When you look up at the sky, what’s the most likely thing you’ll see? A bird. It’s a thing taken for granted. Very few people actually question what a bird is. Birds are aves, specifically avian dinosaurs. One can also call them crocodiles with beaks and feathers and they share the most structural resemblance with them. But then it begs the question, exactly what are birds?
In 1860, a German worker found a limestone fossil. This fossil would later come to be known as the Archaeopteryx lithographica, which would be considered to be the ancestor of birds. From then on, many scientists, like Thomas Henry Huxley, began speculating the similarities between birds and theropod dinosaurs, notably coelurosaurs.
But those ideas didn’t come to flourish back then as hypotheses as A. Walker’s “crocodylomorph” and G. Heilman’s “thecodont” kept things tight for most of the 19th and 20th century. That changed with the advent of Dr. J.H. Ostrom. In 1969, his description of Deinonychusantirrhopus and its similarities to Archaeopteryx gave birth to much more speculation regarding the similarities between aves and dinosaurs.
At one point, it became very difficult to draw the line between birds and non-avian dinosaurs. Some scientists still refuse to accept that birds came from dinosaurs and are trying to prove otherwise but it’s not going well for them. But are birds really dinosaurs? Coelurosaurs are thought the closest to birds. Or rather, birds are considered to be a kind of coelurosaurs. The skeletal morphology described by Gauthier’s analysis show many similarities. The first bird shared many characteristics with the Maniraptora, a well known example being the Velociraptor, that dinosaur that taps its claws in Jurassic Park. The pubis, for one, moved from an anterior to a more posterior place, and bearing a small distal “boot” in birds from coelurosaurs.
The long arms and forelimbs and the clawed ‘hands’, the large eye sockets, flexible wrists with a semi-lunate wrist bone, hollow, thin walled bones, 3-fingered ‘hands’ and 4-fingered ‘feet’ but with 3 supporting fingers, S-shaped neck, similar eggshell microstructure. You know, the small stuff. They count. These resemblances have led to the modern day notion of birds having come from dinosaurs. That said, there arguments that try to disprove these similarities but, these tend to be more iffy than concrete. One argument is the gap in the record between the first known bird and the dromaeosaurs, a kind of sister group of birds. This forgets the obvious fact that other maniraptorancoelurosaurs, such as Ornitholestes, Coelurus, and Compsognathus, are from the Late Jurassic age. If other maniraptorans were there, it logically follows that the ancestors of dromaeosaurs were also there. Remains of possible dromaeosaurs are also from the Late Jurassic.
There are other arguments, like the difference between the the ropod and bird finger development, or lung morphology, or ankle bone morphology. But they all lack substance thanks to the absence of relevant data on extinct theropods, or sometimes they have misinterpretations of anatomy, or sometimes simplify about developmental flexibility, and many more which are easily dispelled.
The facts are always in favor of maniraptorian origin of birds or at least a theropodan origin. When we look at that ostrich chasing that fox for venturing towards its nest in some African savanna, we can see a bit of what it was like back in the Jurassic era. That is to say, most things did change. 150 million years isn’t a short time. Things changed. Birds adapted. And they became what we see them to be as today.