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Archaeopteryx is a dromaeosaur theropod
Getting ready for the Soc Avian Paleo & Evol conference on Mesozoic birds in
June in DC has caused me to restudy Archaeopteryx and protoavian (sensu Paul
1988 PDW) theropods. With all the specimens old and new now available,
virtually the entire skull and skeleton is now known for Archaeopteryx.
Conclusions are as follows.
Archaeopteryx truly is a flying theropod that shows virtually no avian
characters not observed in other theropods (the large, distal, fully reversed
hallux may be the only exception). Contrary to reports otherwise, there is no
evidence for avian skull kinetics, and the postorbital bar was probably
complete. The palate and braincase are entirely theropodian in structure.
Archaeopteryx is not only a theropod, it is a dromaeosaur because it shares a
number of detailed characters only with dromaeosaurs. Some other characters
are found only in the two forms and some basal birds. Some of the
Archaeopteryx-dromaeosaur characters are as follows.
Nasal depressed nasal and snout
Dorsal process of maxilla almost reaches preorbital bar
Preorbital bar slender & straight preorbital in lateral view
Dorsal depression on the ectopterygoid
Diamond shaped supraoccipital
Strongly twisted paraoccipital process (noted by Currie)
HIghly modified tail with hyperdorso-flexible base (condition
approached in troodonts)
Middle finger most robust
Ilium parallalogram shaped (also basal birds)
Pubic peduncle very large & reversed
Ilio-pubic articulation inverted V shape
Pubic shafts are flat plates oriented 140 degrees to each other
In addition, the foot of Archaeopteryx is functionally two toed, with a short
toe II that is hyperextendable.
Many Cretaceous theropods - dromaeosaurs, troodonts, oviraptors - are more
bird-like than Archaeopteryx in many respects, and have a shoulder girdle
that is similar to secondarily flightless birds. All this suggests that avian
flight first evolved in arboreal theropods (where they developed big brains
and forward facing eyes, features not found in flying insects and pteosaurs),
and that some of the flying theropods lost flight. Not knowable at this time
is whether Archaeopteryx was a member of the true bird clade, or was an
independent experiment in flight, or were theropods end and birds begin,
among other matters.