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32: (Case Study) Dinosaur classification

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    Learning Objectives

    After reading this chapter, students should be able to:

    • Identify the two major groups (orders) of dinosaurs, separated based on their hips
    • Describe major subdivisions within each order, like theropods, sauropodomorphs, thyreophora, ornithopods, and marginocephalia.
    It shows how all this dinosaurs are related as described in the chapter
    Figure \(\PageIndex{1}\): The major branches of the dinosaur family tree. (Image by Callan Bentley.)

    Dinosaurs are a diverse group of reptiles and can be classified into various different groups. Understanding these groups can not only help in placing fan favorites like Triceratops and Velociraptor into a greater context, but also to show how different animals are related to one another and evolved from common ancestors. In this case study, we will talk about the seven major groups that include the vast majority of known dinosaurs and how those groups relate to one another.

    Closed structure of a pelvis
    Figure \(\PageIndex{2}\): Ornithischian pelvis with a closed structure. (Image by AdmiralHood on Wikimedia.)

    The most basic subdivision of dinosaurs is based on their hips. This division, first proposed by British paleontologist Harry Sheely in 1888, has traditionally been thought to be at the ‘order’ level in biological classification schemes, but modern research suggests that instead, it may merely be a clade. The hip has three bones: the pubis, ischium, and ilium. Some dinosaurs have a closed hip system in which the pubis and the ischium are close together. These dinosaurs are called bird-hipped dinosaurs, or Ornithischians. This style of hip is more similar to modern birds.

    Pelvis with an open structure
    Figure \(\PageIndex{3}\): Saurischian pelvis with an open structure. (Image by AdmiralHood on Wikimedia.)

    Other dinosaurs have an open structure to their hips with the pubis and ischium far apart and are known as Saurischian dinosaurs. Saurischian means ‘lizard hip’ because these hips are more similar to living lizards. Ironically, the dinosaurs that eventually evolved into birds are the Saurischian (lizard-hipped) dinosaurs whose descendents eventually adopted the closed-hip structure as well 一 a nice example of convergent evolution. These two branches of the dinosaur family tree are traditionally considered sister groups, but it should be noted that not all dinosaurs fall easily into one of these two groups (such as early dinosaurs like Chilesaurus and Nyasasaurus) and some paleontologists debate where to place other dinosaurs (such as the early meat-eating Herrerasaurs). Another complication is that not all scientists agree that with the premise that these are the two main groups. Most notable is a 2017 study by Baron et. al. (link to blog post) that says the traditional tree is all wrong. Since this new phylogeny is not widely accepted, below is the classic and prevailing interpretation of the members of each of the hip-based classifications.

    This page titled 32: (Case Study) Dinosaur classification is shared under a CC BY-NC 4.0 license and was authored, remixed, and/or curated by Callan Bentley, Karen Layou, Russ Kohrs, Shelley Jaye, Matt Affolter, and Brian Ricketts (VIVA, the Virginia Library Consortium) via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.