Classification of organisms that do not fit in other classifications
Wastebasket taxon (also called a wastebin taxon,[1]dustbin taxon[2] or catch-all taxon[3]) is a term used by some
taxonomists to refer to a
taxon that has the purpose of classifying organisms that do not fit anywhere else. They are typically defined by either their designated members' often superficial similarity to each other, or their lack of one or more distinct
character states or by their not belonging to one or more other taxa. Wastebasket taxa are by definition either
paraphyletic or
polyphyletic, and are therefore not considered valid taxa under strict
cladistic rules of taxonomy. The name of a wastebasket taxon may in some cases be retained as the designation of an
evolutionary grade, however.
There are many examples of paraphyletic groups, but true "wastebasket" taxa are those that are known not to, and perhaps not intended to, represent natural groups, but are nevertheless used as convenient groups of organisms. The
acritarchs are perhaps the most famous example. Wastebasket taxa are often old (and perhaps not described with the systematic rigour and precision that is possible in the light of accumulated knowledge of diversity) and populous; further characteristics are reviewed by.[6]
The order
Insectivora has traditionally been used as a dumping ground for placental insectivorous mammals (and similar forms such as
colugos), usually aligned with
carnivorans,
ungulates and
bats. While the core components (
moles,
shrews,
hedgehogs and their close relations) do in fact form a consistent clade,
Eulipotyphla, that is part of
Laurasiatheria with the aforementioned clades, other mammals historically placed in the order have been found to belong to other branches of the placental tree:
tree shrews and colugos are
euarchontans related to
Primates and sometimes grouped in
Sundatheria, while
tenrecs,
golden moles and
elephant shrews are all
afrotheres, probably forming the clade
Afroinsectiphilia. Both of these clades have at times been accused of being wastebasket taxa themselves, grouping superficially similar animals in Euarchonta and Afrotheria, respectively, but they have been more strongly supported by genetic studies.[citation needed]
Vermes is an obsolete taxon of worm-like animals. It was a catch-all term used by
Carl Linnaeus and
Jean-Baptiste Lamarck for non-arthropod invertebrate animals.
Fossil groups that are poorly known due to fragmentary remains are sometimes grouped together on gross morphology or
stratigraphy, only later to be found to be wastebasket taxa, such as the crocodile-like
Triassic group
Rauisuchia.[13]
One of the roles of taxonomists is to identify wastebasket taxa and reclassify the content into more natural units. Sometimes, during taxonomic revisions, a wastebasket taxon 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 abandoned altogether (for example, Simia).[citation needed]
Related concepts
A related concept is that of
form taxon, "wastebasket" groupings that are united by gross morphology. This is often result of a common mode of life, often one that is
generalist, leading to generally similar body shapes by
convergent evolution.[citation needed]
^Chase, Mark W.; Sue Zmarzty; M. Dolores Lledó; Kenneth J. Wurdack; Susan M. Swensen; Michael F. Fay (2002). "When in doubt, put it in Flacourtiaceae: a molecular phylogenetic analysis based on plastid rbcL DNA sequences". Kew Bulletin. 57 (1): 141–181.
Bibcode:
2002KewBu..57..141C.
doi:
10.2307/4110825.
JSTOR4110825.