For many years I have been defending theories that somehow relate the evolution of birds and dinosaurs to arboreality, among them Greg Paul's and Sankar Chatterjee original link of dromaeosaurs to arboreality and Olshevsky's "Birds Came First" scenario. As the long debate trees-down or ground-up rages on, reality (as everything in evolution) might have been a lot more complex than "just so" stories and we have to go back and recapitulate all the new evidence to make the pertinent modifications. New comparative studies and subsequent fossil discoveries complicate familiar hypothetical scenarios:

1.- Mark Norell's new, extremely "furry" juvenile dromaeosaurid ("Dave", as it is known) with at least three different kinds of feathers and "proto feathers" tie up nicely with Richard Prum's proposals on feather evolution in the indispensable volume "New Perspectives on the Origin and Evolution of Birds" from the Proceedings of the Ostrom Symposium.
2.-Sinovenator, a newly published basal tröodontid with surprising opisthopubic pelvis and deep skull, shows that the dromeosaurid pelvis arrangement is probably primitive to dromaeosaurs and tröodontids (not derived). Tröodon and some other maniraptorans dromaeosaur relatives (including Caudipteryx and oviraptorosaurs from the late Cretaceous) had reverted to a more conventionally dinosaurian, less bird-like pelvis.
3.-The SVP presentation of Ken Dial, with his slow motion videos that unmistakably show chicks of several living bird species (not just one, discarding the argument that it might have been an odd behaviour from just one species) "running-up trees" using their wings as air foils to stabilise their bodies against the tree trunk while the legs run in frantic motion. A perfect combination of bipedal running and tree climbing. Interestingly this seems to also back Luis Chiappe's paper in Nature(May 99) "The wing of Archaeopteryx as a primary thrust generator" presenting feathered arms as cursorial take-off" aids and stabilisers.

It's no surprise that (as any other animal) birds and dinosaurs show mosaic evolution. Sankar Chattarjee originally proposed in his book "The Rise of Birds" that maybe the famous dromaeosaur opisthopubic pelvis was advantageous for arboreal animals, flattening the body against the trees and changing the center of gravity. But at the same time, it is obvious that the legs of all theropods show perfect adaptation for terrestrial cursoriality. A strange combination: The lower half of the body is adapted for running and upper half seems to be adapted for perching or flying, or depending on the kind of dinosaur, the forelimbs become simply atrophied "would be" wings.

I think it is time to review Tony Thulborn's old proposal for a new viable, hypothetical scenario on the evolution of birds: Birds are indeed neotenic dinosaurs. And the primitive dinosaurs found alternate evolutionary scenarios in both the ground and the trees. So the long evolutionary path of the Dinosauria could well have been trees-down, ground-up and viceversa all over again... not once, but several times!

Most (if not all) basal juvenile theropods (and maybe all dinosaurs right down to the most basal ones) were covered in insulatory protofeathers. This insulatory coverage evolved originally for insulation purposes of the homeothermic warm-blooded extremely active animals that dinosaurs were. As evolution continued, the insulatory integument evolved in different shapes and varieties as theropods became more arboreal. Richard Prum proposed different evolutionary stages for the development of feathers. The primitive proto-dinosaurs most probably were covered in "quills" or "hair-like" strands similar to pterosaurs. Each of the quills finally started branching, becoming more complex and turning into effective body insulation. There's evidence of almost each one of these steps in the integument found all over the Liaoning Province new feathered dinosaurs. But how did dinosaurs become arboreal? Ground up or trees down? Or trees-trees? Or a combination of the three? A new ground-up evolutionary scenario could well have been like this: It might have been advantageous for dino chicks and juveniles to take refuge from predators in the trees, an inheritance of little running prototheropods (descendants of "rabbit-like" protodinosaurs like the Triassic Marasuchus or Lagosuchus) that became accustomed to run up trees using the matted and elongated integument around their arms as protowings to propel their run-up motion. The system was simple: Run-up a tree using the wings as stabilisers and parachute down (or parachute-jumping from branch to branch, as some lemurs from the genus Propithecus with their matted, rigid hair strands around the arms, do today). Long strands of protofeathery integument became more and more specialised and rigid, developing barbules that actually kept the strands locked together (in a kind of velcro effect). The branching needed for future fully developed feathers. As they became adults, they started losing a lot the insulatory integument and some of the longer strands (if not all, specially in gigantic forms) becoming more and more earthbound and returning to a fully cursorial-only status. Only then protowing motion was turned into effective predatorial strikes (proposed by Gauthier et al as the origin of wings and flapping flight) in the general maniraptoran branch of theropod evolution while in other theropods the forelimbs got atrophied and almost disappeared completely as in the case of advanced tyrannosaurs (still showing high degrees of specialisation). In this evolutionary path, there were branches of dinosaur evolution that actually remained more and more in the trees (as proposed by the "Birds Came First" theory). This would clarify the patterns of mosaic evolution, including lost hand digits and modification of the arms towards becoming wings, that we well know is evident in theropod evolution and specially in the branches of Theropoda leading to Aves.
Hence, birds are dinosaurs that retained more and more their juvenile stages into adulthood.
If you'd like to draw parallels, just think that dinosaur evolution was in many senses very similar to ours: Humans are modified, arboreal apes that retained chimpanzee infantile stages into adulthood and became obligatory bipeds!
All these theories have been proposed before, but baking them in the fire of current evidence is vital; and I think the current evidence keeps supporting them. The open question is now: All these new discoveries... where do they leave the most famous and ancient of all "dinobirds": Archaeopteryx?

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