I spent most of my childhood with books about dinosaurs that played up the ancient beasties as overgrown lizards. The connection between dinosaurs and birds, while kind of flipping obvious once somebody points it out, was not much discussed among laypeople until I was in my teens. (That would be the 1990s, FYI.)
But, among scientists, the idea of a dinosaur-bird relationship is nothing new. In fact, Thomas Henry Huxley was making that connection back in the 1860s. On the Dinosaur Tracking blog, Brian Switek tells the fascinating story of how Huxley started to realize that dinosaurs and birds were related—a discovery that's all the more impressive because he figured it out without the help of some of the key transitional fossils we have access to today.
Huxley did not suggest that birds were the direct descendants of dinosaurs. So much geologic time was unaccounted for, and so few dinosaurs were known, that Huxley could not point to any known fossil creature as the forerunner of birds. Instead he made his argument on anatomical grounds and removed the issue of time. Dinosaurs were proxies for what the actual bird ancestor would have been like, and flightless birds (such as the ostrich and emu) stood in for what Huxley thought was the most archaic bird type. (We now know that Huxley got this backwards—the earliest birds could fly, and flightless birds represent a secondary loss of that ability.) As Huxley went about collecting evidence for his case, though, he also gave dinosaurs an overhaul. They were not the bloated, plodding, rhinoceros-like creatures that Richard Owen had envisioned. Dinosaurs were more bird-like than anyone had imagined.
In October of 1867, Huxley met with John Philips, an English geologist and a curator of Oxford’s museum. As Huxley related in his 1870 paper “Further Evidence of the Affinity Between the Dinosaurian Reptiles and Birds,” Philips wanted to discuss details of marine reptiles called ichthyosaurs in his museum’s collection, but as he and Huxley made their way over toward the displays they stopped to look at the bones of the carnivorous dinosaur Megalosaurus. Then Huxley spotted something peculiar:
As Prof. Phillips directed my attention to one after the other of the precious relics, my eye was suddenly caught by what I had never before seen, namely, the complete pectoral arch of the great reptile, consisting of a scapula and a coracoid ankylosed together. Here was a tangle at once unravelled. The coracoid was totally different from the bone described by Cuvier, and by all subsequent anatomists, under that name. What then was the latter bone? Clearly, if it did not belong to the shoulder-girdle it must form a part of the pelvis; and, in the pelvis, the ilium at once suggested itself as the only possible homologue. Comparison with skeletons of reptiles and of birds, close at hand, showed it to be not only an ilium, but an ilium which, though peculiar in its form and proportions, was eminently ornithic in its chief peculiarities.
Earlier naturalists had made a mistake. They had misidentified the shoulder girdle, and one part of what was thought to be part of the shoulder was actually part of the hip. Another strange piece, previously thought to be a clavicle, also turned out to belong to the pelvis. This rearrangement immediately gave the dinosaur a more bird-like character.
If you look at the bottom of the image at the top of this post, you can see how much the re-arrangement of megalosaurus' parts changed our conception of what the whole creature looked like. Where other scientists saw a lumpy, obese crocodile, Thomas Henry Huxley saw a saber-toothed chicken.
Image: Ballista via CC