Where did birds come from?


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Did birds evolve from dinosaurs?

Othniel Marsh

Othniel Marsh

A lot of people think, that dinosaurs are extinct. This, however, may not be the case, because it is possible, that dinosaurs have descendants: the birds. In the 19th century this theory was presented by Thomas Huxley and Othniel Marsh, but in 1925 Gerhard Heilmann published a book, which criticised this theory.

Only recently, in the last few years, most palaeontologists agreed that dinosaurs were, in fact, the ancestors of birds. Here I would like to explain this theory, by listing arguments for and against it.


Similarities between birds and dinosaurs

Thomas Huxley

Thomas Huxley

The British palaeontologist Thomas Huxley presented a theory in 1868, which claimed that birds are descended from dinosaurs. His theory stated these reasons for it:

  • Only dinosaurs had the same sort of ankle joint as birds
  • Only dinosaurs had the same sort of ilium as birds
  • Some dinosaurs had the same sort of rear foot as birds with a rear-pointing fourth toe (phalanx)
  • Advanced dinosaurs had the same sort of build as birds with a short torso, stable hips, long and very flexible neck and long rear limbs
  • Only dinosaurs and pterosaurs had hollow bones or the air sacks which are connected to a bird-like lung. Pterosaurs are in every other aspect a lot less bird-like than advanced dinosaurs
  • Some dinosaurs had the same sort of hip as birds (ornithischian dinosaurs).
Deinonychus

Deinonychus

In America the palaeontologist Marsh came up with the same idea and although not everyone believed in evolution, this theory was accepted until the 20's.

In 1925 the German palaeontologist Gerhard Heilmann wrote a book in which he accepted that birds and dinosaurs had a lot in common, but that the dinosaurs from which birds are supposed to have descended had no, or extremely small, collarbones. Because the ancestors of the dinosaurs, like birds, had big collarbones, Heilmann argued that birds and dinosaurs had a common ancestor and that birds couldn't have descended from dinosaurs. Other palaeontologists were convinced by this theory and it was believed to be correct until the 1960's.

In the 1960's the American palaeontologist John Ostrom started examining Deinonychus. He discovered that the skeleton was bird-like in every way. When he saw an archaeopteryx skeleton he realised that it was very similar to that of a deinonychus. The Hands, shoulders, hips, thighbone and ankle joints were very similar. Deinonychus had remarkable shoulder blades and hips that were identical to those of archaeopteryx. Only the wrist joints of deinonychus and archaeopteryx are identical to those of birds.

This suggests that birds are the descendants of dinosaurs.

Click here to see a comparison of the hands of dinosaurs and birds.


The changes

Archäopteryx

Archaeopteryx fossil

Of course, all these changes didn't just happen overnight but took many millions of years. We can tell this from the fossils of primitive reptiles which have birdlike collar bones, which dinosaurs did not have. Then in the triassic period the first dinosaurs with a birdlike build appeared, such as Coelophysis. In the jurassic Archäopteryx appeared (see picture). This was a primitive bird with teeth that probably could not fly properly. Then in the cretaceous period birds appeared which were not so primitive, such as Hesperonis and Ichthyornis. Finally, at the end of the cretaceous period, modern birds without teeth appeared.

Which dinosaurs are birds descended from?

Archaeopteryx

How Archaeopteryx may have looked

Dinosaurs are divided into two orders: The Ornithischians (bird-hipped) and Saurischians (lizard-hipped). The order of the Saurischians is divided into the suborders Sauropodomorpha and Theropoda. The Theropods are divided into several infraorders (like Coelurosauria) and several families (like Carnosauria). The Theropods are divided into: the family of Ceratosauria and the infraorder Tetanurae. The Tetanurae are divided into the family of Carnosauria and the infraorder Coelurosauria. The Coelurosauria are divided into the family of the Ornithomimids and the infraorder Maniraptora. The Maniraptora are divided into two infraorders: The infraorder Dromaeosauria and the infraorder Aves (birds).

Click here to see a chart showing dinosaur classification.

What other evidence is there?

Willo

Willo

Another similarity between dinosaurs and birds, which wasn't mentioned by Huxley, is that dinosaurs were probably warm-blooded (like birds). Evidence of this is "Willo, the dinosaur with a heart". Willo is a unique fossil of a Thescelosaurus, because it is the only fossil of a dinosaur where the heart was preserved. This heart had four chambers and only one aorta. This points to a warm-blooded creature like birds and not like cold-blooded reptiles.

Another argument is, that dinosaurs have been found with feathers. This shows that feathers were originally for warmth and then adapted for flying.

Dinosaurs also share other features with birds; they laid eggs in nests and sometimes protected their young. Some species even had nesting colonies and returned to the same place to lay their eggs each year, like some birds today.

According to recent studies of the leg bones of dinosaur hatchlings, some dinosaurs could not stand up for some time after hatching. Like birds today, dinosaurs must have relied on their parents to care for them for some time after they had hatched.

Were dinosaurs warm or cold-blooded?

The theory of the common ancestor

Avimimus

Avimimus was a birdlike dinosaur

Because birdlike dinosaurs had only very small collar bones and birds have very large ones, the theory that birds are descended from dinosaurs was rejected. It was belived that, Dinosaurs had the same ancestors as birds, until the following was proven scientifically: there are genes which are suppressed but exist nevertheless. They can be reactivated in situations when they are needed. Chickens, for example, have a suppressed gene which would allow them to develop teeth if they needed them. Scientists transplanted this gene into another part of the chicken's body where it could be reactivated and the chickens developed teeth. Dinosaurs probably had a suppressed gene which allowed them to develop larger collar bones. This is what deinonychus did, which had a collar bone very similar to that of archäopteryx. Deinonychus also had a hand very like that of archaeopteryx. It is therefore very likely that birds are descended from dinosaurs.

Click here to see the way that birds developed.

So the dinosaurs did not all die out - they merely developed into birds!


If you would like to find out more about this, I suggest you read "The Dinosaur Heresies" by Bob Bakker, which is one of the best books ever written about dinosaurs.