Electron backscatter diffraction is used to characterize the transition interface between aragonite prisms and nacre in two species of freshwater molluscs, Anodonta anatina (Linnaeus, 1758) and Anodonta cygnea (L. 1758). In the duck mussel, Anodonta anatina, the shell thickness is comprised mainly of nacre with only a thin outer prismatic layer, whereas in the swan mussel, A. cygnea, the shell comprises a greater thickness of aragonite prisms relative to the nacreous layer. In A. anatina, the prism-nacre interface is flat and featureless. In A. cygnea, the nacre below the base of the prisms is characterized by distinct concave footholds. In both species, the c-axis of aragonite nanogranules comprising the prisms is parallel with the prism length. This crystallographic orientation, with the c-axis perpendicular to the shell exterior, occurs in the nacreous layer and thus is conserved throughout the shell thickness. In A. cygnea alternate prisms have a crystallographic orientation with the c-axis deviating from the prism long axis by approximately 20 deg. The initial nacreous layer adopts the crystallographic orientation of the preceding prisms and subsequent nacreous laminae are oriented in register with the bulk nacre. This is consistent with the hypothesis that nacre evolved through horizontal partitioning of prisms.
Synopsis
A 1.89-m section excavated from the
Stimulograptus sedgwickii
Biozone (Silurian: Llandovery, Aeronian) at Dob’s Linn, Southern Uplands, Scotland, representing some 0.2 Ma of deposition, shows pronounced, quantitatively established patterns of graptolite abundance and diversity. The most striking signature comprises several marked peaks in abundance of the distinctive taxon
Neolagarograptus tenuis
(Portlock) within this interval. Such ‘bloom events’, if recognized further afield, may provide very fine-scale stratigraphic resolution, seemingly akin to the ammonite biohorizons of the Jurassic. Taxonomic richness as a whole decreases upwards and appears associated with bentonite layers, suggesting that volcanic ash falls might have led to local extinctions of graptolite populations with little or no subsequent recovery. Repetition of the analysis on a duplicate section along-strike at Dob’s Linn showed comparable patterns, demonstrating the robustness of the methodology.
Montañ a Negra is a 121 m cinder cone in the Bandas del Sur region of southern Tenerife. Formed in the Middle Pleistocene, it comprises alternating phonolitic pumice deposits and scoria layers; the latter are extremely fossiliferous with good taphonomical fidelity. 40 Ar/ 39 Ar age determination provides new dates of 302 AE 7.6 ka and 299.9 AE 11.4 ka for the Lower and Upper Aldea Blanca pumice fall deposits, respectively. This chronological constraint allows comparison of the palaeo-habitat with the global climate at the time of pyroclastic activity. Abundant terrestrial gastropod species and rare disarticulated Coleoptera fragments are to be found. The occurrence of the endemic semi-slug genus Plutonia (Family Vitrinidae) is significant in indicating a woodland habitat in the region during the Middle Pleistocene. We suggest that this may have been forest, possibly dominated by laurel, which is in stark contrast to the present-day semi-desert.
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