GEOL 104 Dinosaurs: A Natural History

Fall Semester 2000
Archaeopteryx and the Origin of Birds II: What is a dinosaur (version 4)?

Under cladistics, birds are the descendants of dinosaurs. As such, they are descendants of the most recent common ancestor of Megalosaurus and Iguanodon.
Therefore:

Birds (Aves) are a type of dinosaur!

How do birds fit into dinosaur phylogeny?

Archaeopteryx and later birds have an “S”-shaped neck, simple hinge ankle joints, and long hindlimbs:

  • Birds are ornithodirans

    Archaeopteryx and later birds have an inturned femoral head (and upright stance), and their tibiae, fibulae, and metatarsi are all long and slender:

  • Birds are dinosauriforms

    Archaeopteryx and later birds have a semiopposable manual digit I, reduced (lost) manual digits IV and IV, more than three sacrals, and (in primitive forms) a perforate acetabulum:

  • Birds are dinosaurs

    Archaeopteryx and later birds have posterior cervicals longer than their anterior ones, manual digit II is the longest, and (in primitive forms) a big thumb claw:

  • Birds are saurischians (and not ornithischians!)

    Archaeopteryx and later birds have extremely hollow long bones and an intramandibular joint:

  • Birds are theropods

    Archaeopteryx and later birds have a promaxillary fenestra (lost in more advanced birds), five or more sacrals, a furcula, manual digit V is lost, pedal digits I and V are reduced, and metatarsal I does not contact the ankle:

  • Birds are neotheropods

    Archaeopteryx and later birds have the tooth row anterior to the orbit, a distally stiffened tail, a semilunate carpal block, large hands, have lost manual digit IV, and have a tall ascending process of the astragalus:

  • Birds are tetanurines

    Archaeopteryx and later birds have a maxillary fenestra, complex chambers in their vertebrae, and an obturator process on the ischium:

  • Birds are avetheropods

    Archaeopteryx and later birds have boat-shaped chevrons, a triangular obturator process, and a very tall ascending process of the astragalus:

  • Birds are coelurosaurs

    Archaeopteryx and later birds have enlarged brain size, a bony palate, and fewer than 45 caudals:

  • Birds are maniraptoriforms

    Archaeopteryx and later birds have enlarged sterna, shoulder joints that face laterally, enlarged semilunate carpal blocks, and manual unguals which are very curved and compressed mediolaterally:

  • Birds are maniraptorans

    Archaeopteryx and later birds have tail which is very mobile proximally but very stiff distally, a backwards-pointing pubis, and (in primitive forms) a retractable pedal digit II:

  • Birds are eumaniraptorans

    Avialae (the taxon comprised of Archaeopteryx and more advanced birds) share the following synapomorphies:

    Many features that characterize modern birds evolved long after Archaeopteryx. These include:

    All these features worked out by skeletal relationships. New discovery of feathered coelurosaurs leads to new interpretation of these “avian” features:

    Coelurosauria seems be characterized by feathers, and Maniraptora by vaned feathers.

    So, tyrannosaurids and ornithomimosaurs were most likely feathered (or at least descendants of feathered ancestors)!!

    Since feathers are found in non-avian, non-flying dinosaurs, feathers must have evolved for something other than flight. Possibilities include:

    Feathers were thus symplesiomorphies of birds, NOT synapomorphies!

    Flight origins:
    Two primary models are:

    Still considerable work being done on modern flyers and gliders, so the models for flight origin are still being worked out.

    During Cretaceous, a great burst of bird diversity, including:

    Because of the cladistic position of Aves, we now know that Dinosauria is not extinct! (In fact, it outnumbers mammal species 2:1).

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