Sunday, 9 January 2011

Arboreal Raptors did not look like this:

The Primeval version of an arboreal Dromaeosaur is very far from the truth (to our knowledge anyway), but the fact that similar animals did exist relates very nicely to my previous post. This is part 2 of my series of posts on the possibility of Dromaeosaurs being neoflightless birds.
Above; Sinornithosaurus milleni; a real arboreal Dromaeosaur from China. At 1.2 meters long and with a fluffy integument it was a far cry from the critter in Primeval.

Arboreal Dromaeosaurs; especially those found with feathers, support the possibility that Dromaeosaurs were related to birds. It even indirectly strengthens the idea that they may be descended from them.

Creatures like Sinornithosaurus were small early Cretaceous feathered Dromaeosaurs that probably lived in trees quadrupedally. This is suggested by the fact that the arms are very similar in length to the legs and that they therefore had comparatively equal roles in locomotion. The curved claws would have been effective for gripping branches. Also interesting is the disparity in length between the fingers. The middle is of course the longest. In most arboreal animals the digits are of equal length. This suggests that the arms used to serve a different purpose, be it flying or prey apprehension, and that Sinornithosaurus was only recently a climber. Even though it had long arms and large branching feathers on its arms they lack barbules which could hold them together, making flying impossible. A ground dwelling mode of life for Sinornithosaurus is also impractical, as its long leg and arm feathers would have impeded swift movements on the ground. But what suggests that Sinornithosaurus may have evolved from a flier and not a runner?

Firstly the third metacarpal is bowed posterolaterally. This increases the surface area for feather attachment in the hand. The fact that this feature is more prominent in Sinornithosaurus than the flying or at the very least gliding Archaeopteryx suggests that Sinornithosaus may have descended from even more advanced fliers than Archaeopteryx. Another feature of the hand that supports this argument is the widening of the first phalangeal of the middle finger. This was another feather support related feature. These two features so prominent in more advanced birds and present in Sinornithosaurus are wanting in Archaeopteryx.

Also interesting is the presence of ossified uncinate processes on the ribs of the flightless Velociraptor. These make the body more rigid and therefore maneuverable in the flight of birds. They also help in bracing the ribs together when breathing with a complex air sac system. This flight related feature is ironically lacking in Archaeopteryx. I must point out that uncinate proceses are not essentially a flight related character, but if Velociraptor was more basal to Archaeopteryx I would expect to see them in the latter. The ossified Dromaeosaur sternum is much more developed than that of Archaeopteryx. The sternum of Velociraptor is much longer and more developed than that of Archaeopteryx. It possesses ossified sternal ribs which further brace the body cavity in Dromaeosaurs, but not in Archaeopteryx.

All of these flight related features present in the pectoral complex of arboreal and terrestrial Dromaeosaurs are intriguing and make the idea of a flying Dromaeosaur ancestor even more substantiated.

For more on Dromaeosaurs and the idea that they may be Neoflightless see my posts titled : Where are the neoflightless Archaeopterygiformes? and Were Deinonychosaurians and Avialians descended from an arboreal, sickle toed, four winged ancestor?

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