Birds are one of the most distinctive and transformed groups of modern vertebrates.
Technical name for birds is Aves, and “avian” means “of or concerning birds”.
Birds have many unusual synapomorphies among modern animals:
Difficult to find relatives using only modern animals (turtles have modified necks and toothless beaks, but otherwise very different; bats fly and are warm-blooded, but are clearly mammals; etc.)
With discovery of fossils, other potential relations: pterosaurs had big brains, “S”-shaped neck, hinge-like foot, but wings are VERY different.
In 1859, Darwin published the Origin; some used birds as a counter-example against evolution, as there were apparently know transitional forms between birds and other vertebrates. In 1860, a feather (identical to modern birds' feathers) was found in the Solnhofen Lithographic Limestone of Bavaria, Germany: a Late Jurassic formation. The following year, the skeleton of this feather-bearer was found.
This creature was named Archaeopteryx lithographica: ancient wing of the Lithographic limestone.
Like modern birds, it had:
Many saw Archaeopteryx as transitional between “normal” reptiles and birds. This specimen was bought by the British Museum (Owen's place of work).
A second, and much better, specimen was found in 1877: this one was bought by the Humbolt Museum in Berlin. This specimen showed that the skull DID have teeth!
(During the 20th Century, five more Archaeopteryx skeletons have been found: all from the Solnhofen Limestone).
With the discovery of these primitive bird specimens, could now better compare birds
with potential ancestors:
Thomas Huxley (Darwin's protégé) and O. C. Marsh suggested birds might be dinosaur descendants (as they shared many features). Huxley showed that bird ancestors were clearly archosaurs, since Archaeopteryx had an antorbital fenestra (among other features).
During late 19th Century, the dinosaurian origin of birds was the best option.
During the early 20th Century, a new hypothesis was suggested. In 1910, discovery of Euparkeria (a very primitive archosaur about the size of a cat). Its discover suggested it might have been “…very near the ancestor of the Dinosaurs, Pterodacyles (sic), Birds and Crocodiles”.
This idea was amplified by artist and natural historian Gerhard Heilmann. Heilmann examined lots of Mesozoic archosaurs. He considered coelurosaurian dinosaurs to have the most shared features with birds, but thought that they didn't have furculae or clavicles (and therefore didn't have anything that could evolve into bird clavicles). He settled instead on Euparkeria, suggesting that it was sufficiently primitive to be a bird ancestor. Book The Origin of Birds published in 1927 in English (earlier Dutch version not as influential). Became THE main hypothesis by far from the 1920s until the 1970s/1980s.
(Heilmann also championed a particular theory of the origin of avian flight: from quadrupedal ground dweller to quadrupedal tree climber to semiquadrupedal glider to bipedal flier).
In 1970, John Ostrom was examining a supposed pterosaur specimen in the Tyler Museum in Haarlem, the Netherlands. Recognized it was really an Archaeopteryx (had actually been found in 1855, long before the better specimens), but also recognized that the manus was almost identical in every detail to that of Deinonychus!
Ostrom documented many features found in Archaeopteryx also found in various theropods, most especially coelurosaurs, most especially Deinonychus:
Ostrom also revised an earlier idea that flight arose from fully terrestrial bipeds rather than going through a tree-dwelling gliding stage; noted that birds show no sign of using their hindlimbs in flight, whereas almost all gliders do.
In 1980s and 1990s cladistic work helped establish the relationships of birds WITHIN the dinosaurs. In mid-to-late 1990s, discovery of other coelurosaurian dinosaurs with feathers!!
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