The similarities between dinosaurs and birds has long been noted, but it is only comparatively recently that they have been fully studied, leading many to conclude that not only do birds share physical similarities with dinosaurs, but their relationship goes much deeper. It is now commonly accepted that birds evolved from dinosaurs, and to understand why the "birds are dinosaurs" hypothesis has grown it is necessary to look at how well the methodology of looking at the similarities between dinosaurs and birds, as well as the fossil evidence supports this view.
Dinosaurs and Birds: Methodology
Until the 1960's, all features were considered important when studying the evolutionary relationships between species. Then a German entomologist, Willy Hennig determined that as organisms evolve and their physical characteristics change, it is only those features that remain the same that are relevant. This system, known as Cladistics, or Phylogenetic Systematics, is now the standard methodology for measuring the characteristics a species shares with other organisms, and has been a primary tool in developing the understanding of the similarities between dinosaurs and birds.
In 1969 the work of John Ostrom described the similarities between Archaeopteryx (a Jurassic bird), and Deinonyclus, a theropod. He noted twenty two features both species shared which helped shape the current thinking on the relationship between dinosaurs and birds. Following his work, Jaque Goultier's studies, using the Cladistics method became the first well accepted work to produce compelling evidence that dinosaurs and birds do not just share similarities but are related. It has since been shown that there are ninety features that relate birds to dinosaurs, and it is now widely accepted that birds most likely evolved from a turkey sized theropod.
Dinosaurs and Birds: Their Similarities
Since dinosaurs are reptiles, the physical similarities they share with birds are reptilian. For instance, birds have scales on their feet just like reptiles. In fact the tissue that produces feathers is similar to tissue that produces scales. Also like reptiles birds lay eggs, and the soft anatomy of birds (their internal organs) is again very similar to that of reptiles.
Thanks to the work of people like Ostrom and Goultier we now know there are many skeletal similarities between dinosaurs and birds. Some of the main similarities include:
1/ Large eye sockets
2/ Elongated arms and forelimbs
3/ A flexible wrist
4/ Hollow, thin walled bones
5/ Four toed feet, supported by three main toes
6/ Elongated bones between the ankles and toes
7/ A similar eggshell micro structure
8/ A fused collar bone to form a wishbone (furcula)
9/ An anterior positioned pelvis.
In both birds and theropods the pelvis is shifted from an anterior position to a more posterior position with a small "distal boot"
10/ Respiratory system.
Birds and dinosaurs are also thought to share a similar respiratory system with pulmonary air sacs that would have allowed them to force air through their lungs twice.
For many paleontologists the list of similarities provides overwhelming evidence that birds are related to dinosaurs, and each new fossil discovery is seen as important evidence to substantiate this.
The Fossil Evidence
Although there have been many exciting fossil finds to support the "birds are dinosaurs" hypothesis, it is still far from conclusive whether birds did in fact evolve from dinosaurs, or whether they instead share a common ancestor. The features of theropods are well known to paleontologists, so the kinds of fossils of most interest are ones that share both theropod characteristics as well as those of birds. Early examples of bird fossils are also important to demonstrate an evolutionary link between dinosaurs and birds.
Archaeopteryx
The first Archaeopteryx fossil was found in 1861 in the Jurassic sediments of Southern Germany. It is 145 million years old and was crow sized. Of the seven known Archaeopteryx fossils, two have feathers, although its other features are distinctly dinosaur like. For instance it has teeth, a long bony tail and claws on its wings.
Its bird like features includes a furcula and bird like feet. Archaeopteryx also had hollow bones and asymmetrical feathers which is a strong indication of flight. Since it shares both theropod and bird characteristics, whilst it is considered a bird, Archaeopteryx is seen as a "transitional form" between dinosaurs and birds.
Other Dinosaur Fossils With Bird Features
Unenlagia
Unenlagia is a theropod found in Pategonia and is 88 million years old. It had arms that folded in a bird like manner.
Oviraptor
This species is known from a number of fossils and has been found brooding over its nest much like a modern emu.
Rahonavis
Fossils from this theropod are around 80 million years old. It is more bird like than Archaeopteryx but it still retains dinosaur features such as a slashing claw.
Bird Fossils With Dinosaur Features
There are also a number of bird fossils that show dinosaur features. For instance, Mononykus, found in Mongolia
was a flightless bird that had a single digit hand instead of wings. A sparrow sized bird fossil known from Spain had a more developed shoulder blade than Archeopteryx but it still had teeth.
Protoavis
This is a controversial fossil specimen because if it is a bird it does not fit within the current thinking that birds evolved from theropods. Protoavis dates from the Triassic period, some 100 million years before the appearance of theropods. Only fragments have been found but depending on whom you believe it may have had features suitable for flight, such as a keel-like sternum, and strong wing muscles. Its skull was also light and it is thought to have had a brain similar to modern birds.
The Chinese Fossils
There have been a number of fascinating Chinese fossil finds to connect bird evolution with dinosaurs, particularly from the Liaoning Province. There have been specimens found with proto-feathers (or dino-fuzz), the first example of a pygostyle (parson's nose), as well as the first know beak.The two finds that caused the most excitement were Protoarchaeopteryx and Caudipteryx and they are believed to provide conclusive evidence for the evolution of birds from theropods.
Protoarchaeopteryx had a tail covered with six to eight inch proto-feathers as well as wing feathers. Its hands were longer than its arms and it had powerful legs with three forward pointing toes. Caudipteryx also had feathers on its arms and most of its body and tail. Some of the feathers were downy but others were more structured with quill-like vanes. Of the ninety recognized similarities between dinosaurs and birds, these two specimens were found to have eighty.
Dinosaur and Birds: Opposing Views
Most of the points raised by those who oppose the hypothesis that birds evolved from dinosaurs concentrate on the differences between them. For instance it has been noted that a dinosaur's ribcage is compressed from side to side whereas a bird's ribcage is compressed from to back. The Cladists argue that it is only the similarities between species that are important because these are the characteristics unchanged by evolution.
It has also been contended that birds are more likely to have evolved from a tree dwelling (arborial) animal than a ground dwelling (cursorial) species. This is known as the "trees down v the ground up" debate. The supporters of the "birds are dinosaurs" theory say that it is possible birds evolved from a theropod leaping and jumping after insects, and point to species thought to have survived this way.
The fossil evidence has also been contested. The order of the fossils shows that theropods are found in the Cretaceous sediment and bird fossils in the Jurassic sediment. If birds had evolved from dinosaurs then this order should be reversed since the Jurassic period predates the Cretaceous period. From this standpoint the fossil evidence is only coherent if birds and dinosaurs shared a common ancestor rather then birds evolving from dinosaurs.
It has also been suggested that the proto-feathers of the Chinese fossils are not feathers at all but fossilized impressions of lizard-like decomposed skin and its supporting tissues.
In spite of these objections most paleontologists agree that birds did evolve from dinosaurs, probably a turkey sized theropod, and the evidence from Cladistics, the most widely used methodology supports this position. However, an inescapable fact is that the earliest bird-like species such as those found in the Chinese fossils appear after Archaeopteryx (the first accepted bird fossil), and so cannot be ancestors of something that lived sixty million years earlier. For the Cladists the matter is completely settled but for those that contest the "birds are dinosaurs" hypothesis, they would not agree.
References
http://www.unc.edu/news/archives/oct 05/feducci100705.htm
http://www.ucmp.berkeley.edu/dia psids/avians.htmlhttp://www.abc.net.au/science/slab/ dinobird/story.htm