A recent article, “How Birds Got Their Wings,” also here, here and here describe work by Dececchi and Larsson (2013) that noted scaling of forelimb vs hind limb took a big turn with Archaeopteryx and kin (Fig. 1). They note, as forelimbs lengthened, they became long enough to serve as an airfoil, allowing for the evolution of powered flight. Shorter legs would have aided in reducing drag during flight — the reason modern birds tuck their legs as they fly — and also in perching and moving about on small branches in trees.
That’s all well and good, but its not the key. It’s one step following the key.
The key is flapping. That’s a behavior that definitely leads to flight. Having long forelimbs vs. hind limbs is also found in primates, chalicotheres and sloths. They don’t fly. Even the “flying lemur” Cynocephalus had long arms. All it can do is glide because it doesn’t flap.
The key to flapping is a locked down coracoid.
As we learned earlier with Cosesaurus (Fig. 1), a locked down coracoid is one thing pterosaurs and birds share. Cosesaurus had fibers trailing its forelimbs (Ellenberger 1993, Peters 2011) but its forelimbs were too short to fly. Nevertheless, it could flap because it had a locked down coracoid. And that was a secondary sexual trait (behavior) that led to more of the same in three distinct directions in Sharovipteryx, Longisquama and basal pterosaurs.
with long forelimbs and a tall, narrow, locked down coracoid include oviraptorids, dromaeosaurids, alvarezaurs and birds all taxa that phylogenetically follow Archaeopteryx.
They have a locked down tall clavicle that serves the same function. We don’t know when they started flapping because the closest known fossils of prebats are all skull material only.
Dececchi TA and, Larsson HCE 2013. Body and Limb Size Dissociation at the Origin of Birds: Uncoupling Allometric Constraints Across a Macroevolutionary Transition. Evolution 67(9):2741 DOI:10.1111/evo.12150