Wastebin taxon

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Wastebin taxon (also called a wastebasket, or dustbin taxon) is a term used in taxonomic circles that refers to a taxon that has the sole purpose of classifying organisms that don't fit anywhere else. 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.

Examples of wastebin taxa include Simia, Rhynchocephalia, Carnosauria, Megalosaurus and Thecodontia. Sometimes, during taxonomic revisions, the wastebin 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).

Related is the concept of form taxon. Form taxa are "wastebin" 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. 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 taxa of today.