I am delighted with the recent revolution in scientific thought about dinosaurs—that they did not die out after all, but that some of them evolved into birds. For one thing, it’s a revolution that I have observed happening for most of my life. Like many turnabouts in thought, it takes twenty to thirty years to go from a notion to a hypothesis to an almost fully proven theory (meaning a testable explanation with predictive powers). This is not an unheard-of idea: Huxley suggested around 1870 that birds were descended from dinosaurs, but finally solid evidence is coming in!
The recent exhibit at the Royal Ontario Museum outlined the process step by step and included some surprises as well. Birds’ hollow bones help them to fly. But the tyrant lizards such as Allosaurus were the ones that developed hollow bones as they grew large and needed to retain strength without excess weight. When smaller dinosaurs began to run or glide and fly, they already had hollow bones that gave them lightweight strength. We often see pre-adaption in evolution: a trait developed for one reason is “re-purposed” to serve another.
And feathers did not have to evolve “with flight in mind.” Feathers could provide thermal insulation, protection from the sun, stabilization for running or climbing, colour schemes for camoflage or display, seasonal colour changes, and finally gliding surfaces before being adapted for powered flight.
And the history of the feather is extending farther back in time: pterodactyls, which flew with wings of skin, are now thought to have been covered with some down, which might have been protofeathers. The skin shows markings like gooseflesh, not like hair follicles.
We now have “missing link” fossils of various kinds where the shoulder girdle and symmetrical feathers show that the dinosaur did not fly. And we have “birdlike” fossils where the shoulder girdle and asymmetrical feathers show that the dinosaur did fly.
This would be a good controversy to teach about evolution, because it is more or less settled, because children are fascinated by dinosaurs, and because it shows how evidence changes conclusions and makes a stronger theory.