Wastebasket taxon

Wastebasket taxon (also called a wastebin taxon or dustbin taxon) is a term used in taxonomic circles to refer to a taxon that has the sole purpose of classifying organisms that do not fit anywhere else. They are typically defined by their lack of one or more distinct character states or by their not belonging one or more other taxa. Wastebasket taxa are by definition either paraphyletic or polyphyletic, and are therefore not considered to be valid taxa under modern rules of taxonomy.

The familiar category of invertebrates is an "everything-else" category left when vertebrates have been selected out: "invertebrates" include animals from all the phyla save Chordata and share no distinctive common ancestry. Other examples of wastebasket taxa include the Protista, Carnosauria, Thecodontia, and Tricholomataceae. Sometimes, during taxonomic revisions, the wastebasket taxa can be salvaged after doing thorough research on its members, and then imposing tighter restrictions on what continues to be included. Such techniques "saved" Carnosauria and Megalosaurus. Other times, the taxonomic name contains too much unrelated "baggage" to be successfully salvaged. As such, it is usually dumped in favour of a new, more restrictive name (for example, Rhynchocephalia or Thecodontia), or abandoned altogether (for example, Simia).

A related concept is that of form taxon. Form taxa are "wastebasket" groupings that are united by a common mode of life, often one that is generalist, in consequence acquiring generally similar body shapes by convergent evolution. Ediacaran biota — whether they are the precursors of the Cambrian explosion of phyla or are unrelated to any modern phylum— can only be grouped in "form taxa" as yet. Other examples include the seabirds and the "Graculavidae". The latter were initially described as the earliest family of Neornithes but are nowadays recognized to unite a number of unrelated early neornithine lineages, several of which probably later gave rise to the "seabird" form taxon of today.