Heilmann and The Thecodont Hypothesis
A turning point came in the early twentieth century with the writings of Gerhard Heilmann of Denmark. An artist by trade, Heilmann had a scholarly interest in birds and from 1913 to 1916 published the results of his research in several parts, dealing with the anatomy, embryology, behavior, paleontology, and evolution of birds. His work, originally written in Danish as Vor Nuvaerende Viden om Fuglenes Afstamning, was compiled, translated into English, and published in 1926 as The Origin of Birds.
Like Huxley, Heilmann compared Archaeopteryx and other birds to an exhaustive list of prehistoric reptiles, and also came to the conclusion that theropod dinosaurs like Compsognathus were the most similar. However, Heilmann noted that birds had clavicles (collar bones) fused to form a bone called the furcula ("wishbone"), and while clavicles were known in more primitive reptiles, they had not yet been recognized in dinosaurs. Since he was a firm believer in Dollo's law, which states that evolution is not reversible, Heilmann could not accept that clavicles were lost in dinosaurs and re-evolved in birds. He was therefore forced to rule out dinosaurs as bird ancestors and ascribe all of their similarities to convergent evolution. Heilmann stated that bird ancestors would instead be found among the more primitive "thecodont" grade of reptiles. Heilmann's extremely thorough approach ensured that his book became a classic in the field, and its conclusions on bird origins, as with most other topics, were accepted by nearly all evolutionary biologists for the next four decades.
Clavicles are relatively delicate bones and therefore in danger of being destroyed or at least damaged beyond recognition. Nevertheless, some fossil theropod clavicles had actually been excavated before Heilmann wrote his book but these had been misidentified. The absence of clavicles in dinosaurs became the orthodox view despite the discovery of clavicles in the primitive theropod Segisaurus in 1936. The next report of clavicles in a dinosaur was in a Russian article in 1983.
Contrary to what Heilmann believed, paleontologists now accept that clavicles and in most cases furculae are a standard feature not just of theropods but of saurischian dinosaurs. Up to late 2007 ossified furculae (i.e. made of bone rather than cartilage) have been found in all types of theropods except the most basal ones, Eoraptor and Herrerasaurus. The original report of a furcula in the primitive theropod Segisaurus (1936) was confirmed by a re-examination in 2005. Joined, furcula-like clavicles have also been found in Massospondylus, an Early Jurassic sauropodomorph.
Famous quotes containing the word hypothesis:
“The wheels and springs of man are all set to the hypothesis of the permanence of nature. We are not built like a ship to be tossed, but like a house to stand.”
—Ralph Waldo Emerson (18031882)