It is interesting to analyze the dromaeosaurids and the troodontids.
Let's begin with the dromaeosaurids.
The dromaeosuarids fall into two groups. One group includes deinonychus, microraptor, graciliraptor and utahraptor etc. This group dates to around 125 mya. If we look at the characteristics of this group we find flying bird characteristics such as aerodynamic flight (asymmetric) feathers.
The other group of dromaeosuarids includes dromaeosaurus, saurornitholestes, bambiraptor, mahakala, unenlagia and velociraptor etc. This group dates to around 75 mya. In this second group we find flightless bird characteristics, such as only symmetric feathers.
And we find troodon and saurornithoides etc from around 75 mya with flightless bird characteristics.
The fossil record (even if incomplete) tells a clear story. Flying birds were from very early on, with flightless birds later (closer to today).
Supporting info (in alphabetic order):
ReplyDeleteANCHIORNIS
http://en.wikipedia.org/wiki/Anchiornis
As in other early paravians such as Microraptor, Anchiornis had large wings, made up of pennaceous flight feathers attached to the arm and hand (as in modern birds) as well as flight feathers on the hind legs, forming an arrangement of fore and hind wings.
MICRORAPTOR
http://en.wikipedia.org/wiki/Microraptor
Unusual even among early birds and feathered dinosaurs, Microraptor is one of the few known bird precursors to sport long flight feathers on its feet as well as its forearms and hands.
UNENLAGIA
http://en.wikipedia.org/wiki/Unenlagia
Makovicky et al. suggested that the 'flying raptor' Rahonavis is also a member of this group, which would mean that either Unenlagia is secondarily flightless, having evolved from flying, Rahonavis-like ancestors, or that bird-like flight evolved at least twice[1]
VELOCIRAPTOR
http://en.wikipedia.org/wiki/Velociraptor
All dromaeosaurids have also been referred to the family Archaeopterygidae by at least one author (which would, in effect, make Velociraptor a flightless bird).[5]