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Origin of birds

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Huxley, Archaeopteryx and early researchEdit

Scientific investigation into the origin of birds began shortly after the 1859 publication of Charles Darwin's Origin of Species, the ground-breaking book which described his theory of evolution by natural selection.[1] In 1860, a fossilized feather was discovered in Germany's Late Jurassic solnhofen limestone. Christian Erich Hermann von Meyer described this feather as Archaeopteryx lithographica the next year,[2] and Richard Owen described a nearly complete skeleton in 1863, recognizing it as a bird despite many features reminiscent of reptiles, including clawed forelimbs and a long, bony tail.[3]

Biologist Thomas Henry Huxley, known as "Darwin's Bulldog" for his ferocious support of the new theory of evolution, almost immediately seized upon Archaeopteryx as a transitional fossil between birds and reptiles. Starting in 1868, Huxley made detailed comparisons of Archaeopteryx with various prehistoric reptiles and found that it was most similar to dinosaurs like Hypsilophodon and Compsognathus.[4][5] The discovery in the late 1870s of the iconic "Berlin specimen" of Archaeopteryx, complete with a set of reptilian teeth, provided further evidence. Huxley was the first to propose an evolutionary relationship between birds and dinosaurs, although he was opposed by the very influential Owen, who remained a staunch creationist. Huxley's conclusions were accepted by many biologists, including Baron Franz Nopcsa,[6] while others, notably Harry Seeley,[7] argued that the similarities were due to convergent evolution.

Heilmann and the thecodont hypothesisEdit

Heilmann 1916 fighting proaves

Heilmann's hypothetical illustration of a pair of fighting 'Proaves' from 1916

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.[8] 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 possessed 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.[9] 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.[10]

Clavicles are relatively delicate bones and therefore in danger of being destroyed or at least damaged beyond recognition. Nevertheless clavicles had been found in theropod dinosaurs before Heilman wrote his book, but had gone unrecognized.[11] The absence of clavicles in dinosaurs became the orthodox view despite the discovery of clavicles in the primitive theropod Segisaurus in 1936.[12] The next report of clavicles in a dinosaur was in 1983, and that was in a Russian article published before the end of the Cold War.[13]

Contrary to what Heilman 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 nearly all types of theropods except the most basal ones, Eoraptor and Herrerasaurus.[14] The original report of a furcula in the primitive theropod Segisaurus (1936) has been confirmed by a re-examination in 2005.[15] Joined, furcula-like clavicles have also been found in Massospondylus, an Early Jurassic sauropodomorph.[16]

Clavicles and even fully developed furculae have since been identified in numerous other non-avian theropods.[17][18]

Since then furculae have been reported in a wide range of theropods, Allosaurus[19]

FurculaeEdit

At one time, it was believed that dinosaurs lacked furculae (fused left and right clavicles, or "wishbones"). This was considered an overwhelming argument to refute the dinosaur ancestry of birds by Heilmann (1926). However, it has been shown since then that numerous tetanuran theropod species indeed have a furcula,[20] apparently a tetanuran innovation. The presence of a furcula even in Allosaurus, a relatively basal tetanuran, has been confirmed, and in an Early Jurassic theropod[21] and Late Triassic coelophysids, among others. There is evidence that the evolution of the furcula was well underway when the earliest dinosaurs were diversifying; joined, furcula-like clavicles are known from Massospondylus, an Early Jurassic sauropodomorph.[16]


