Phylum

A phylum (plural: phyla) is a taxonomic rank below Kingdom and above Class. "Phylum" is equivalent to the botanical term division.

Although a phylum is often spoken of as if it were a hard and fast entity, no satisfactory definition of a phylum exists. Consequently the number of phyla varies from author to author. The relationship of phyla is increasingly well known, and larger clades can be erected to contain many of the phyla.

Informally, phyla can be thought of as grouping animals based on general body plan, developmental or internal organizations. For example, though seemingly divergent, spiders and crabs both belong to Arthropoda, whereas earthworms and tapeworms, similar in shape, are from Annelida and Platyhelminthes, respectively. Although the International Code of Botanical Nomenclature allows the use of the term "phylum" in reference to plants, the term "Division" is almost always used by botanists.

The best known animal phyla are the Mollusca, Porifera, Cnidaria, Platyhelminthes, Nematoda, Annelida, Arthropoda, Echinodermata, and Chordata, the phylum to which humans belong. Although there are approximately 35 phyla, these nine include over 96% of animal species. Many phyla are exclusively marine, and only one phylum, the Onychophora (velvet worms) is entirely absent from the world's oceans–although ancestral oncyophorans were marine.

The origin of phyla has traditionally been interpreted as a sudden and rapid event early in the Cambrian period, known as the Cambrian explosion. However, this interpretation stemmed from an incomplete knowledge of the fossil record, and a circular definition of a phylum; organisms of the time were mainly similar to, but not strictly members of, modern phyla. The significance of this event depends on (1) for how long modern phyla had existed prior to the Cambrian–fossil embryos and new interpretations of the Ediacara biota suggest that there may be a fair Precambrian root to the phyla; and (2) how soon phyla appeared in the Cambrian, a factor that depends on both the definition of a phylum and our interpretation of early fossils which may not display vital characteristics–for instance, non-mineralised parts of organisms rarely preserve.

The magnitude of the event was also overestimated as early authors felt it necessary to erect a new phylum for any organism that could not be accommodated in modern phyla. This approach is misleading and unhelpful; by one definition, such organisms do not fall into any phylum, but are classified as "aunts" of a phylum.

Defining a phylum
At the most basic level, a phylum can be defined in two ways: as a group of organisms with a certain degree of morphological or developmental similarity (the phenetic definition), or a group of organisms with a certain degree of relatedness (the phylogenetic definition). Attempting to define a level of the Linnean hierarchy without referring to relatedness is an unsatisfactory approach, but the phenetic definition is more useful when addressing questions of a morphological nature–such as how successful different body plans were.

The largest objective measure in the above definitions is the "certain degree"–how unrelated do organisms need to be to be members of different phyla? Implicitly, the general definition is that they should be closely enough related that it is clear that they are more closely related to one another than to any other group. However, this too is problematic, as the definition is a function of our current knowledge about relationships. As more data becomes available, particularly from molecular studies, we are better able to judge the relationships between groups; therefore the phyla will eventually be merged as it becomes apparent that they are related to one another; for instance, the onychophora and tardigrada are now accepted as stem group arthropods; by the general definition, these three phyla should be combined.

This has led to calls for the concept of a phylum to be abandoned in favour of cladistics, a method in groups are placed on a "family tree" without any formal ranking of group size. So as to provide a handle on the size and significance of groups, a "body-plan" based definition of a phylum has been proposed by paleontologists Graham Budd an Sören Jensen. The definition was posited by paleontologists because it is extinct organisms that are typically hardest to classify, because they can be extinct off-shoots that diverged from a phylum's history before the characters that define the modern phylum were all acquired.

By Budd and Jensen's definition, phyla are defined by a set of characters shared by all their living representatives. This has a couple of small problems – for instance, characters common to most members of a phylum may be secondarily lost by some members. It is also defined based on an arbitrary point of time (the present). However, as it is character based, it is easy to apply to the fossil record. A more major problem is that it relies on an objective decision of which group of organisms should be considered a phylum.

Its utility is that it makes it easy to classify extinct organisms as "stem groups" to the phyla with which they bear the most resemblance, based only on the taxonomically important similarities. However, proving that a fossil belongs to the crown group of a phylum is difficult, as it must display a character unique to a sub-set of the crown group. Further, organisms in the stem group to a phylum can bear all the aspects of the "body plan" of the phylum without all the characters necessary to fall within it. This weakens the idea that each of the phyla represents a distinct body plan.

Based upon this definition, which some say is unreasonably affected by the chance survival of rare groups, which vastly increase the size of phyla, representatives of many modern phyla did not appear until long after the Cambrian–as late as the Carboniferous in the case of the Priapulids.