GEOL 104 Dinosaurs: A Natural History

Fall Semester 2006
The Dinosaur Family Tree: What is a dinosaur?

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

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

    Most primitive known ornithischian is Pisanosaurus from the Ischigualasto Formation (early Late Triassic) of Argentina: about 1 m long.

    All other, more derived ornithischians have a pubis that points backwards and ossified tendons along the top of the vertebrae.

    Other primitive ornithischians include Early Jurassic Lesothosaurus and the Heterodontosauridae:

    Heterodontosauridae ("different toothed"):

  • Retain the long grasping hands (in most other ornithischians the hands are short and are non-grasping)
  • Short, deep, powerful skulls
  • Long considered a type of ornithopod, and also suggested as sister group to Marginocephalia
  • Oldest is a possible Late Triassic Argentine specimen; greatest diversity is in Early Jurassic of southern Africa; last known species in early part of Early Cretaceous
  • Small (1 m long) bipedal animals

    Advanced ornithischians include two major clades: Thyreophora, and Neornithischia.

    Thyreophora ("sheild 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

    Neornithischians are diagnosed by asymmetrical enamel on their teeth, giving them an ever-sharpening surface. Among the neornithischians are various primitive forms like Early Jurassic Strombergia and Middle Jurassic Agilisaurus and Hexinlusaurus, plus the advanced diverse Ornithopoda and Marginocephalia.

    Ornithopoda ("bird foot"):

  • Specialized jaws (similar features seen in advanced heterodontosaurids):
  • Early ornithopods small (1 m long) bipedal animals; later forms increase in size and retain bipedality, but develop more sophisticated chewing jaws

    Marginocephalia ("ridge head"):

  • Characterized by shelf of bone extending back over occipital region of skull
  • Primitive forms are bipedal; in one branch derived forms become very large and obligate quadrupeds

    The other major clade of dinosaurs is Saurischia:

  • Defined as Megalosaurus and all taxa closer to Megalosaurus than to Iguanodon
  • Diagnosed by:

    The most primitive saurischians are the Herrerasauria:

  • Carnivorous bipedal dinosaurs, up to about 4 m long
  • Most completely known form is Herrerasaurus of the Ischigualasto Fm. (early Late Triassic of Argentina)
  • Has longer clutching fingers, shared with theropods (see below)
  • Has an intramandibular joint between the dentary and postdentary bones
  • Herrerasaurs were once thought to be true theropods, but have only two sacrals (a pre-dinosaur feature), have manual digit III longer than manual digit II (a pre-eusaurischian feature), and have relatively short necks (another pre-eusaurischian feature). Recent studies place them as a group primitive saurischians rather than true theropods.

    Also in Ischigualasto Fm., another very primitive saurischian: Eoraptor. Overall, one of the least-transformed of the Triassic dinosaurs, and probably the best approximation known to the ancestral dinosaur form. Recent studies place it outside of Eusaurischia, but within Saurischia.

    The remaining part of Saurischia, called Eusaurischia, is recognized by:

  • Elongate necks where posterior cervicals are longer than anterior cervicals
  • Manus with an enlarged digit I ungual (thumb claw)
  • Manual digit II is the longest in the hand
  • Some eusaurischians retain the primitive carnivorous condition; others have leaf-shaped teeth and were herbivores.

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

    Sauropodomorpha ("sauropod form"):

  • Characterized by:
  • Herbivores (early forms maybe omnivores)
  • Primitive forms are facultative bipeds; later forms were so large they were obligate quadrupeds

    Theropoda ("beast foot"):

  • 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
  • Has an intramandibular joint between the dentary and postdentary bones
  • Promaxillary fenestra (an extra opening in the maxilla between the naris and the antorbital fenestra)
  • Five or more sacrals
  • Furcula (clavicles fused into wishbone)
  • Manual digit V lost: hand has only four fingers
  • Pedal digits I and V reduced, so foot is functionally three-toed

    By Late Triassic good fossils are known of Sauropodomorpha, Eoraptor, Herrerasauridae, and Theropoda. Ornithischia only poorly known then.

    With the Triassic-Jurassic extinction event, most potential dinosaur rivals (therapsids, pseudosuchians, etc.) were removed, and Dinosauria became the dominant group on land: the Fourth Amniote Radiation.

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    Last modified: 27 July 2006