Archive for the ‘Hardware’ Category

WHY DID BIRDS APPEAR? FEATHERS!

March 8, 2020

Sounds like a joke, but the idea is (new and) serious: when flying in forests, feathered wings are superior to membrane wings. 

Why birds, indeed? The skies were already fully occupied. Pterosaurs (“Wing-lizards”) had dominated the skies for 80 million years. Pterosaurs were the best fliers which ever were, so good that human computation, for the first century after their discovery established they couldn’t possibly fly: some of them were twenty times too heavy, at least. The (heaviest flying bird today is the Kori Bustard (Ardeotis kori) of Africa: males weigh about 18kg, females about half that. A swan once was found, weighting 23 kilograms: it was not clear that it could fly. The largest bird ever to fly were the Teratorns (a type of Condor), the largest of which, Argentavis Magnificens, from Argentina as its name indicates, had a wingspan up to 7 metres, and weighed up to 75kg.

The largest azhdarchid pterosaurs of the Cretaceous, such as Quetzalcoatlus and Hatzegopteryx, had wingspans exceeding 10 m (33 ft), and perhaps 12 m (39 ft) or more. Mass estimates for these azhdarchids are around 200–250 kg (440–550 lb) and their estimated height on the ground was roughly analogous to an elephant or small giraffe. They could launch themselves from flat ground, using a spectacular jumping technique, now known in detail. They were also predatory, and represent a degree of horrors not yet depicted in horror movies (but now it will come). The origins of birds are much more amusing:

Ready to take flight: 155 million year old: Dinosaur Bird prototype Paraves Xiaotingia shows that several features, including long, robust forearms, with lots of long feathers, that were pre-Bird, indeed. It also is equipped with pants which it used as a second pair of wings (that was typical of early birds).

So let’s backtrack. Some small pterosaurs lived in forests, among trees, flying here and there. Scurrying below, 155 million years ago, were dinosaurs. Some of the dinosaurs looked pretty much like chickens with feathered arms much more powerful than chicken wings, but also claws to climb up trees. Early birds learned to glide and parachute from trees before achieving full-fledged flight. They descended from arboreal parachuters and gliders, similar to flying squirrels. Naturally the feathers on their arms grew to accommodate ever more gliding. Except that, having feathers, dinosaur-birds could augment the area of these protowings in a way squirrels cannot. Once so equipped dinosaur-birds (technically called pre-birds, proto-aves) thrived more than Pterosaurs, as they could fly better among vegetation than Pterosaurs. Feathered, clawing dinosaurs had risen to the occasion, and successfully competed.

If this theory is correct, one should observe, in the paleontological record, the progressive disappearance of pterosaurs in forests colonized by birds, were they were out-competed by birds, and then, 50 million years later, bats… which had another trick up their ears… [1]

These are interesting considerations bearing more general philosophical lessons:  first that hardware choices can have long term consequences. Pterosaurs had wings made of skin, muscles and fibre. Right, they had no need of flight feathers. But, it’s not just that they didn’t evolve them… Pterosaurs had a kind of down, but couldn’t turn it to flight feathers, from the structure of their existing wings. It’s a case of HARDWARE INERTIA. Birds started from clean slate, or rather, clean arms…  [2]

Then something extraordinary happened: after they took flight birds evolved very powerful brains. Or was it like feathers, a much older trait? If so, then dinosaurs were really smart. In any case, we can see form following function. Birds took to flight, showing that, if there is a will, there is a wing, and a prayer.

Patrice Ayme

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[1] So what of bats, then? Bats evolved probably 100 million years ago, or so, in tropical areas… echo-locating moths have been found at 85 years Before Present, showing bats were solidly established by then (also bats are close to no other mammal group, so had long evolved).  Just like birds had the trick of having feathers, which pterosaurs couldn’t easily evolve, the bats had large mobile ears, something dinosaurs had not evolved. And thus bats could evolve echolocation. That was an advantage over birds, which compensated for their more fragile wings (comparable to pterosaur wings). 

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[2] Pterosaurs had down like feathers to help keep them warm, just like many dinosaurs. The feathers they had are small and tufty. They possessed a dense filamentous covering the entire body, at least in anurognathids, and were probably for insulation, tactile sensing, signalling and aerodynamics. Fossils found as long ago as the 1840s revealed that pterosaurs had fur on their head and bodies. Palaeontologists came up with the term “pycnofibres” to describe it, to distinguish it from the hair of mammals and the feathers of birds. In recently discovered fossils, these pycnofibres are exceptionally well preserved. Much of the head, body and limbs of these pterosaurs were covered by hair-like filaments, just we have long thought was the case. Microscopic studies of the pterosaur feathers revealed pigment-containing structures called melanosomes, whose shape suggests known fossil pterosaurs were a gingery brown colour in life. The ones well-studied were small animals that probably flitted from tree to tree in forests, and fed on insects.