Why are such different animals as fish amphibians reptiles birds and mammals grouped into a single phylum

Since they share common body plan (triploblastic and bilateral symmetry) and also have common key characters such as presence of notochord at any stage of their life cycle.

Explanation:

Organisms such as fish amphibians reptiles birds and mammals are grouped under phylum Chordata as they share common body plan and key characters. They all are triploblastic with three primary germ layers, bilateral symmetrical organisms that have the organ system level of body organization. These organisms have notochord, tubular nerve cord and pharyngeal gill slits at any stage of their life.  

Hence, they are grouped under the same phylum.  

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explanation: