The Origin of Species had only been published for two years before the first discovery of Archaeopteryx lithographica, (the fossil in question still resides in London's Natural History Museum, and I have seen it, so nyah,) and it was a powerful strike in favour of the Darwinian model of evolution. This post will concern sexually reproducing organisms; asexual critters are a bit more complicated.
Evolution describes branching lineages radiating outwards from species of organisms, which will themselves branch outwards ad infinitum unless a particular lineage goes extinct (which happens extremely frequently.) Isolated populations within a species will evolve into different organisms. The species to which these populations belong may continue to exist; it is only each isolated population which will evolve in a given way, separated from and unable to breed with the rest of their species, and the ancestral species common to these branches may or may not itself go extinct.
Transitional species are those which link what are generally regarded as two different kinds of organisms. Our understanding of gradualism in evolution makes the notion of 'kind' problematic: evolutionary adaptations and features are accumulated by isolated populations over large periods of time (though in geologic terms this may be an extremely short period of time relative to the age of the Earth) and changes from generation to generation are extremely small. Therefore the difference from one species to its descendent - and its close relative lineages - can also be extremely minor, and the methodology by which we group animals into 'species' is often case dependent on what a particular scientist or group of scientists is looking at.
So calling Archaeopteryx the 'first bird' was a bit of a cheeky assertion, because the evolution from non avian dinosaur to modern bird took many transitional species and lineages (the vast majority of which have gone extinct,) and the difference between bird and non avian dinosaur is a very blurry distinction indeed. Archaeopteryx probably never gave rise to any contemporary lineage of bird, and many of the strongest demarcations between bird and reptile have found their way into both sides of the fence. Due to the powerful morphological (that is, structural and functional) similarities between birds and dinosaurs modern phylogenic analyses (that is, analyses which describe evolutionary history and relationships,) group birds as a clade within the dinosauria; birds are dinosaurs.
Now I mentioned that Archaeopteryx probably isn't the ancestor to any contemporary lineage of bird. What makes Archaeopteryx transitional is the blend of features normally associated with two broad classes of animals, rather than its necessarily being a stepping stone from extinct reptile to modern bird. It is closely related to the late Jurassic animals which would give rise to modern birds, and probably descended from the same common ancestor (or they are descended from closely related lineages at least.) Ultimately, we may never discover the true ancestor to common birds which was hopping, gliding or flying about at the same time as Archaeopteryx - and even if we did, we will not know that it is the common ancestor for certain. But our knowledge of evolution means that this ultimately doesn't matter. Archaeopterx as a transitional serves as powerful proof of the relatedness of birds and reptiles, though we now know of a couple of other birds which also strongly support this relationship.
In summary Archaeopteryx shares many features previously thought reserved solely for birds and reptiles respectively, and is therefore a jolly good demonstration that animals are in a continuous state of change. A few of these are, broadly:
- Bones with air sacs
(nb. All of these features have been found in some Theropod dinosaurs subsequent to Archaeopteryx's discovery. Ergo these could all feasibly settle neatly in 'transitional features'.)
Reptilian and generally non-avian features:
- Lack of beak
- Dinosaurian brain shape
- Vertebrae meets rear of skull (All contemporary birds, like humans, have vertebrae which enter the bottom of the skull.)
- Bony tail
- Flexible wrist joint
- Pubic bone shape and structure
- Two free fingers, one fused (adult bird digits are fused.)
With the latest description of a tenth Archaeopteryx fossil this link is even stronger, outdating a couple of the points in the Talk Origins bird-reptile comparison I linked at the top of this post. Remember the skeletal distortions in other fossils I mentioned earlier? A large part of the confusion and ambiguity these caused has been cleared up with the description of this fossil, which showed the top of the skull and wonderfully preserved the specimen's feet. The feet indicate that rather than the grasping, opposable first digit of modern birds, Archaeopteryx had a reduced toe facing the same direction as the rest of its feet, as per Theropods. Rather than the perching second digit of birds, it had an extendable toe, like the big claw on Velociraptor. Combined these suggest that Archaeopteryx was less arboreal than previously thought, that it spent most of its time on the ground and would have been rather awkward on branches. The skull and brain are even closer to those of dromaeosaurs than previously thought - narrow and tapering, though still blunter than dromaeosaurs.
The connection between birds and dinosaurs is made stronger with each new discovery, and is almost universally accepted amongst palaeontologists. And old friends to evolution are still turning up strong new evidence in its favour.
More on the new fossil:
New Scientist (May eventually require subscription.)
Dr Myers of Pharyngula also did stuff. Good stuff. Foot and skull details are graphically shown and explained.
-The Rev. Schmitt.