Wastebasket taxon
Wastebasket taxon (also called a wastebin taxon,[1] dustbin taxon[2] or catch-all taxon[3]) is a term used in some 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 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 to be 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.
Examples
- The familiar category of invertebrates is an "everything-else" category, comprising all animals without backbones.
- The Colubridae family of snakes traditionally are defined as not a natural group.
- The kingdom Protista is composed of all eukaryotes that are not animals, plants or fungi, leaving to the protists all single-celled eukaryotes.[4]
- The Tricholomataceae is a fungal group, composed of the white-, yellow-, or pink-spored genera in the Agaricales not already classified as belonging to the Amanitaceae, Lepiotaceae, Hygrophoraceae, Pluteaceae, or Entolomataceae.[5]
- Carnosauria and Thecodontia are fossil groups, banded together back when the limited fossil record did not allow for a more detailed scheme.
- Condylarthra is an artificial clade into which ungulate mammals not clearly within Perissodactyla or Cetartiodactyla were traditionally shoved. Many of these groups, like Meridiungulata or Protungulatum, may not represent laurasithere mammals, while others like phenacodontids have been clearly established as early odd-toed ungulates.[6][7]
- 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 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 compose the euarchontan 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 taxons themselves, grouping superficially similar animals in Euarchonta and Afrotheria respectively, but they have been more strongly supported by genetic studies.
Wastebasket taxa in science
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.[8]
One of the roles of taxonomists is to identify wastebasket taxa and reclassify the content into more natural units. 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).
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.
The term wastebasket taxon is sometimes employed in a derogatory fashion to refer to an evolutionary grade taxon.
See also
References
- ↑ Friedman, M.; Brazeau, M.D (7 February 2011). "Sequences, stratigraphy and scenarios: what can we say about the fossil record of the earliest tetrapods?". Proceedings of the Royal Society. B. 278 (1704): 432–439. doi:10.1098/rspb.2010.1321. PMID 20739322. Retrieved 15 April 2013.
- ↑ Hallam, A.; Wignall, P. B. (1997). Mass extinctions and their aftermath. Oxford [England]: Oxford University Press. p. 107. ISBN 978-0-19-854916-1.
- ↑ Monks, N. (2002). "Cladistic analysis of a problematic ammonite group: the Hamitidae (Cretaceous, Albian-Turonian) and proposals for new cladistic terms". Palaeontology. 45 (4): 689–707. doi:10.1111/1475-4983.00255.
- ↑ Whittaker RH (January 1969). "New concepts of kingdoms or organisms. Evolutionary relations are better represented by new classifications than by the traditional two kingdoms". Science. 163 (3863): 150–60. doi:10.1126/science.163.3863.150. PMID 5762760.
- ↑ Young AM. (2002) Brief notes on the status of Family Hygrophoraceae Lotsy. Australasian Mycologist 21(3):114–116.
- ↑ Naish, Darren (8 August 2013). "Phenacodontidae, I feel like I know you". Tetrapod Zoology. Scientific American. Archived from the original on 10 March 2014.
- ↑ Cooper, L. N.; Seiffert, E. R.; Clementz, M.; Madar, S. I.; Bajpai, S.; Hussain, S. T.; Thewissen, J. G. M. (2014-10-08). "Anthracobunids from the Middle Eocene of India and Pakistan Are Stem Perissodactyls". PLoS ONE. 9 (10): e109232. doi:10.1371/journal.pone.0109232. PMC 4189980
. PMID 25295875. - ↑ Nesbitt, S. J. (2003). "Arizonasaurus and its implications for archosaur divergence". Proceedings of the Royal Society B. 270 (Suppl. 2): S234–S237. doi:10.1098/rsbl.2003.0066. PMC 1809943. PMID 14667392.