February Geology and GSA Today media highlights

The oldest widely accepted mollusks are a group of cap-shaped fossil shells known as helcionellids. Helcionellids are usually very small (1-2 millimeters), which has contributed to the belief that ancestral mollusks were millimeter-scale animals. Mollusks are usually born with a shell which accompanies them their whole life. As they grow in size, so does the shell, but only at the margins; therefore, the juvenile shell always remains visible at the tip of the adult one. Martí Mus et al. have discovered a new, large (2 centimeter) mollusk from the Cambrian in Spain which has preserved, at the tip of its large shell, a tiny cap identical to the small helcionellids previously found only as "free" elements in Cambrian rocks. Considering the mode of growth of molluskan shells, Martí Mus et al.'s finding shows that at least some small helcionellids represent juvenile (or maybe even larval) stages of species that reached a centimeter-scale adult size. Therefore, contrary to a widely held wisdom, ancestral mollusks may have been relatively large animals.

Extinction and environmental change across the Eocene-Oligocene boundary in Tanzania
Paul Pearson et al., School of Earth and Ocean Sciences, Cardiff University, Main Building, Park Place, Cardiff CF10 3YE, UK. Pages 179-182.

The transition between the Eocene and Oligocene epochs (between approximately 33.5 and 40 million years ago) was a time of extreme climatic upheaval as a warm "greenhouse" climate state, which had prevailed for many millions of years, gave way to a glaciated one in which a large ice cap appeared on Antarctica for the first time. It was also a time of widespread extinction, in the oceans and on land, as the world's biota adjusted to the new conditions. In their paper, Pearson et al. discuss a new, highly detailed record of these events that has been discovered in southern Tanzania. It comes from drill cores through ancient marine muds that originally accumulated on the sea floor but are now exposed on land. The muds contain abundant and exquisitely preserved microscopic fossils, which preserve a record of changes in tropical marine life and environments through the climate transition. The new records show that the marine extinctions peaked relatively early, well before maximum glacial conditions set in, and involved both the plankton and organisms from shallow-water reef environments. These new data will help geologists locate the epoch boundary more precisely than before.

Tenuous connection between high-silica rhyolites and granodiorite plutons
Allen F. Glazner et al., Department of Geological Sciences, CB# 3315, University of North Carolina, Mitchell Hall, 104 South Road, Chapel Hill, North Carolina 27599-3315, USA. Pages 183-186.

Rhyolite makes up some of the largest eruptions known on Earth. For decades it has been assumed that rhyolite (a magma that is rich in silicon and commonly forms explosive eruptions) forms as the last liquid in a crystallizing body of granite, and therefore rhyolite and granite are intrusive/extrusive equivalents of one another. In this paper, Glazner et al. geochemically show that late-stage liquids in granites have trace-element compositions that are quite distinct from erupted rhyolites. Thus, rhyolite must form by some other mechanism.