How do we know that dinosaurs were related to lizards and birds?

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Jesse asked the Naked Scientists:
Hey Naked Scientists,

Here's a question: How do we know that dinosaurs were related to lizards and birds? If all we have is bones, couldn't they have been related to mammals or something?

Ithaca, NY, USA

What do you think?


Offline Don_1

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There are significant differences between reptilian and mammalian skeletons.

The teeth and lower jaw bone are a good example. Reptiles have teeth which are all the same conical shape, though they may vary in size, while mammals have different teeth for different purposes. Canines, molars and incisors. The mammalian lower jaw is comprised of a single bone while the reptilian lower jaw is comprised of several bones. Also, the reptilian skull has a small hole, known as the 'third eye' which mammals do not have.

The mammals probably evolved from reptiles before the age of the dinosaurs, but the dinosaurs were the 'top dogs' and so came to prominence above the odd few mammals. Synapsids of the late Permian period show a distinct move away from their reptilian cousins.

In the case of the tritylodonts, the two remaining sections of the jaw bone were almost fused together and it had specialised teeth for different purposes. By the time the Cynognathus came along, in the lower Triassic period, it is difficult to place this skeleton alongside the reptiles. It has become distinctly mammalian.

So you see, the bones can tell much about the animal.

Ooo, I should add, there are other skeletal differences.
« Last Edit: 12/11/2009 14:10:40 by Don_1 »
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Offline LeeE

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Ooo, I should add, there are other skeletal differences.

I believe that the hips are one of these other skeletal differences, or similarities, depending on which way you look at it.
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Offline Nizzle

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I think the hips of the (biped) dino skeletons is what proves to us that birds are descendants of dinos since they're very similar
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