When looking back to the phylogenetic tree from which all species stem from, one of the groups that is often over looked is the Cyclostomata. This group is one of the earliest members of the vertebrates who’s name literally translates to “circle mouth” thus referring to their circular jaws. This group breaks down further into two families, Myxiniformes (hagfish) and Petromysontiformes (lampreys). This passage will serve to discuss the similarities and differences between Myxiniformes, Petromysontiformes, and the rest of the vertebrates.
To start, it is important to have a general idea of the ancestral characteristics that each family share. For example, all three of the above families have a braincase, neural crest cells, a tripartite brain and cranial nerves. They also have an inner ear with at least one semicircular duct, and optic tectum, a muscularized gut tube, and respiratory gills with hemoglobin. These shared ancestral characteristics are referred to as symplesiomorphies and can be seen in every vertebrate living today.
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