This question originally appeared on Quora: Are there any "transitional life forms" which have not been debunked by modern science?
Answer by Glenn Anderson, Principle Software Engineer at OnLive
The whole concept of "transitional fossil," unfortunately, has a sort of linear connotation. Like, there is a sequence of species, one having evolved into the next and so on, but there's a hole in the sequence that needs to be filled. But that's not really how evolution works.
Then we have the fossil record. In reality, what we have are a bunch of bones (and things like feather and skin impressions, gastroliths, trace fossils, etc). It is the job of scientists to organize, categorize and arrange these fossils into a taxonomy, using the same tools they use for living species.
To do this requires a great deal of knowledge about comparative anatomy, and because there is so much to know, often times there will be specialization. For example, one scientist may specialize in bat anatomy.
Like living species, extinct species are categorized according to traits that they share in exclusion to other groups of species. It's a bit more complicated than that, of course. There is a whole science around building these taxonomies, called. Cladistics uses algorithmic and statistical methods to determine these taxonomies, based on a matrix of informative characters.
For fossils, these characters would include information about the size and shape and robustness of various bones, the presence of certain features, like a bump, or an opening for a vein or nerve. The number of cusps on a tooth. Lots of stuff that these comparative anatomists are all crazy about. It's easier for living species because there is so much more data available, such as DNA.
So, using the fossils, scientists can derive a taxonomy for these extinct species. This taxonomy will show things like species A and species B, shared a common ancestor and are more closely related to each other than they are to species C. And what they find are patterns. You have what are called derived traits (). These are traits that are unique to a certain branch of the taxonomy (this is called a clade). And you have basal traits ( ), these are traits that are present in a containing clade. For example, mammals have a single jaw bone. This is a derived trait. The group in which they are nested, has three jaw bones. Three jaw bones is the basal state. In mammals, the bones that used to be part of the jaw, migrated to become the inner ear bones. is a fossil species that shows a stage intermediate between the derived state of the inner ear bones, and the three bone jaw.
Start by looking at a, b, c, and d in this diagram. Highlighted in blues and purples at the back of the jaw are these small bones in Morganucodon (a) and Yanoconodon (b). In d is the jaw of , a large Cretaceous mammal. Don't miss c--that small object is the collection of middle ear bones from Ornithorhyncus, better known as the platypus.
And that is what a transitional fossil is. It is a fossil that shows the development of character states from the basal to the more derived. Is Yanoconodon the grandpappy of all mammals. I don't know, maybe. There is really no way to determine direct lines of ancestry. But we have a pretty good idea, based on the more derived state of the inner ear bones, of where it fits on the branches that lead to mammals. Our direct ancestors would have had inner ear bones close to this at some stage.
So, are there examples of transitional species in the fossil record? Hell yeah, lots of them. Yanoconodon is just one.