Evidence is presented in support of the following phylogenetic hypotheses: (1) Sipuncula are the sister taxon of Mollusca; (2) the two aplacophoran taxa, Neomeniomorpha (= neomenioids) and Chaetodermomorpha (= chaetoderms), are monophyletic with a common neomenioid-like ancestor, and of the two taxa, Chaetodermomorpha are more derived; (3) Aplacophora and Polyplacophora are sister taxa and form a clade, Aculifera; (4) Aculifera are the sister group of the remaining extant mollusks, Conchifera; and (5) Aplacophora are progenetic Aculifera. The evidence is based on homologies of early and late embryological development, adult morphologies, and molecular analyses. Embryological development in sipunculans and mollusks shows a close relationship between them, and embryological development of the shell separates Aculifera and Conchifera. Adult morphologies indicate: (1) monophyly of Aplacophora; (2) sister-group relationship between Aplacophora and Polyplacophora; (3) a molluscan plesiomorphy of nonsegmented serial replication of organs; and (4) progenesis in Aplacophora. Molecular evidence supports the embryological and morphological relationships between Sipuncula and Mollusca. Mollusca are thus hypothesized to be coelomate Eutrochozoa, which share an ancestor that probably had serial replication of organs. Differences in size and structure of the coelom among Eutrochozoa are hypothesized to have been brought about by changes in the timing and the process of cavitation of the mesodermal bands that arise from cell 4d. Through the process of progenesis Aplacophora retained an ovoid embryological shape and several internal structures that, although they appear to be in a primitive state, are actually secondarily derived as is quadrant D specification during early cleavage.

Summary:Odontogriphus omalus was originally described as a problematic non-biomineralized lophophorate organism. Here we reinterpret Odontogriphus based on 189 new specimens including numerous exceptionally well-preserved individuals from the Burgess Shale collections of the Royal Ontario Museum. This additional material provides compelling evidence that the feeding apparatus in Odontogriphus is a radula of molluscan architecture comprising two primary bipartite tooth rows attached to a radular membrane and showing replacement by posterior addition. Further characters supporting molluscan affinity include a broad foot bordered by numerous ctenidia located in a mantle groove and a stiffened cuticular dorsum. Odontogriphus has a radula similar to Wiwaxia corrugata but lacks a scleritome. We interpret these animals to be members of an early stem-group mollusc lineage that likely originated in the Neoproterozoic Ediacaran Period, providing support for the retention of a biomat-based grazing community from the late Precambrian until at least the Middle Cambrian.

Macrobenthos of the deep, northern Gulf of Mexico (GoM) was sampled with box cores (0.2 m 2 ) along multiple cross-depth transects extending from depths of 200 m to the maximum depth of the basin at 3700 m. Bathymetric (depth) zonation of the macrofaunal community was documented for 6 major taxa (a total of 957 species) on the basis of shared species among geographic locations; 4 major depth zones were identified, with the 2 intermediatedepth zones being divided into east and west subzones. Change of faunal composition with depth reflects an underlying continuum of species replacements without distinct boundaries. The zonal patterns correlated with depth and detrital particulate organic carbon (POC) export flux estimated from remotely-sensed phytoplankton pigment concentrations in the surface water. The Mississippi River and its associated mesoscale eddies, submarine canyon, and deep sediment fan appear to influence the horizontal zonation pattern through export of organic carbon from the ocean surface and the adjacent continental margin. On the local scale, near-bottom currents may shape the zonation pattern by altering sediment grain size, food availability, and larval dispersal. This study suggests a macroecological relationship between depth, export POC flux, and zonation; parsimonious zonal thresholds need to be tested independently for other continental margin ecosystems.KEY WORDS: Northern Gulf of Mexico · Deep sea · Macrofauna · Zonation · Biogeography · Community structure · POC export flux · Macroecology Resale or republication not permitted without written consent of the publisherThaumastasoma species 521 (Crustacea; Isopoda; Nannoniscidae), a typical species of the lower continental slope.

As the original molluscan radula is not known from direct observation, we consider what the form of the original radula may have been from evidence provided by neomenioid Aplacophora (Solenogastres), Gastropoda, Polyplacophora, and the Cambrian fossil Wiwaxia corrugata (Matthews). Conclusions are based on direct observation of radula morphology and its accessory structures (salivary gland ducts, radular sac, anteroventral radular pocket) in 25 species and 16 genera of Aplacophora; radula morphogenesis in Aplacophora; earliest tooth formation in Gastropoda (14 species among Prosobranchia, Opisthobranchia, and Pulmonata); earliest tooth formation in four species of Polyplacophora; and the morphology of the feeding apparatus in W. corrugata. The existence of a true radula membrane and of membranoblasts and odontoblasts in neomenioids indicates that morphogenesis of the aplacophoran radula is homologous to that in other radulate Mollusca. We conclude from p redness of salivary gland ducts, a divided radular sac, and a pair of anteroventral pockets that the plesiomorphic state in neomenioids is bipartite, formed of denticulate bars that are distichous (two teeth per row) on a partially divided or fused radula membrane with the largest denticles lateral, as occurs in the genus Helicoradomenia. The tooth morphology in Helicoradomenia is similar to the feeding apparatus in W. corrugata. We show that distichy also occurs during early development in several species of gastropods and polyplacophorans. Through the rejection of the null hypothesis that the earliest radula was unipartite and had no radula membrane, we conclude that the original molluscan radula was similar to the radula found in Helicoradomena species.

Six species in three genera of Prochaetodermatidae are described from over 650 stations and 5200 specimens in the Atlantic and north Pacific Oceans from depths between 500 and 7300 m. Included are all species in the North American Basin and all species in Chevroderma n.g. Three principal characters differentiate prochaetodermatid Species and genera: spicules, radula, and body shape. Family membership is defined by radula and jaws, spicule morphology determines genus, and species are described by spicules and radula. Mean body shape describes populations of species. Interference colors produced by the aragonite spicules indicate spicule thickness and symmetry. The variation in Prochaetoderma yongei n. sp., described in detail, establishes the taxonomic base on which to judge the morphological limits of a prochaetodermatid species. Spaihoderma n.g. and Chevroderma n.g. differ from each other and from the genus Prochaetoderma in spicule morphology. P. yongei and S. clenchi n. sp. are widespread northwestern and eastern Atlantic continental slope and abyssal rise species. C. turnerae and C. gauson n. spp. are abyssal species, the former occurring throughout the Atlantic, the latter only in the northern West European Basin. C. scalpellum n. sp. is a slope species of restricted range in the eastern Atlantic. C. whitlatchi n. sp. is a wide-ranging abyssal and hadal species of the northern east and mid-Pacific. A wide geographic range is correlated with a vertical depth distribution greater than 1500 m. All species are patchy in distribution but particular species can be numerically dominant and occur at high densities locally, e.g., up to 400 m for P. yongei and 178 m for C. whitlatchi. In the north Atlantic, greatest numerical abundances and lowest diversity of Prochaetodermatidae occur in the North American Basin.