Shell microstructure of the early bivalve Pojetaia and the independent origin of nacre within the mollusca

Abstract: Pojetaia and Fordilla are the oldest bivalve molluscs, occurring in roughly co‐eval rocks from the Tommotian, and are the only undisputed, well‐known bivalves from the Cambrian. New specimens reveal that Pojetaia had a laminar inner shell microstructure reminiscent of the foliated aragonite of modern monoplacophorans, and the same is true for Fordilla. A similar shell microstructure is seen in Anabarella and Watsonella, providing support for the hypothesis that they are the ancestors of bivalves. Foliated aragonite shares many similarities with nacre, and it may have been the precursor to nacre in bivalves. No cases of undisputed nacre occur in the Cambrian, in spite of much shell microstructure data from molluscs of this time period. Thus, although considered by many to be homologous among molluscs, we conclude that nacre convergently evolved in monoplacophorans, gastropods, bivalves, and cephalopods. This independent origin of nacre appears to have taken place during, or just prior to, the Great Ordovician Biodiversification Event and represents a significant step in the arms race between predators and molluscan prey.     

Shell microstructures of the Early Cambrian Anabarella and Watsonella as new evidence on the origin of the Rostroconchia

Internal moulds of the laterally compressed monoplacophoran Anabarella plana Vostokova, 1962, and likely earliest rostroconch Watsonella Grabau, 1900, from the Early Cambrian of the Siberian Platform, show similar microstructures. The moulds are covered with a thin phosphatic crust replicating the inner morphology and microstructure of the shells. The shells were completely removed during etching of the samples in 10% acetic acid, except for some moulds of Watsonella sp., which retained an incompletely preserved and recrystallized wall. Three types of microtexture were found in moulds of Anabarella and Watsonella : polygonal, spiny and step-wise. The polygonal texture is well exposed in the apical area and dorsal margin and is interpreted to represent a prismatic outer layer. The polygons can be partially overlapped by spiny and stepwise textures that may represent a crossed-lamellar inner layer. Prisms and lamellae were first-order structural units, probably consisting of fibers. The similar shell microstructures of Anabarella and Watsonella , especially at the dorsal margins, support the hypothesis that a laterally compressed monoplacophoran such as Anabarella plana was the first evolutionary step from monoplacophorans towards Early Cambrian bivalves via the earliest rostroconch-like Watsonella.     

Gastropod biodiversity at the 'Evolution Canyon' microsite, lower Nahal Oren, Mount Carmel, Israel

Twenty-six species of gastropods (terrestrial, shell-bearing snails and slugs) were recorded at the 'Evolution Canyon' microsite, lower Nahal Oren, Mount Carmel, Israel. Twenty-five species were recorded at the temperate, mesic north-facing slope (NFS) and 20 species at the xeric south-facing slope (SFS). Out of these species, six were NFS specific ( Pilorcula raymondi hebraica , Euchondrus septemdentatus , Monacha crispulata , Pyramidula rupestris hierosolymitana , Truncatellina haasi , and Vitrea contracta ) and one was SFS specific ( Prolimax eustrictus ). The interslope difference was probably partly due to missing forest species at the SFS in comparison with the NFS. Twenty-two species were Levantine endemics (84.6%) and four species were more widely distributed in the Palaearctic region (15.4%). The Levantine species are inhabitants of the mesic and mainly mountainous regions, but four species ( Granopupa granum , Calaxis hierosolymarum , Cecilioides acicula , and Helix engaddensis ) also penetrate the deserts. Seven species ( Buliminus labrosus , H. engaddensis , Levantina spiriplana caesareana , Metafruticicola fourousi , Monacha syriaca , Sphincterochila cariosa , and Xeropicta vestalis joppensis ) were significantly more abundant on the SFS than on the NFS. The local physical microclimatic sharp divergence leads to gastropod adaptive interslope biotic divergence caused by natural selection.  © 2008 The Linnean Society of London, Biological Journal of the Linnean Society , 2008, 93 , 147–155.     

New data on molluscs and their shell microstructures from the Middle Cambrian Gowers Formation, Australia

Abstract: Numerous new cases of preserved shell microstructure were discovered in molluscs from the Middle Cambrian Gowers Formation (Ptychagnostus atavus/Peronopsis opimus Zone, Floran Stage) in the Georgina Basin, Australia. The new data provide further evidence that, by the Middle Cambrian, molluscan shell microstructures were diverse, and many molluscs had a complex shell with multiple types of shell microstructure. In addition, many new occurrences of laminar microstructures are described herein. For many, the nature of these laminar microstructures is not known, but in three species the microstructure is foliated calcite, and in at least two the microstructure is more likely to have been calcitic semi‐nacre, a type of microstructure known in brachiopods and bryozoans but unknown in modern molluscs. This commonality among these three closely related lophotrochozoans underscores a similar mechanism of biomineralization. Moreover, these observations suggest a prevalence of calcite‐shelled lineages among molluscs from the Middle Cambrian, a time of calcite seas. In addition, the broad occurrence of laminar, nacre‐like microstructures in many of these fossils reveals how widespread these strong (fracture‐resistant) microstructures were in Middle Cambrian molluscs. Additionally, a few specimens of Yochelcionella preserve imprints of a bilaterally symmetrical pair of muscle scars. New taxa described here include Corystos thorntoniensis gen. et sp. nov., Yochelcionella snorkorum sp. nov., Yochelcionella saginata sp. nov., and Anhuiconus? agrenon sp. nov.     

Changes in tree sapling composition within powerline corridors appear to be consistent with climatic changes in New York State

Despite emerging evidence that on‐going climate change is affecting species physiology, distribution, and phenology, there are few studies that examine changes in tree sapling establishment as a response. Changes in tree species composition can be expected due to increasing temperatures, with subsequent effects on future forest compositions. This study's objective was to examine changes in relative density of tree species assemblages within powerline corridors from 1975 to 2003 in New York State. Powerline corridors in New York are commonly surrounded by forests, which creates constant tree invasion pressure within a perpetual old‐field environment. This unique combination of factors allowed us to examine tree sapling establishment in a nearly constant environment over a 28‐year period, utilizing manova and PCA as primary statistical analyses. Tree species dynamics varied across the four ecological provinces within New York over time. Northern pioneer species (Betula populifolia, Fraxinus americana, Prunus serotina, and Tilia americana) declined across the state over the past 28 years, while the southern pioneer species (Betula lenta, Liriodendron tulipifera, and Sassafras albidum) increased in the hot continental division. In the warm continental division, the pine‐hemlock assemblage increased in the Northeastern Mixed Forest Province, while aspen‐birch increased in the Adirondack Highlands Forest Province, likely due to increases in precipitation. It appears that climate change may have had some influence on tree sapling composition that could affect future vegetation management decisions and expectations in powerline rights‐of‐way and forests.     

The anatomy,taphonomy, taxonomy and systematic affinity of Markuelia: Early Cambrian to Early Ordovician scalidophorans

Abstract: Markuelia is a vermiform, annulated introvertan animal known as embryonic fossils from the Lower Cambrian to Lower Ordovician. Analysis of an expanded and revised dataset for Introverta shows that the precise position of Markuelia within this clade is dependent on the taxa included. As a result, Markuelia is assigned to the scalidophoran total group to reflect uncertainty as to whether it is a stem‐scalidophoran or a stem‐priapulid. The taxonomy of the genus is revised to provide an improved taxonomic framework for material assigned to Markuelia. Five species are recognized: M. secunda Val’kov, M. hunanensis Dong and Donoghue, M. lauriei Haug et al., M. spinulifera sp. nov. and M. waloszeki sp. nov. Finally, the preservation of Markuelia is evaluated in the light of both the taphonomy of the fossil embryos themselves and the experimental taphonomy of the priapulid Priapulus caudatus, which has been proposed as both a close relative and an anatomical analogue of Markuelia.     

A new genus of cave‐dwelling microcrustaceans from the Dinaric Region (south‐east Europe): adaptations of true stygobitic Cladocera (Crustacea: Branchiopoda)

We revise ‘true’ stygobitic cladocerans and lift three species from Alona Baird, 1843 (Cladocera: Chydoridae). Species of Brancelia gen. nov. are inhabitants of saturated karst, collected in pools of residual water in the amphibious zones of a few caves in the Dinaric Region, Europe. All species are blind (regression of eye and ocellus), have elongated sensorial equipment (aesthetascs) and a short rostrum, reduced antennal spines, and a globular body. In contrast to earlier hypotheses, there is no epigean chydorid taxon from the Palaearctic that can be linked to Brancelia gen. nov. The new genus may be an offshoot of six‐limbed Aloninae, but a littoral‐benthic ancestor is not apparent and most likely extinct. Evolution of Brancelia gen. nov. parallels that of other subterranean Cladocera like Phreatalona Van Damme, Brancelj & Dumont, 2009. We discuss the functional morphology of Brancelia gen. nov. and compare its adaptations to a subterranean life mode with those of Phreatalona. © 2011 The Linnean Society of London, Zoological Journal of the Linnean Society, 2011, 161 , 31–52.