Phylogeny of Malacostraca is the evolutionary relationships of the largest of the six classes of crustaceans, containing about 40,000 living species, divided among 16 orders. Its members display a great diversity of body forms. Although the class Malacostraca is united by a number of well-defined and documented features, which were recognised a century ago by William Thomas Calman in 1904, the phylogenetic relationship (the evolutionary tree) of the orders which compose this class is unclear due to the vast diversity present in their morphology. Molecular studies have attempted to infer the phylogeny of this clade, resulting in phylogenies which have a limited amount of morphological support. To resolve a well-supported eumalacostracan phylogeny and obtain a robust tree, it will be necessary to look beyond the most commonly utilized sources of data (nuclear ribosomal and mitochondrial sequences). Features The Malacostraca is assumed to be monophyletic due to several common morphological traits which are present throughout the group and due to molecular studies that have also confirmed it. William T. Calman in 1904 and 1909 described these common morphological features and introduced the major taxonomic subdivisions of the Malacostraca which are still in use today: he divided the Malacostraca in two subclasses the Phyllocarida and the Eumalacostraca, which is further subdivided into four superorders: Eucarida, Peracarida, Hoplocarida and Syncarida. W.T. Calman coined the term caridoid facies for the common eumalacostracan (shrimp-like) features; the most important of these is the constant number of segments in each tagma: members of this class have five segments in the cephalon, eight thoracic segments (thoracomeres) and six segments in the pleon and possess a telson, which forms a characteristic tail fan when the uropods are present. Many other characteristic features are present but their presence varies amongst lineages; one notable ancestral feature which varies is the carapace, which may be absent, reduced or well developed covering the whole cephalothorax. Furthermore, Richter, S., & Scholtz, G. (2001) list five separate unique eumalacostracan features which taken together form a strong argument in favour of the monophyly of the Eumalacostraca. However debate arises in the relationship between the subdivisions of the Malacostraca, due to the presence of several contrasting features. The traditional basal malacostracans The Phyllocarida is a group of about 36 small marine species that are distributed across planet and possess a characteristic large bivalve carapace and an elongated abdomen with no uropods. This group is believed to be