In a way, this is more of a philosophical post than a paleontological one. From slightly different perspectives, either there are many ‘transitional fossils’, or there is no such thing. And both those views are consistent with the fossil record!
I’ve certainly said “There’s no such thing as a transitional fossil” before. What did I mean by that? Well, no species of living thing is ever ‘on the way’ to a different species, ‘in transition’ from one thing to another. Each living thing – and each population of living things, which is the unit evolution works on – is simply living. Simply being as well adapted to its particular ecological niche as it can possibly be, in order to live, move, breed and pass on its genes.
So a fossil of a living thing is, in a very real sense, not ‘between’ two other species. It is simply itself.
At the same time, in retrospect, we can reconstruct the ‘tree of life’ – the sequence from simpler to more complex organisms over time. (It’s worth noting that, more recently, this reconstruction is a dynamic process that includes DNA evidence as well as the fossil record.) That sequence is not linear: it is branching, and has many, many dead ends. The fact that there are more complex organisms doesn’t mean the simpler ones go away: bacteria, viruses and archaea are still with us today.
So, in the sense that we can reconstruct the sequence, in a sense any fossil that is not the fossil of a present-day species can be thought of as a transitional fossil, since that species had both ancestor species and successor species.
What is usually thought of as a transitional fossil, of course, is something that has obvious features of both its ancestors and its successors. Archaeopteryx, for example, is a bird-like dinosaur with teeth and feathers. It seems likely now that it was more of a dead-end than a transitional species, but it is the kind of thing we think of. (It is also believed now that many more dinosaurs had feathers than first thought: they’re just less easily fossilised than bones.)
There are fossils of limbed species that we believe are ancestors of modern whales, and modern whales have vestigial hips that suggested their ancestors had limbs. We could even argue that whales are ‘transitional’ to future ocean-going species in which those vestiges have completely disappeared.
As usual with these little posts of mine, checking out the Wikipedia page on ‘transitional fossils’ will add a lot more detail and a lot of examples, and googling the term will… well, frankly, lead you to a lot of examples but also a lot of ill-informed claims that such things have never been observed.
For ease of navigation I will include links to each of the other posts in this series at the bottom of each post.
Why I think it’s important to understand evolution
Cosmogenesis, abiogenesis and evolution
Evolution and entropy
Facts, Theories and Laws
Radiometric dating and deep time
Four Forces of the Universe
Probability and evolution
Species and ‘baramin’, macro- and micro-evolution
Mitochondrial Eve and Y-chromosomal Adam
Complexity – irreducible and otherwise