Because the term "fish" is defined negatively, and excludes the tetrapods (i.e., the amphibians, reptiles, birds and mammals) which descend from within the same ancestry, it is paraphyletic, and is not considered a proper grouping in systematic biology. The traditional term pisces (also ichthyes) is considered a typological, but not a phylogenetic classification.

Fish do not represent a monophyletic group, and therefore the "evolution of fish" is not studied as a single event.

Fish are a paraphyletic group: that is, any clade containing all fish also contains the tetrapods, which are not fish. For this reason, groups such as the "Class Pisces" seen in older reference works are no longer used in formal classifications.

Traditional classification divide fish into three extant classes, and with extinct forms sometimes classified within the tree, sometimes as their own classes

What are we supposed to do?

find the hidden fish. big fan

Dunkleosteus?(still stranged out by the question)

Edited 4/25/2015 02:57:06

Super secret puzzle, find the hidden fish.

in the ocean?

https://www.youtube.com/watch?v=bpIm4SWOI34

Blocked