<p>First, let me link a variety of popular science articles which can give you some background explanation. Here are <a href="https://www.scientificamerican.com/article/how-dinosaurs-shrank-and-became-birds/" rel="nofollow noreferrer">Scientific American</a>, <a href="http://www.smithsonianmag.com/science-nature/dinosaurs-living-descendants-69657706/?no-ist" rel="nofollow noreferrer">Smithsonian</a>, and <a href="http://www.audubon.org/magazine/january-february-2015/which-came-first-dinosaur-or-bird" rel="nofollow noreferrer">Audubon</a> magazines. </p>

<p>The 'smoking gun' evidence linking birds and dinosaurs is the evolution of feathers. Feathers are a feature that set the birds apart morphologically from the rest of the tetrapods, and indeed from the rest of all animals. All birds have feathers and nothing that isn't a bird has feathers. So a logical place to start looking for the origin of birds is the origin of feathers. </p>

<p>Since the dawn of modern paleontology, there had always suspected links between dinosaurs and birds due to the fossils of <a href="https://en.wikipedia.org/wiki/Archaeopteryx" rel="nofollow noreferrer">Archaeopteryx</a> found in the 1860s. <a href="https://en.wikipedia.org/wiki/Sauropsida#Huxley_and_the_fossil_gaps" rel="nofollow noreferrer">Thomas Huxley</a> claimed the two groups were related as early as 1863. But it wasn't until recent decades that the fossil record started being filled in with definitive links between Archaeopteryx and therapod dinosaurs, such as more species intermediate between birds and therapods and fossil impressions indicating that some therapods had feathers. </p>

<p>The rest is just filled in by logical deduction, or <a href="https://en.wikipedia.org/wiki/Occam&#39;s_razor" rel="nofollow noreferrer">Occam's Razor</a>. There is clear evidence that all birds have feathers, and no other animals alive today have feathers. There is now clear evidence that therapods had very similar skeletons to birds with feathers about 150 million years ago. There is also evidence that many therapods had feathers, a trait otherwise confined to birds.</p>

<p>Let our null hypothesis be that the origin of birds is unknown, and our alternative hypothesis be that birds evolved from dinosaurs. Given the similarities noted above (and, keep in mind, the evidence is much stronger for scientists who specialize in this area), do you think there is enough evidence to reject the null hypothesis and conclude that birds evolved from dinosaurs? If not, are there any plausible alternative hypotheses?</p>

<p>The great thing about science is that you never have to take someone else's word for it. Everyone has the right to be a skeptic, just keep an open mind and look at the evidence and you can draw your own conclusions. </p>

CC BY-SA 3.0
2016-11-03T19:09:32.280Z
Feathers are rarely preserved, and yet we now have specimens from every major theropod group with feathers. Sometimes it was more of a downy covering - eg. Tyrannosaurs, but you get the idea. Yes cuddly T Rex :-)
winwaed
CC BY-SA 3.0
2016-11-03T20:19:39.323Z
Feathers doth not a cuddly make, neither for [birds](http://i2.wp.com/listverse.com/wp-content/uploads/2013/02/IMG_9167.jpg?resize=600%2C409), nor for [dinos](http://i.imgur.com/IsPnSWe.jpg).
kingledion
CC BY-SA 3.0
2016-11-04T05:05:13.213Z
If therapods were the ancestors of modern birds, how did they escape the extinction mechanism/event that killed off all of the other dinosaurs, in order to evolve into pigeons and such?
Jessica
CC BY-SA 3.0
2016-11-04T11:08:36.817Z
That is a great question. It is not completely clear why any one set of creatures survived mass extinction events while other sets did not. But to clarify a bit, the therapods that evolved into birds were already quite birdlike by the time of the K-T extinction event. So far as we know, the set of therapods that were very small survived, all other, larger ones did not. We see something similar with other land creatures (small mammals and lizards survived, larger dinosaurs did not). So the reason probably has something to do with size.
kingledion
CC BY-SA 3.0
2016-12-10T14:45:02.723Z
feathers are such an unusual and unique structure that it is unbelievably unlikely to have evolved twice.
John
CC BY-SA 3.0
2016-12-10T20:05:16.140Z
@kingledion: I'd say that the feathered creatures that survived the K-T extinction WERE birds.  IOW, the theropod/bird split was tens of millions of years prior to the extinction. There were 4 distinct lineages of birds that survived; several other lineages didn't.  One recent theory is that the survivor's beaks allowed them to peck out buried seeds:  http://www.bbc.com/news/science-environment-36102018
jamesqf
CC BY-SA 3.0
2016-12-12T08:56:57.067Z
In addition to feathers there is also the furcula (see [Nesbitt et al. 2009](https://www.researchgate.net/profile/Sterling_Nesbitt/publication/23996833_The_Theropod_Furcula/links/5432250e0cf277d58e983aa9.pdf) for instance).
plannapus
CC BY-SA 3.0
2016-11-03T15:18:45.397Z
2016-11-03T15:18:45.397Z
kingledion
<p>One factor that helps you survive mass extinctions is small size becasue there are more of you and you can repopulate faster. Early birds are among the smallest dinosaurs. </p>

