Most people who are even casually interested in dinosaurs have heard that this group of "terrible reptiles" went extinct at the end of the Cretaceous Period, around 66 million years ago. Sadly, this means that you can't see a Velociraptor or Stegosaurus today. However, we now know that not all dinosaurs went extinct. In fact, you can probably catch a glimpse of an entire flock of living dinosaurs just by looking out your window.
Birds Are Dinosaurs
When you hear that birds are dinosaurs, as you likely have over the past few years, you might think that's misleading, or even disappointing. You might say, "Really? THIS is what became of the mighty dinosaurs?" But it's true! Birds are not just related to dinosaurs, or similar to dinosaurs, but are actually classified as dinosaurs themselves. Specifically, birds are a type of theropod, the group of bipedal, usually carnivorous dinosaurs that includes species like Tyrannosaurus, Velociraptor, and Allosaurus. This might come as a surprise; however, birds and dinosaurs have a lot more in common than you may realize. For example:
The presence of feathers is very widespread across theropod dinosaurs. Feathers have even been found on Yutyrannus, a large dinosaur related to Tyrannosaurus.
Like birds, dinosaurs are believed to have been warm-blooded.
Both theropods and birds have hollow bones.
We now know that many theropods had a bird-like respiratory system. Although since the dinosaurs did it first, you could say that birds have a theropod-like respiratory system!
Theropod dinosaurs also have a furcula, also known as a wishbone. (Yes, even Tyrannosaurus had a wishbone!)
Many theropods brooded their nests, insulating their eggs to keep them warm.
The observation that birds are very similar to dinosaurs isn't new. When the first fossil dinosaur tracks were found across New England in the 1800s, the word "dinosaur" had not been coined yet, and the first fossils officially recognized to be from dinosaurs had yet to be discovered. Rather, these tracks were thought to have been left by giant birds. Scholars even dubbed the first track discovered "Noah's Raven." Today, we know these tracks were left by theropod dinosaurs.
Fossil footprint left by a large theropod
In the early 1900s, a scientist named Gerhard Heilmann was investigating the origins of modern birds. He was trying to figure out what sort of animal they most likely descended from. He looked at many groups of reptiles including crocodilians, pterosaurs, and dinosaurs. While he ultimately concluded that birds had not descended from any of these groups. he noticed that they were awfully similar to some theropod dinosaurs, including the famous Archaeopteryx.
Knowing why Heilmann concluded that birds did not descend from dinosaurs is very important. In his research, Heilmann studied the skeleton of a semi-famous dinosaur known as Oviraptor. Like all theropods, Oviraptor had a wishbone. However, the original discoverer, Henry Osborn, misidentified this bone as something else (he thought it was a bone called the interclavicle). Heilmann came to the same conclusion. If Oviraptor's wishbone had originally been identified correctly, or if the error had been caught by Heilmann, we may have made the connection between birds and dinosaurs a long time ago!
Above: Drawing of an Oviraptor skeleton, with the misidentified wishbone highlighted.
Even today, there are many fossils that blur the line between birds and non-avian dinosaurs (the term used to specify when we're talking about all dinosaurs that aren't birds). When species like Caudipteryx (below) and Anchiornis were discovered, they sparked debate over what side of the "dinosaur/bird" line they fell. The consensus currently rests on Caudipteryx being a dinosaur and Anchiornis being a bird, but there have been scientific papers published arriving at opposite conclusions for each. While the relationships of these animals are eventually sorted out by finding more and better specimens, these fossils show that there really isn't an obvious line between birds and dinosaurs. Rather, these two groups exist on a continuum.