The Gans-Northcutt Theory of the New Head considered. Fossils also help, but Chuck will talk about this in another lecture. Phylogenetic tree of vertebrates and their close relatives Below is a very simplified phylogenetic tree of deuterostomes , a super-phyletic group which the vertebrate subphylum of the chordate phylum is a part. I've labeled the deuterostome and chordate nodes on the tree. This phylogeny is based on both adult and embryonic morphological and molecular data.
Cephalochordates are the next closest relative to the vertebrates, the vertebrate sister group .
Tunicates and echinoderms are more distantly related vertebrate outgroups . The sister group is the most closely related outgroup to a given group. Remember that these terms only have meaning relative to the phylogenetic node of which you are speaking. If a character is present in the vertebrates and an outgroup, this character is plesiomorphic to the vertebrates. If it is only in vertebrates, it is apomorphic to vertebrates.