BSCI392
11-28-07
Walking Theropods

Theropoda (Late Triassic - Recent) includes the classic bipedal carnivorous dinosaurs plus some interesting surprises. Theropods represent an interesting combination of diversity and uniformity:

Literary evolution of theropod phylogeny.

  • Older literature broke theropods down into Both "groups" ended up being polyphyletic. Oops.
  • And yet cladists have struggled with theropod phylogeny, also. A cladist of the mid 1990s would have confidently divided them into "Ceratosauria" and Tetanurae. Alas, recent analyses all have broken Ceratosauria, as originally conceived, into a paraphyletic grade. (Note: We still use the name, but apply it to a much smaller group.)

    Survey of Theropod Diversity

    Eoraptor. (Late Triassic) Near the base of the theropod tree:

    Coelophysoidea. (Late Triassic - Early Jurassic):

    Ceratosauria (in the strict sense) (Early Jurassic - Latest Cretaceous):

    Spinosauroidea (Middle Jurassic - Late Cretaceous): Includes one of the two first dinosaurs to be studied - Megalosaurus - (not highly specialized), as well as specialized long-snouted forms. Generally speaking, spinosauroids reflect derived features that they share with other groups of more derived theropods:

  • Apart from these, derived spinosauroids had their own unique features, including:

    Carnosauria (Late Jurassic - Late Cretaceous): Includes such classics as the Late Jurassic North American Allosaurus. During the last twenty years, a great many other carnosaurs have become well known. Carnosaurs share with coelurosaurs (the other more derived theropods) a key feature:

  • Apart from these, carnosaurs are characterized by complex extra openings in the bones of the snout.
  • They include the largest known carnivorous dinosaur and largest biped in history - Giganotosaurus (right)

    Coelurosauria (Middle Jurassic - Recent): Includes a great diversity of small - medium-sized animals, including birds. Indeed, coelurosaurs seem, ancestrally, to have been small creatures like Sinosauropteryx. Synapomorphies include:

  • Examples of relatively primitive coelurosaurs include:

    Paleobiological issues:

    Size - how big can a biped be?

    Although all theropod lineages except Coelophysoidea and Eoraptor (if it counts) produced very large animals, members of Spinosauroidea, Carnosauria, and Coelurosauria have members that compete for the title of largest biped ever:

    What's interesting is that they all weigh in at roughly six tons. Could this represent the mechanical limit for a biped? What are the limiting factors? Some possibilities: