World Congress of Malacology Antwerp ... - Unitas Malacologica

Interpreting the last molluscan Unionoidea from the Cretaceous of North Dakota
Hartman, Joseph H. 1 ; Burton-Kelly, Matthew 1 ; Sweet, Arthur R. 2
1. University of North Dakota, Department of Geology and Geological Engineering,
81 Cornell Street Mail Stop 8358, Grand Forks, ND 58202 USA,
Email: joseph_hartman@und.edu
2. Survey of Canada, 3033 33 rd Street N.W., Calgary, AB T2L 2A7 Canada,
Email: asweet@nrcan.gc.ca
Arguably, the best-sampled and constrained Cretaceous-Paleogene (K/P) continental mollusk-bearing
strata anywhere are in the Williston Basin of Montana and North Dakota. Even so, few unionoid
mussels are known within a few meters below or tens of meters above the K/P boundary. The
Hartman base-level change hypothesis is that sedimentological conditions changed decidedly prior to
the K/P boundary and persisted into the Paleocene. A corollary to this hypothesis is the loss of highly
sculptured continental taxa just prior to the K/P impact event, thus contradicting the likely survival of
unionoids according to the Robertson et al. sheltering hypothesis. A low-diversity assemblage
(Locality L6516, Section M8608) dominated by anodontine unionoids and a Viviparidae species is
documented in a 7-cm-thick layer, 90.5 cm above the base of the Ludlow Member (Fort Union
Formation) and 63 cm below the K/P boundary (identified on the basis of a well-preserved and
diverse suite of angiosperm pollen). Extant anodontines are thin-shelled and most often found in
shallow, quiet-water environments. L6516 mussels are preserved as external molds in pale yellowishbrown
(10 YR 6/2) silty, very fine-grained sandstone, rich in plant debris. Most are found with their
valves closed or in butterfly-open position, indicating that subsequent transport was limited to
nonturbulent conditions. This locality raises specific questions: 1) Is this type of preservation more
common than previously suspected? Can anodontine or other mussels be located by a systematic
search of specific likely lithologies? and 2) Does the presence of an anodontine morphology in this
stratigraphic interval indicate a shift in depositional regime as expected by the base-level change
hypothesis? The five described Late Cretaceous “Anodonta” taxa currently shed no light on the
problem; they are without sedimentological context, and three are based on poor material. This record
of interesting end-Cretaceous taxa raises far more questions than it answers.
Diversity of microgastropod fauna in Northwestern Pacific: A case study of Heterostrophans in
Japan
Hasegawa, Kazunori
National Museum of Nature and Science, 4-1-1 Amakubo, Tsukuba City, Ibaraki 305-0005, Japan,
Email: hasegawa@kahaku.go.jp
In spite of remarkably high diversity among medium to large molluscs in northwestern Pacific
waters, which have been relatively well surveyed, only a little is known about the micromolluscan
fauna of this region. Researchers undertaking ecological or faunal studies often encounter
undescribed species even in the intertidal zone, and this can hinder the progress of their respective
studies. For instance, a preliminary field study showed that among 14 species of gastropod occurring
on the red algae Gracilaria verrucosa in the upper intertidal zone on the Miura Peninsula in Honshu,
Japan, five species (including the most dominant one) were regarded as being undescribed.
‘Micro’-species (less than 5 mm in adult body length) occur intermittently throughout the Mollusca,
but are especially concentrated in several specific groups, such as the Scissurellidae, Rissooidea and
Heterostropha in the Gastropoda. The Heterostropha are the most basal and possible paraphyletic
members of the diverse clade Heterobranchia, and comprise rather unfamiliar families that had
previously been assigned to ‘Prosobranchia’ or are recently discovered. Most of these families have
not attracted much attention from taxonomists, with a few exceptions such as the Pyramidellidae and
Architectonicidae, and many species in the northwestern Pacific remain unstudied. In this
presentation, species of several heterostrophan families such as the Rissoellidae, Cimidae,
Omalogyridae and Hyalogyrinidae from Japanese waters are introduced, with special emphasis on
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