on top and two pointing backward.
Sauropods and theropods fall under Saurischia (yes, confusingly birds are not in the bird-hipped group, but are in the reptile-hipped group) while the rest fall under Ornithischia. Pterosaurs (Mesozoic flying reptiles) are closely related to dinosaurs but are not dinosaurs.ĭinosaurs all belong to one of two groups: Saurischia (meaning “reptile-hipped”) and Ornithischia (meaning “bird-hipped”). This group includes sauropods (the long-necked dinosaurs like Apatosaurus), theropods (the bipedal carnivorous dinosaurs like Tyrannosaurus and also birds), thyreophorans (the armored dinosaurs), ornithopods (the duck-billed dinosaurs), ceratopsians (the horned dinosaurs), and some other, less well-known groups. It was historically defined as the last common ancestor of Megalosaurus and Iguanodon and all its descendants, which is just a different way of describing the same group. This clade is usually defined nowadays as the last common ancestor between Triceratops and modern birds, and all its descendants. Are you intimidated by the overly-detailed phylogenetic trees and endless jargon you find on Wikipedia when you’re trying to learn about a specific dinosaur? Are you overwhelmed by the sheer number of genera and do you struggle to figure out which groups are actually important? Or do you just want to get familiar with dinosaurs without much overhead? Then this is the post for you! Here I will go over the major groups within Dinosauria and how they’re related to one another.
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