ON THE CLASSIFICATION OF EUMALACOSTRACA

Traditional views of eumalacostracan phylogeny are not in accord with current understanding of the fossil record. Another approach results from identification of basic morphotypes upon which radiations were built in the course of eumalacostracan history. Random association of characters (absence or varying conditions of the carapace, biramous or uniramous thoracopods, and presence or absence of subthoracic brood pouch of some form) yields an array of 16 morphotypes, of which 10 are recognized in the fossil and Recent record. The resultant "paper animals" combined with an analysis by cladistic methods yield six possible phenograms of eumalacostracan phylogeny. An evolutionary uncertainty principle precludes definitive choice among these patterns to produce a cladogram and taxonomy. However, a taxonomic system is advocated with the subjectivity associated with uncertainty clearly acknowledged. Perhaps one of the greatest noncontroversies in crustacean evolution has been the phylogeny and interrelationships of the Eumalacostraca. Since Calman (1904, 1909) it has been taken for granted that the higher malacostracans could be classified as discreet superorders which were derived from an ancestral type neatly delineated by a set of characters known as the caridoid facies. However, Calman's synthesis was the result of several decades of debate over how malacostracan groups were related. Boas (1883) visualized a single line arising from the phyllopods and giving off the malacostracan groups at successive levels. Claus (1885) envisioned three branches arising from a "eumalacostracan" stock: leptostracans, stomatopods, and "urschizopods," and this was essentially Grobben's (1892) position as well, except that Grobben had the stomatopods as the earliest offshoot from the "urschizopods.' Haeckel (1896) derived his "proschizopods" from the Leptostraca and in turn treated these as an ancestral stock for five lines: mysids, cumaceans, other "peracarids," "eucarids," and stomatopods. Calman's caridoid concept, however, exerted a stablizing influence on all succeeding speculations on higher malacostracan phylogeny, with the result that subsequent phyletic schemes for these groups have been essentially similar (Giesbrecht, 1913; Grobben, 1919; Balss, 1938; Siewing, 1956, 1963; and Brooks, 1962). Of these, Siewing came to be a focus of a certain school (viz., Fryer, 1964; Hessler, 1969) which essentially holds that all eumalacostracan superorders arose from a central stem. Schram (1969a, b) recently suggested that the Hoplocarida had nothing to do with other Eumalacostraca, but rather had an independent origin separate from the Eumalacostraca sensu stricto. This latter position was objected to by Burnett and Hessler (1973) but supported by Reaka (1975), Bowman and Abele (in press), and Kunze (personal communication). Through all this, the essential stability and supposed interrelationships of the Eumalacostraca sensu stricto, viz., syncarids, eocarids, peracarids, pancarids, and eucarids has remained relatively constant. The Eocarida and Syncarida were interpreted as near the base of the eumalacostracan line; and supposedly from somewhere within the eocaridan stock the Eucarida and Peracarida were derived, with the Pancarida seeming to bear some relationships to peracarids (some authors placing them as a separate superorder and others as an order of peracarids). This traditional arrangement of orders is outlined in Fig. 1.