We've been working our way across through the tree of life in the past few editions of Phylogeny Friday. Last week we took a look at the evolutionary relationships of the animals, and we realized that many of the branching orders are extremely difficult to resolve. Today we're going to zoom in on one of those branches: the vertebrates. Why? Because we're vertebrates and we want to know about ourselves. Also, we know more about this taxon than any other taxon. These are the same rules than have governed us throughout our journey.
After last week's disappointingly unresolved tree, I have a slightly better offering for you today. As of now, we have a good idea about the evolutionary relationships of many vertebrates. There are also some relationships that we just can't figure out. Here's what we know (from this paper from 2003):
The most basal division we can make is between jawless fish (Agnathans) and jawed vertebrates (Gnathostomata). It's unclear whether Agnathans are a monophyletic or paraphyletic taxon, so that basal node is unresolved. The jawed vertebrates can be split between cartilaginous fishes (sharks, rays, and skates) and bony fish (Osteichthyes).
The bony vertebrates are made up of ray-finned fishes, lobe-finned fishes, and tetrapods. The ray-finned fishes are the most dominant group of vertebrates, filling up the oceans, lakes, seas, and rivers. But all of these (and every vertebrate) is a "fish". You wouldn't normally think of terrestrial vertebrates (amphibians, reptiles, and mammals) as fish, unless you think like me. However, in order for fish to be a monophyletic taxon (and I will tolerate nothing less than monophyletic taxa), all vertebrates must be called fish.
Of all the vertebrates, you are probably most familiar with the tetrapods -- amphibians, reptiles, and mammals. We are fairly confident about the relationship of these three taxa, but our understanding of the relationships within these taxa is not as good. We do know that birds are the only living descendents of dinosaurs, and crocodiles are their closest living relatives. One big debate surrounds the position of turtles within reptiles. The relationship of the three groups of mammals (placentals, marsupials, and monotremes) is also unresolved.
What did we learn? We're fishes. The most common fish/vertebrate are the ray-finned fish. There's still a bit of work left to do before we can resolve the entire vertebrate tree. Next week: the mammals.
Meyer A, Zardoya ÂR. 2003. Recent advances in the (molecular) phylogeny of vertebrates. Annu. Rev. Ecol. Syst. 34: 311-338. doi: 10.1146/annurev.ecolsys.34.011802.132351