<p>Other indicators of their relationship besides feathers which Kingledon covers are eggshells, dinosaurs and birds both have hard shelled eggs with identical microstructure. most dinosaurs have round spherical eggs but theropods have asymmetric elongate eggs just like birds. </p>

<p>The shape of the hand and forearm of early birds and maniraptoran dinosaurs are almost identical. both theropods and aves also have air-sac based breathing. Honestly in anatomy maniraptoran dinosaurs and early birds blur together so seamlessly there really is no compulsion other than a direct relationship.</p>

<p>what throws most people off is the tail, but early birds have long tails (and teeth). we see a similar evolutionary path in pterosaurs, flying vertebrates start with long tails for passive stability but as they become better at flying shorten it to gain maneuverability and use better active control for stability. </p>

<p>I was hoping to post this comparison of the skeletons of late maniraptorans and early birds done by Scott Hartman but the resolution sucks. Hopefully you can zoom in and see how seamlessly one blends into the other.</p>

<p><a href="https://i.sstatic.net/0XdSk.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/0XdSk.jpg" alt="I was hoping to post this comparison of late maniraptorans and early birds but the resolution sucks."></a></p>

CC BY-SA 3.0
2016-12-10T15:05:34.563Z
2016-12-10T15:05:34.563Z
John
<p>It seems to be an accepted fact that Theropod dinosaurs evolved into present-day birds, but I am still leery of this theory. Can biologists and paleontologists really compare these distinct animals and conclusively show that they are "one-in-the-same" creature, based upon certain morphological similarities?  Is this idea "set in stone?"</p>

CC BY-SA 3.0
2016-11-03T15:15:52.057Z
Not only morphological features, but also DNA: https://www.allaboutbirds.org/earliest-beginnings-of-bird-evolution-brought-into-focus-with-new-dna-analysis/
arkaia
<paleontology><evolution>
Dino-birds or "Pigeon-o-saurus"
Jessica
# Dino-birds or "Pigeon-o-saurus"

(Asked by: username_0 on 2016-11-03)

It seems to be an accepted fact that Theropod dinosaurs evolved into present-day birds, but I am still leery of this theory. Can biologists and paleontologists really compare these distinct animals and conclusively show that they are "one-in-the-same" creature, based upon certain morphological similarities? Is this idea "set in stone?"

**Tags:** paleontology, evolution

> **Comments:**
> **username_1**: Not only morphological features, but also DNA: https://www.allaboutbirds.org/earliest-beginnings-of-bird-evolution-brought-into-focus-with-new-dna-analysis/  

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## 2 answers

### Answer by username_2 on 2016-11-03. Score: 7

First, let me link a variety of popular science articles which can give you some background explanation. Here are [Scientific American](https://www.scientificamerican.com/article/how-dinosaurs-shrank-and-became-birds/), [Smithsonian](http://www.smithsonianmag.com/science-nature/dinosaurs-living-descendants-69657706/?no-ist), and [Audubon](http://www.audubon.org/magazine/january-february-2015/which-came-first-dinosaur-or-bird) magazines.

The 'smoking gun' evidence linking birds and dinosaurs is the evolution of feathers. Feathers are a feature that set the birds apart morphologically from the rest of the tetrapods, and indeed from the rest of all animals. All birds have feathers and nothing that isn't a bird has feathers. So a logical place to start looking for the origin of birds is the origin of feathers.

