Eumalacostraca

Abstract: Crustaceans have been an important component of marine diversity and biomass since the earliest Phanerozoic. With a relatively well-documented fossil record, they provide an excellent subject for a continuous study of disparity (approxiamtely bodyplan variety) from the Cambrian to the Recent. A data base of 135 morphological characters forms the basis for cladistic and morphospace studies at the ordinal and sub-ordinal level. Gross cladistic topology is: (Eumalacostraca+Hoplocarida vs Maxillopoda) vs Phyllopoda (paraphyletic). Each of these groups is of approximately equal disparity, and occupies a distinct region of the morphospace plot. A few problematical fossils (e.g. Waptia and Odaraia ) fall close to the base of the tree. Comparison of the cladogram with stratigraphic range data indicates the location of probable ghost lineages, and randomization procedures provide a statistical test of the goodness of fit of a given set of stratigraphic ranges to a given tree topology. Disparity indices are calculated at series and stage intervals. Observed range data indicate that Cambrian disparity was approximately one third its present level. The Earliest Ordovician saw a marked decrease, with an increase and subsequent plateau through rest of the period. Increases through the Silurian and Devonian corresponded to the radiation of branchiopods, cephalocarids, and latterly the Eumalacostraca and Hoplocarida. By the end of the Carboniferous, observed disparity had reached over four fifths of Recent levels, and the remaining history of the group saw a gradual but slightly irregular increase up until the end of the Tertiary. Indices of disparity incorporating ghost lineages exhibit less marked peaks and troughs, with fewer perturbations overall. Cladistically-implied disparity in the Lower Cambrian is estimated at three quarters of that in the Recent. Rarefaction is used to compare actual levels of disparity at each time interval with the mean for a similar number of taxa selected randomly from the list of all realized bodyplans. Most intervals preserved a range of forms more disparate than the mean of random samples drawn from the pool of all the taxa considered. From the Triassic to the Recent this difference was intermittently significant. Once occupied, extremes of morphospace tend not to fall vacant again.

Abstract: The relation of the Leptostraca to other groups of Crustacea has long been a problem of interest, although the alliance with the Eumalacostraca has been amply justified (Claus, 1872, 1888, Calm an, 1909). Claus discussed the essentially Malacostracan form of the appendages of Nebalia , and the mode of feeding by the aid of these appendages has been shown (Cannon, 1927) to be a specialized modification of the type shown by the simpler Malacostraca (Cannon and Manton, 1927, Some of the apparent differences between the Leptostraca and the Eumalacostraca have recently been shown to be differences of degree rather than of kind. Thus the seventh abdominal segment of Nebalia is found also in the embryo mysid, but is partially or completely fused with the sixth segment in the adult Lophogaster and Hemimysis respectively (Ma n to n, 1928,a and b). Thus the basal number of abdominal segments in the Eumalacostraca as well as the Leptostraca may be seven. The presence of a large caudal furca in Nebalia may also be a difference of degree, if this furca is homologous with the embryonic furca of a mysid (Manton, 1928, a). Other differences, such as the presence of a fully formed carapace adductor muscle, of cephalic liver lobes, etc., require further investigation. Of the resemblances between the Leptostraca and Eumalacostraca, the mode of development of the former is said to resemble that of the Mysidacea, but the embryology of no Leptostracan has been adequately followed, and the recent work on mysid development has considerably modified many previous views on this subject.

Abstract: Schram, F. R. (Department of Zoology, Eastern Illinois University, Charleston, Illinois 61920) 1977. Paleozoogeography of Late Paleozoic and Triassic Malacostraca. Syst. Zool. 26:367-379.-Paleozoogeographic patterns of Late Paleozoic through Triassic marine and freshwater malacostracans, interpreted in light of Croizat's vicariance model of biogeography, lend significant insights into the distribution of living malacostracans, as well as events in the Late Paleozoic and early Mesozoic radiation of these crustaceans. The Hoplocarida and Eumalacostraca had a tropical Laurentian range which became well established by Late Carboniferous time. With the suturing of the supercontinent Pangaea in the Permian, the former Laurentian marine crustaceans appear to have spread into the nearshore and freshwater habitats of Gondwana and Angara. By the late Perrmian and Early Triassic a decapod eumalacostracan fauna became established in tropical and subtropical regions of Pangaean marine habitats, while the Late Paleozoic hoplocaridan and eumalacostracan fauna became extinct or was restricted to geographic and ecologic refugia (which in contemporary terms is largely expressed as a Gondwana and deep-sea distribution). [Malacostraca; paleozoogeography; vi-