The image shows what is called a cladogram. It is a
tree-like structure which represent evolutionary
relationships. It starts with a single root and
splits into two taxa which themselves might split.
From the root the first to split of is Petromyzontoidea,
then Elasmobranchii, then Actinopteryaii,
then Dipnoi, and then Amphibia. The remaining taxa splits
into two groups: the Mammalia and a second taxon which splits
into Testudinata and another taxon. That taxon first
has Lepidosauria split off and then splits into Crocodilia
and Aves.
