Form classification is the classification of organisms based on their morphology, which does not necessarily reflect their biological relationships. Form classification, generally restricted to palaeontology, reflects uncertainty; the goal of science is to move "form taxa" to biological taxa whose affinity is known.[1]
Strictly defined, form taxonomy is restricted to fossils that preserve too few characters for a conclusive taxonomic definition or assessment of their biological affinity, but whose study is made easier if a binomial name is available by which to identify them.[2] The term "Form classification" is preferred to "Form taxonomy"; taxonomy suggests that the classification implies a biological affinity, whereas in fact form classification is about giving a name to a group of morphologically-similar organisms that may not be related.[1]
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Form taxa are groupings that are based on common overall forms. Early attempts at classification of labyrinthodonts was based on skull shape (the heavily armoured skulls often being the only preserved part). The amount of convergent evolution in the many groups lead to a number of polyphyletic taxa.[3] Such groups 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 the fossil record, or are unrelated to any modern phylum — can currently only be grouped in "form taxa". 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.
A "parataxon", or "sciotaxon" (Gr. "shadow taxon"), is a classification based on incomplete data: for instance, the larval stage of an organism that cannot be matched up with an adult. It reflects a paucity of data that makes biological classification impossible.[1] A sciotaxon is defined as a taxon thought to be equivalent to a true taxon (orthotaxon), but whose identity cannot be established because the two candidate taxa are preserved in different ways and thus cannot be compared directly.[1]
In paleobotany, the term is occasionally substituted for the more correct term "organ taxon", meaning a group of fossils of a particular part of a plant, such as a leaf or seed, whose parent plant is not known because the fossils were preserved unattached to the parent plant.[4] Names given to organ taxa may only be applied to the organs in question - and cannot be extended to the entire organism.[2] However, because a form genus is erected on morphological grounds (which do not change when its affinity is known), a form genus that can eventually be assigned to a higher biological group should not be renamed.[5]
Whilst organ genera can potentially be assigned to a family (even if the other parts of the plant are unknown), form genera usually cannot : although they may be referrable to higher categories (e.g. "Fungi" or "Animalia").[2]
The part of the plant is often, but not universally, indicated by the use of a suffix in the generic name:
"Form taxon" can more casually be used to describe a wastebasket taxon: either a taxon that is not a natural (monophyletic) group but united by shared plesiomorphies, or a presumably artificial group of organisms whose true relationships are not known, being obscured by ecomorphological similarity. Well-known form taxa of this kind include "ducks", "fish", "reptiles" and "worms".