Are Birds Modern Dinosaurs?

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  via Quora

  Looking at the picture above, it's not hard to see the similarities between the two creatures: both use their legs to get around, defend themselves and kill prey; both are flightless and don't get much use from their wings/arms; both have similar physical features like feathers and horn-like protrusions on their heads. 

  But the creatures depicted here are millions of years apart from each other. The one on the left is a Corythoraptor jacobsi, a genus of theropod dinosaurs. On the right is a cassowary, found in northeast Australia and parts of New Guinea. 

  What is going on here? Why are these two animals that live 70 million years apart so similar? Is the cassowary a modern day descendant of the Corythoraptor jacobsi (crested raptor)? Are we living among dinosaurs? The short answer is yes. 

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  So how exactly are birds considered dinosaurs? It all goes back to the family tree. Take humans, for example. We are direct descendants from mammals. To refresh your mind, mammals must meet the following criteria: mammals have hair or fur growing from the skin, female mammals have mammary glands that produce milk for feeding the young, mammals have three bones in the middle ear for transmitting sound to the inner ear, and they have a single bone on each side of the lower jaw. We meet all these criteria, as do cats, dogs and elephants. Therefore, humans are mammals (although more intelligent than other mammals). 

  In the same way, birds are firmly rooted in the dinosaur family tree. All species of birds today are descendants from one lineage of dinosaur: theropod dinosaurs. There are many characteristics that bolster this fact. Theropods and modern birds both have hollow bones, three toed limbs, are bipedal (meaning they stand on two legs), have shortened forelimbs in relation to their hind legs, and many more similarities. 

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  So how did two-legged theropod dinosaurs evolve into the harmless chicken? First of all, evolution happens over a long period of time, and a lot can change in a few millions or billions of years. The first primitive birds were very dinosaur-like, with teeth, reptilian nails and claws on their hands. Many theropod dinosaurs actually had feathers. There are a few features all modern birds have that some theropods had too, like unfused shoulder bones, toothless beaks and forelimbs longer than their hind limbs. 

  But theropods really started to become birds when the Archaeopteryx evolved around 150 million years ago. Fossils show that the creature weighed two pounds, was 20 inches in length, and had feathers on its body with long tail plumage. The shape of it's skeleton also suggested that it was capable of flight, although it still had small claws at the end of it's wings. From there, the rest is history (literally). 

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  Eventually, dinosaurs with different features grouped off and away into their own genuses. The most important separation is of the avian theropods away from the non-avian theropods - or in other words, those that could fly and those that couldn't. 

  After the evolution of flight, the bones in bird-dinosaurs hands stiffened up and became the structure we see today to support wings. Non-avian dinosaurs became extinct at the end of the Cretaceous period while bird-dinosaurs continued to evolve and branch out into their own species. Today, there are 10,000 species of birds, from penguins to cassowaries to eagles to hummingbirds. But they all come from the theropod dinosaurs that hatched Archaeopteryx.

  So the next time you see a bird, whether it's a pigeon darting around your feet, a seagull eyeing your sandwich, or a crow plotting your death, remember the pretty cool fact that you are living among dinosaurs. Although it might not feel like it, especially when you see pigeons wearing tiny cowboy hats or sombreros, we are much more connected to our ancient past than we realize. 

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