Looking at the family tree of birds, there are three important aspects to keep in mind.
- Early birds like Archaeopteryx are extremely similar in anatomy to advanced non-avian dinosaurs like Velociraptor. For example, both had long bony tails and teeth.
- Early in bird history, there were flying birds and non-flying birds. This great variety of forms included flightless diving animals like Hesperornis and small active flyers like Apsaravis.
- Birds, with many of the advanced features we consider to be characteristic of modern birds, lived at the same time as the non-avian dinosaurs.
Today, Archaeopteryx is the most primitive known bird. ©AMNH Library
So the characters unique to birds must have evolved very early in the split from non-avian theropods.
Before we continue, you may find it interesting to know that deciphering the family tree of animals at the transition from non-avian dinosaurs to birds is one of the hottest topics around. It is the focus of extensive efforts by scientists on many parts of the globe. Most investigators agree that small bipedal dinosaurs called troodontids and dromaeosaurs are the non-avian dinosaur groups most closely related to birds. These small dinosaurs have many things in common with modern birds and even more in common with primitive birds like Archaeopteryx.
Archaeopteryx is the most primitive known bird and therefore most primitive known avian dinosaur. The first specimen was discovered in the 1850s, just about the same time that Charles Darwin published On the Origin of Species. This book was one of the most influential ever published in that it laid out the basic principles of evolution by natural selection. Darwin’s book stated that evolution was the great sum of small changes; in this way it predicted that transitional forms would be found between modern species. Archaeopteryx, as was immediately recognized by Thomas Huxley, was just one of these physical links between non-avian dinosaurs and birds. It was a close relative of birds in that it had a wishbone and a reversed first toe on its foot. (This is the toe that allows some birds to perch.)
It also had feathers. Some of the feathers were asymmetric, meaning that the front edge of the feather was shorter than the back edge. Such feathers indicate that Archaeopteryx had some capacity for flight. Yet at the same time, it had many characteristics that are more primitive and more like non-avian theropods. For example, Archaeopteryx had a long bony tail similar to dromeosaurs, while in modern birds this tail is shortened and fused into the triangular piece called the pygostyle. Archaeopteryx also had teeth, three separate front fingers, and a breastbone that lacked a keel, like many non-avian theropods (except T. rex and oviraptorids). In modern birds, the teeth are lost, and the three fingers and the crescent-shaped bone in the wrist are all fused into a single bone in the hand.
Linking Birds and Dinosaurs
Small non-avian theropods, like this troodontid, are very close to the ancestry of birds. ©Brian Franczak
To demonstrate that non-avian dinosaurs and birds are related, scientists needed fossils of both advanced dinosaurs and primitive birds to compare characters. But at the time Archaeopteryx was discovered, no good specimens of small, advanced dinosaurs had been found. When they were found at the end of the 20th century, they showed that, except for the relative length of the forelimb, there is very little difference between primitive birds like Archaeopteryx and advanced theropods like the dromaeosaur Velociraptor. Both have:
- wrists that contain a crescent-shaped bone
- hands like most other advanced theropods, with three fingers in which the middle finger is the longest
Part of the reason that Archaeopteryx was so important is that for a long time it was the only well-preserved early bird fossil. All of that is now changed. As we speak, there are several newly found fossils that document transitional body plans from non-avian dinosaur to modern bird. The best way to look at these is from the perspective of a family tree. Moving up the genealogic ladder from Archaeopteryx to modern birds, the next early bird is Confuciosornis. This animal was found about six years ago. Now there are nearly a thousand specimens. Unlike Archaeopteryx, it lacks a long tail and has no teeth. Nevertheless, it still has three separate bones in its hand. Some Confuciusornis specimens preserve long tail feathers, like those of a bird of paradise. These feathers may represent the breeding plumage of males.
After Confuciusornis there is a line of primitive birds that leads directly to modern birds. Included in this diversity from the Mesozoic era are birds like Hesperornis—a four-foot-tall water bird that lived about 70 million years ago in the seaway that bisected North America. Hesperornis could not fly, and it did have teeth. Another particularly interesting animal is Apsaravis, a small, robin-size flying bird from Mongolia. This little animal was an active flyer that undoubtedly flew above the heads and skittered around the feet of its non-avian dinosaur relatives.
But the dominant group of birds that lived during the Mesozoic was the enantiornithines, otherwise known as “opposite birds.” These animals had an extensive distribution and diversity during the age of dinosaurs. Some of them even had teeth, some were probably good flyers, and others more closely resembled ground birds.
The diversification of the kinds of birds that we see today occurred rather late in bird history. The first representatives of birds that belong to the modern groups are loons, grebes, and similar waterfowl, whose fossils have been found at the end of the Cretaceous, just before the non-avian dinosaurs became extinct.