ReferencesEdit

  1. ^ Darwin, Charles R. (1859). On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life. London: John Murray. p. 502pp. http://darwin-online.org.uk/content/frameset?itemID=F373&viewtype=side&pageseq=16. 
  2. ^ von Meyer, C.E. Hermann. (1861). "Archaeopteryx lithographica (Vogel-Feder) und Pterodactylus von Solnhofen" (in German). Neues Jahrbuch für Mineralogie, Geologie und Paläontologie 1861: 678–679. 
  3. ^ Owen, Richard. (1863). "On the Archeopteryx [sp] of von Meyer, with a description of the fossil remains of a long-tailed species, from the lithographic stone of Solenhofen [sp]". Philosophical Transactions of the Royal Society of London 153: 33–47. doi:10.1098/rstl.1863.0003. 
  4. ^ Huxley, Thomas H. (1868). "On the animals which are most nearly intermediate between birds and reptiles". Annals of the Magazine of Natural History 4 (2): 66–75. 
  5. ^ Huxley, Thomas H. (1870). "Further evidence of the affinity between the dinosaurian reptiles and birds". Quarterly Journal of the Geological Society of London 26: 12–31. 
  6. ^ Nopcsa, Franz. (1907). "Ideas on the origin of flight". Proceedings of the Zoological Society of London: 223–238. 
  7. ^ Seeley, Harry G. (1901). Dragons of the Air: An Account of Extinct Flying Reptiles. London: Methuen & Co.. p. 239pp. 
  8. ^ Nieuwland, Ilja J.J. (2004). "Gerhard Heilmann and the artist’s eye in science, 1912-1927". PalArch's Journal of Vertebrate Palaeontology 3 (2). http://www.palarch.nl/wp-content/ver_2004_3_2.pdf. 
  9. ^ Heilmann, Gerhard (1926). The Origin of Birds. London: Witherby. p. 208pp. 
  10. ^ Padian, Kevin. (2004). "Basal Avialae". in Weishampel, David B.; Dodson, Peter; & Osmólska, Halszka (eds.). The Dinosauria (Second ed.). Berkeley: University of California Press. pp. 210–231. ISBN 0-520-24209-2. 
  11. ^ For example in 1923, 3 years before Heilmans's book, Roy Chapman Andrews found a good Oviraptor fossil in Mongolia, but Henry Fairfield Osborn, who analyzed the fossil in 1924, mis-identified the furcula as an interclavicle; described in Paul, G.S. (2002). Dinosaurs of the Air: The Evolution and Loss of Flight in Dinosaurs and Birds. JHU Press. ISBN 0801867630. http://books.google.com/books?id=OUwXzD3iihAC&pg=PA9&lpg=PA9&dq=oviraptor+furcula&source=web&ots=H-AeT6WeoR&sig=K8QkvBqB7RDf5aq0eZh8oKpEMto#PPA9,M1. 
  12. ^ Camp, Charles L. (1936). "A new type of small theropod dinosaur from the Navajo Sandstone of Arizona". Bulletin of the University of California Department of Geological Sciences 24: 39–65. 
  13. ^ In an Oviraptor: Barsbold, R. (1983). "[Carnivorous dinosaurs from the Cretaceous of Mongolia". Trudy Soumestnaya Sovetsko-Mongol'skaya Paleontogicheskaya Ekspeditsiya 19: 1–117.  (in Russian!) See the summary an pictures at "A wish for Coelophysis". http://www.hmnh.org/archives/2007/10/11/a-wish-for-coelophysis/. 
  14. ^ Lipkin, C., Sereno, P.C., and Horner, J.R. (November 2007). "THE FURCULA IN SUCHOMIMUS TENERENSIS AND TYRANNOSAURUS REX (DINOSAURIA: THEROPODA: TETANURAE)". Journal of Paleontology 81 (6): 1523–1527. doi:10.1666/06-024.1. http://jpaleontol.geoscienceworld.org/cgi/content/extract/81/6/1523.  - full text currently online at "The Furcula in Suchomimus Tenerensis and Tyrannosaurus rex". http://www.redorbit.com/news/health/1139122/the_furcula_in_suchomimus_tenerensis_and_tyrannosaurus_rex_dinosauria_theropoda/index.html.  This lists a large number of theropods in which furculae have been found, as well as describing those of Suchomimus Tenerensis and Tyrannosaurus rex.
  15. ^ Carrano, M,R., Hutchinson, J.R., and Sampson, S.D. (December 2005). "New information on Segisaurus halli, a small theropod dinosaur from the Early Jurassic of Arizona". Journal of Vertebrate Paleontology 25 (4): 835–849. doi:10.1671/0272-4634(2005)025[0835:NIOSHA]2.0.CO;2. http://www.rvc.ac.uk/AboutUs/Staff/jhutchinson/documents/JH18.pdf. 
  16. ^ a b Yates, Adam M.; and Vasconcelos, Cecilio C. (2005). "Furcula-like clavicles in the prosauropod dinosaur Massospondylus". Journal of Vertebrate Paleontology 25 (2): 466–468. doi:10.1671/0272-4634(2005)025[0466:FCITPD]2.0.CO;2. 
  17. ^ Chure, Daniel J.; & Madsen, James H. (1996). "On the presence of furculae in some non-maniraptoran theropods". Journal of Vertebrate Paleontology 16 (3): 573–577. 
  18. ^ Norell, Mark A.; & Makovicky, Peter J. (1999). "Important features of the dromaeosaurid skeleton II: Information from newly collected specimens of Velociraptor mongoliensis". American Museum Novitates 3282: 1–44. http://hdl.handle.net/2246/3025. 
  19. ^ Chure, D. J. and Madsen, J. H. (September 1996). "On the presence of furculae in some non-maniraptoran theropods". Journal of Vertebrate Paleontology 16 (3): 573–577. 
  20. ^ Included as a cladistic definer, e.g. (Columbia University) Master Cladograms or mentioned even in the broadest context, such as Paul C. Sereno, "The origin and evolution of dinosaurs" Annual Review of Earth and Planetary Sciences 25 pp 435-489.
  21. ^ Matthew R. Carrano, John R. Hutchinson and Scott D. Sampson, 2005. "New information on Segisaurus halli, a small theropod dinosaur from the Early Jurassic of Arizona" Journal of Vertebrate Paleontology 25.4, (December 2005) pp 835-849.

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