Under cladistics, birds are the descendants of dinosaurs.
As such, they are descendants of the most recent common ancestor of Megalosaurus
and Iguanodon.
Therefore:
How do birds fit into dinosaur phylogeny?
Archaeopteryx and later birds have an S-shaped neck, simple hinge ankle joints,
and long hindlimbs:
Archaeopteryx and later birds have an inturned femoral head (and upright stance),
and their tibiae, fibulae, and metatarsi are all long and slender:
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:
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:
Archaeopteryx and later birds have extremely hollow long bones and an
intramandibular joint:
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:
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:
Archaeopteryx and later birds have a maxillary fenestra, complex chambers in
their vertebrae, and an obturator process on the ischium:
Archaeopteryx and later birds have boat-shaped chevrons, a triangular obturator
process, and a very tall ascending process of the astragalus:
Archaeopteryx and later birds have enlarged brain size, a bony palate, and fewer
than 45 caudals:
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:
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:
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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