Birds, an apparent heir of a predominant figure in the Mesozoic fauna, maintain inherited dominance among terrestrial vertebrates, featuring bi-, for some even tri-, mode of locomotion – aerial and terrestrial.
How their ability of aerial locotion might be evolved had been a fascinating subject with a decade-long history of debate. A basic stance to this subject took dichotomy in approach: between “ground up” and “trees down”. Reinvigorated by a proposal of so- called “insect net” hypothesis around the mid-1970s, the subject got settled down without reconciliation between two parties, and limited to sporadic references today. How, then, did they end up? Which one could appear more plausible? None – or both. Each has own merits and difficulties, which are not amenable to calibrate superiority based on a single criterion. One thing, however, can be affirmed: primitive birds might have been able to take off from the ground. For those who championed a terrestrial origin of bird flight obtained an elegant and convincing argument with a bunch of experimental data, which can never be overestimated in the discussion history – the ontogenetic transitional wing hypothesis, or OTW hypothesis. This suggests that birds evolved incrementally by using their half-developed wings to run up steep surfaces (WAIR or Wing-Assisted Incline Running).
In the meanwhile, ones equivalent to that hypothesis in arboreal settings have so far failed to emerge. This doesn’t necessarily mean the demise of arboreal hypothesis, but in effect, it may come back with different perspectives in future.
Re-visiting “Insect net” hypothesis.
Deinonychus might inspire Ostrom into a new perspective on small-sized dinosaurs. A downsized predator was an easier model on which origins of birds and their flight were to be addressed.
What did his hypothesis aim at? That was an ambition to resolve mysteries surrounding two origins, distinct nature but intrisically linked one with another, of birds and their locomotor innovation – powered flight.
His argument went: precursors of modern birds were to be found in one of the theropodan ramifications. Being carnivoran (insectivorous) and small, a predator should have assumed to be an agile lightly-built hunter. Its forelimbs were preferably employed to capture small preys. With an aid by rudimentary feathers (on the hand in his paper), an occurrence of flapping action for wing stroke thus might be envisioned through its behavioral characteristics.
His unique proposal was pounced on by contenders, dragging the discussion down to feud occasionally, and invited their striking back: gliding is the one that might make powered flight become easier.
Criticism levelled at his idea abounded, though ineffectively or unfoundedly, whereas its significance was largely unnoticed:
- Aroused interest and encouraged further investigations (followed by nearly three-decades-long discussion with widely ranging themes).
- Paved the way for further exploring terrestrial settings, providing various clues for modelling hypotheses (followed and developed by various authors). – forelimb use in hunting meant development of forelimb length for a wing, of voluminous pectoral muscle and forelimbs covered with feathers. Terrestrial adaptation could be helpful for running take-off. Bipedalism in an ancestral form, rather than (arboreal) quadrupedalism, is more parsimonious anatomically, and so on.
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Hiroki Mori
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