Symplesiomorphies in Vertebrates

        To identify differences across families, it is important to have a general idea of the ancestral characteristics that each family share. For example, Myxiniformes, Petromysontiformes, and the rest of the vertebrates all have a braincase, neural crest cells, a tripartite brain and cranial nerves. They also each have an inner ear with at least one semicircular duct, an optic tectum, a muscularized gut tube, and respiratory gills with hemoglobin. Other shared characteristics include a pituitary gland that is divided into an adenohypophysis and a neurohypophysis portion as well as the use of water for respiration and feeding behaviors. These shared ancestral characteristics that are carried out throughout a taxon are referred to as symplesiomorphies.