Consider otters. They are members of the Mustelidae family (mink, stoat, weasel, marten, badger, wolverine, skunk (??), ferret, etc.). All species are semi-aquatic. The sea otter rarely comes ashore.
Although we cannot predict the future evolution of any species, since any or all otter species might re-adapt to life on land, otters appear to be in transition from land to water environment.
Consider the pinnipeds, the seals, sea lions and walruses. These too, like the otters, are carnivores and closely related to land mammals in the suborder Caniformia, which includes bears, dogs, pandas, the Mustelidae, raccoons and coatis. They too seem to be in a transition between land and water. While the bone structure of their limbs is the same as any other carnivore, externally they appear as fins.
quote:I think it's worth noting that most transitional fossils aren't strictly speaking actually transitionals between the groups they're transitional between. We have no way of knowing whether a fossil is in the direct line of ancestry or a side branch closely related to that direct line. The existence of the transitionals we find confirms the existence of the direct transitionals, but they are likely not to be those direct transitionals themselves.
So the real tree probably looks a bit like this:
Dinosaurs---->Bird transitional--->Birds \ -->Archaeopteryx rather than like this:
Dinosaurs---->Archaeopteryx--->Birds This represents an additional layer of interpretation, but - as Dr A has explained - such interpretation is not arbitrary.
Absolutely true. To continue with your example, Archaeopteryx is an example of a transitional species. But no one can state for certain a specific ancestor, nor can any descendant species be identified. Nevertheless, Archy is obviously descended from a line of maniraptoran theropods. In fact, two examples were incorrectly classified as Compsognathus, a small maniraptoran dinosaur. There are only a few differences in the skeletons. In fact, paleontologists think that Compsognathus was probably also feathered, but the feathers were not preserved since Compsognathus was terrestrial and less likely to end up at the bottom of an anaerobic lagoon. Moreover, there are only two fossil specimens that have been clasified as Compsognathus, where there are 10 specimens of Archaeopteryx plus a perfectly preserved feather.
If you take away the feathers, it takes an expert to distinguish between the dinosaur Compsognathus and the bird Archaeopteryx.