Since the dawn of modern paleontology, there had always suspected links between dinosaurs and birds due to the fossils of [Archaeopteryx](https://en.wikipedia.org/wiki/Archaeopteryx) found in the 1860s. [Thomas Huxley](https://en.wikipedia.org/wiki/Sauropsida#Huxley_and_the_fossil_gaps) claimed the two groups were related as early as 1863. But it wasn't until recent decades that the fossil record started being filled in with definitive links between Archaeopteryx and therapod dinosaurs, such as more species intermediate between birds and therapods and fossil impressions indicating that some therapods had feathers.

The rest is just filled in by logical deduction, or [Occam's Razor](https://en.wikipedia.org/wiki/Occam's_razor). There is clear evidence that all birds have feathers, and no other animals alive today have feathers. There is now clear evidence that therapods had very similar skeletons to birds with feathers about 150 million years ago. There is also evidence that many therapods had feathers, a trait otherwise confined to birds.

Let our null hypothesis be that the origin of birds is unknown, and our alternative hypothesis be that birds evolved from dinosaurs. Given the similarities noted above (and, keep in mind, the evidence is much stronger for scientists who specialize in this area), do you think there is enough evidence to reject the null hypothesis and conclude that birds evolved from dinosaurs? If not, are there any plausible alternative hypotheses?

The great thing about science is that you never have to take someone else's word for it. Everyone has the right to be a skeptic, just keep an open mind and look at the evidence and you can draw your own conclusions.

> **Comments:**
> **username_3**: Feathers are rarely preserved, and yet we now have specimens from every major theropod group with feathers. Sometimes it was more of a downy covering - eg. Tyrannosaurs, but you get the idea. Yes cuddly T Rex :-)  
> **username_2**: Feathers doth not a cuddly make, neither for [birds](http://i2.wp.com/listverse.com/wp-content/uploads/2013/02/IMG_9167.jpg?resize=600%2C409), nor for [dinos](http://i.imgur.com/IsPnSWe.jpg).  
> **username_0**: If therapods were the ancestors of modern birds, how did they escape the extinction mechanism/event that killed off all of the other dinosaurs, in order to evolve into pigeons and such?  
> **username_2**: That is a great question. It is not completely clear why any one set of creatures survived mass extinction events while other sets did not. But to clarify a bit, the therapods that evolved into birds were already quite birdlike by the time of the K-T extinction event. So far as we know, the set of therapods that were very small survived, all other, larger ones did not. We see something similar with other land creatures (small mammals and lizards survived, larger dinosaurs did not). So the reason probably has something to do with size.  
> **username_4**: feathers are such an unusual and unique structure that it is unbelievably unlikely to have evolved twice.  
> **username_5**: @username_2: I'd say that the feathered creatures that survived the K-T extinction WERE birds.  IOW, the theropod/bird split was tens of millions of years prior to the extinction. There were 4 distinct lineages of birds that survived; several other lineages didn't.  One recent theory is that the survivor's beaks allowed them to peck out buried seeds:  http://www.bbc.com/news/science-environment-36102018  
> **username_6**: In addition to feathers there is also the furcula (see [Nesbitt et al. 2009](https://www.researchgate.net/profile/Sterling_Nesbitt/publication/23996833_The_Theropod_Furcula/links/5432250e0cf277d58e983aa9.pdf) for instance).  

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### Answer by username_4 on 2016-12-10. Score: 0

One factor that helps you survive mass extinctions is small size becasue there are more of you and you can repopulate faster. Early birds are among the smallest dinosaurs.

Other indicators of their relationship besides feathers which Kingledon covers are eggshells, dinosaurs and birds both have hard shelled eggs with identical microstructure. most dinosaurs have round spherical eggs but theropods have asymmetric elongate eggs just like birds.

The shape of the hand and forearm of early birds and maniraptoran dinosaurs are almost identical. both theropods and aves also have air-sac based breathing. Honestly in anatomy maniraptoran dinosaurs and early birds blur together so seamlessly there really is no compulsion other than a direct relationship.

what throws most people off is the tail, but early birds have long tails (and teeth). we see a similar evolutionary path in pterosaurs, flying vertebrates start with long tails for passive stability but as they become better at flying shorten it to gain maneuverability and use better active control for stability.

I was hoping to post this comparison of the skeletons of late maniraptorans and early birds done by Scott Hartman but the resolution sucks. Hopefully you can zoom in and see how seamlessly one blends into the other.

[![I was hoping to post this comparison of late maniraptorans and early birds but the resolution sucks.](https://i.sstatic.net/0XdSk.jpg)](https://i.sstatic.net/0XdSk.jpg)


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