section_path: Evolutionary history/External relationships
section_name: External relationships
paragraph_text: External relationships
  
  Originally, the "Notochordata hypothesis" suggested that the Cephalochordata is the sister taxon to Craniata (Vertebrata). This group, called the Notochordata, was placed as sister group to the Tunicata (Urochordata). Although this was once the leading hypothesis, studies since 2006 analyzing large sequencing datasets strongly support Olfactores (tunicates + vertebrates) as a monophyletic clade, and the placement of Cephalochordata as sister-group to Olfactores (known as the " Olfactores hypothesis "). As chordates, they all share the presence of a notochord, at least during a stage of their life cycle.
  
  The following cladogram summarizes the systematic relationships between the Olfactores (vertebrates and tunicates) and the Cephalochordata.