Birds and Reptiles

More accurately, 'bird' is not a scientific term, but a common one, and doesn't have any clear scientific definition. From an eviolutionary point of view, recognising that change is gradual, it's not too important where the exact line is between protobird and bird.In scientific nomenclature, the birds are 'Aves', and the most common definition of Aves is the one that Dr. Adequate says - the common ancestor of Archaeopteryx and modern birds and all its descendants. This isn't the only definition though, and to try and clarify issues like this and give us clear, unambiguous definitions of groups of organisms, a mighty project is under way called the Phylocode. This will give a standard definition for each taxon so that, in theory at least, anyone working in biology would be able to check exactly what you do and don't mean by 'Aves' (when it will ever be ready is an open question).Under some of the proposed definitions, Archaeopteryx wouldn't make it into Aves, but whether or not you wanted to carry on calling it an early bird is a matter of taste. Common terms for animals always hit problems when you go back towards distant ancestors, that's why some bioogists decided they needed this unambiguous series of definitions for scientific nomenclature.Edited by caffeine, : No reason given.

People came up with the idea that birds evolved from dinosaurs because of the many features that birds share with dinosaurs, specifically with a particular group of dinosaurs, the theropods. Like theropods, birds support most of their weight on just three toes while standing on the ground, with the first toe reversed to point backwards. Particular groups of theropods share more with birds than others - the ischium (part of the hip) is shorter than other theropods, they lose some of the digits on their hand, and the collar bone is fused with the interclavicle - a diamond shaped bone at its base.These are a few big examples that I've listed, but the complete list of shared features between the skeletons of birds and theropods is long and detailed. It's true that there are still a few palaeontologists who argue that birds aren't theropods, but their position is becoming less supportable the more that's discovered - for example the indisputable prescence of vaned feathers in some theropods, and what might be proto-feathers in other theropods thought to be less closely related to birds.