GEOL 104 Dinosaurs: A Natural History

Fall Semester 2000
The Dinosaur Family Tree: What is a dinosaur (version 3)?

Dinosauria defined as all descendants of most recent common ancestor of Iguanodon and Megalosaurus.

Dinosauria is divided into two clades, Ornithischia and Saurischia

Ornithischia (“bird hips”):

  • Defined as Iguanodon and all taxa closer to Iguanodon than to Megalosaurus
  • Diagnosed by presence of the predentary bone (an extra bone joining the two anterior ends of the dentary, forming a beak) and five or more sacrals
  • All known ornithischians were most likely herbivores, with leaf-shaped teeth

    Ischigualasto Formation of Argentina (Late Triassic): oldest and most primitive ] known dinosaurs.

    Most primitive known ornithischian is Pisanosaurus from the Ischigualasto.

    All other, more derived ornithischians have a pubis that points backwards (increases size of gut, allows better digestion of plant material).

    Advanced thyreophorans include three major clades: Thyreophora, Ornithopoda, and Marginocephalia.


  • Name means “shield bearer”
  • Armored dinosaurs: diagnosed by presence of rows of dermal ossifications
  • Early thyreophorans small (1 m long) bipedal animals; later forms much larger and quadrupedal, with heavier armor


  • Name means “bird foot”; a bit odd, as their feet aren't really bird-like
  • Specialized jaws:
  • Early ornithopods small (1 m long) bipedal animals; later forms increase in size and retain bipedality, but develop more sophisticated chewing jaws


  • Name means “ridge head”
  • Characterized by shelf of bone extending back over occipital region of skull
  • Absent from fossil record until Middle Jurassic, and all but one known are Cretaceous
  • Primitive forms are bipedal; in one branch derived forms become very large and obligate quadrupeds
  • Marginocephalian skulls show a lot of suggestions of display and/or combat

    The other major clade of dinosaurs is Saurischia:

  • Defined as Megalosaurus and all taxa closer to Megalosaurus than to Iguanodon
  • Diagnosed by:
  • Some saurischians retain the primitive carnivorous condition; others have leaf-shaped teeth and were probably herbivores.

    Saurischia is divided into two major clades, Sauropodomorpha and Theropoda.


  • Name means “sauropod form”; Sauropoda means “lizard feet”, even though their feet do not look much like lizard feet!
  • Characterized by:
  • Herbivores (early forms maybe omnivores)
  • Primitive forms are facultative bipeds; later forms were so large they were obligate quadrupeds
  • Were the largest herbivores ever to live on land; by the end of the Late Triassic had surpassed all previous land living animals in size, and kept on going…

    From rocks of the same age as the Ischigualasto Fm., but in nearby Argentina, comes Saturnalia, the most primitive known sauropodomorph. Possibly older sauropodomorphs come from the ?Middle Triassic of Madagascar, but the age of the rocks is uncertain.


  • Name means “beast foot”, after the curved claws
  • Characterized by extremely hollow limb bones
  • Most retain the primitive condition of bladelike, serrated teeth, indicating they were carnivores
  • All, from smallest to largest, were obligate bipeds
  • Became the largest terrestrial carnivores of all time, but began at about 1 m long, and even by end of Late Triassic were dwarfed by rauisuchians

    In Ischigualasto Fm., a possible very primitive theropod Eoraptor. However, is so primitive it might be a non-theropod saurischian, or a non-dinosaurian dinosauromorph.

    Another Ischigualasto Fm. dinosaur is Herrerasaurus, member of Herrerasauridae:

  • Has longer clutching fingers, shared with neotheropods (see below)
  • Has an intramandibular joint between the dentary and postdentary bones
  • Herrerasaurids were probably true theropods, but have only two sacrals (a pre-dinosaur feature), have manual digit III longer than manual digit II (a pre-saurischian feature), and have anterior cervicals as long or longer than posterior cervicals (another pre-saurischian feature)
  • So, they MIGHT be a non-theropod saurischian group

    True, unquestioned theropods form the clade Neotheropoda

  • Name means “new theropods”
  • Have many synapomorphies:

    By Late Triassic good fossils are known of Ornithischia, Sauropodomorpha, Eoraptor, Herrerasauridae, and Neotheropoda.

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