Why are such different animals as fishes amphibians reptiles birds and mammals grouped in to a single phylum?

You're right to wonder why such diverse creatures are grouped into the same phylum, Chordata. It might seem counterintuitive, but there's a reason for it! While they might look and behave very differently, these animals share some key, fundamental characteristics during at least some stage of their development:

Key Chordate Features:

* Notochord: A flexible rod that supports the body. While present in the embryo, it's usually replaced by a backbone (vertebrae) in most chordates.

* Dorsal Hollow Nerve Cord: A tube of nerve tissue running along the back, which develops into the brain and spinal cord.

* Pharyngeal Slits: Openings in the throat region that are used for filter feeding in aquatic chordates. While they may be present only in the embryonic stage in some groups, they are a defining feature of chordates.

* Post-Anal Tail: A tail extending beyond the anus. This can be a defining feature in early stages of development, even if it is reduced or absent in some adult chordates.

Think of it like this: Imagine you're looking for a common ancestor of all these groups. While their adult forms are very different, the early stages of their development reveal these common features that point to a shared ancestry.

Why it matters: Classifying organisms based on shared ancestry helps us understand their evolutionary relationships and provides a framework for studying how diversity arose on Earth.

It's important to remember: Phyla are just one level of classification. Within Chordata, there are many sub-groups (classes, orders, etc.) that further categorize animals based on additional shared characteristics.