Even if you despise science, and hate palaeontology with special fervour, you have probably noticed that these days, pretty much everyone accepts that birds descended from dinosaurs. You may have also noticed that a large number of theropods are now regularly depicted with feathers. You probably looked at your calendar realized that it wasn’t April Fools Day, and wondered what was going on. How did this epiphany come about?
We ended part one with Thomas Huxley crushing Richard Owen’s reputation, and riding high with his cohorts on a wave of dinosaur-bird theories. However, trouble was appearing on the horizon, in the form of Gerhard Heilmann, a Danish doctor-turned-artist-turned-palaeontologist.
Heilmann became interested in birds, and between 1913 and 1916, published a series of articles on the evolution of birds in Dansk Ornitologisk Forenings Tidskrift (Journal of the Danish Ornithological Society). These were collected, and Vor Nuværende Viden om Fuglenes Afstamning was published in 1916. In 1926, an English version was published, under the name Origin of Birds.
Heilmann agreed with earlier work that demonstrated striking similarities between theropods and modern birds, but threw the proverbial wrench into the works. He asserted (incorrectly), that theropods did not have clavicles, the bones that fuse together in birds to form the wishbone. Since earlier reptiles had possessed clavicles, Heilmann assumed that theropods had lost them through evolution, and, according to Dollo’s Law, would not be able to re-evolve them.
Therefore, he concluded, birds must have evolved from earlier reptiles. He declared them descendants of Thecodont reptiles, a “Wastebasket Taxon” that existed mainly for reptiles that didn’t fit neatly into any of the categories palaeontologists had established. For Heilmann, it was clavicles or bust. The number of striking similarities between birds and theropods were chalked up to evolutionary convergence.
Scientists loved Heilmann’s work. It was detailed, and methodical. The hypothesis laid out in Origin of Birds formed the basis of the next five decades of thought on the origin of birds. This period was apparently not the height of scientific thought. For instance, the discovery in 1936 of clavicles on Segisaurus, a theropod, didn’t put a dent in the Thecodont hypothesis.
In fact, no compelling evidence existed for the Thecodont hypothesis, but it was very difficult to disprove, because of the nature of the Thecodont group. Because it is a “Wastebasket Taxon,” its members are not bound together by a set of similarities, but simply by the fact that they don’t fit into any other group. Therefore, there are no reasonable criteria to use in comparing modern birds and Thecondonts.
This made the Thecodont hypothesis a good fallback as soon as any doubts were raised about a different, more testable, theory. It was a very safe position to have, and for almost fifty years, palaeontologists largely stuck with it.
This is part two of a three part series on the history of research into the link between birds and theropod dinosaurs. Part three will be posted tomorrow.
Wikipedia: Dollo’s Law - http://en.wikipedia.org/wiki/Dollo's_law
Dino-Bird Relationships - http://www.ucmp.berkeley.edu/diapsids/avians.html
The Origin and Early Evolution of Birds - http://www.ucmp.berkeley.edu/diapsids/avians.html
Wikipedia: Thecodont - http://en.wikipedia.org/wiki/Thecodont