Refined Ordovician timescale reveals no link between asteroid breakup and biodiversification.

The catastrophic disruption of the L chondrite parent body in the asteroid belt c. 470 Ma initiated a prolonged meteorite bombardment of Earth that started in the Ordovician and continues today. Abundant L chondrite meteorites in Middle Ordovician strata have been interpreted to be the consequence of the asteroid breakup event. Here we report a zircon U-Pb date of 467.50±0.28 Ma from a distinct bed within the meteorite-bearing interval of southern Sweden that, combined with published cosmic-ray exposure ages of co-occurring meteoritic material, provides a precise age for the L chondrite breakup at 468.0±0.3 Ma. The new zircon date requires significant revision of the Ordovician timescale that has implications for the understanding of the astrogeobiologic development during this period. It has been suggested that the Middle Ordovician meteorite bombardment played a crucial role in the Great Ordovician Biodiversification Event, but this study shows that the two phenomena were unrelated.

Increased gibberellin biosynthesis in transgenic trees promotes growth, biomass production and xylem fiber length.

In most tree-breeding programs worldwide, increasing the trees' growth rates and stem volumes and shortening their rotation times are important aims. Such trees would yield more biomass per unit area. Here we show that overexpressing a key regulatory gene in the biosynthesis of the plant hormone gibberellin (GA) in hybrid aspen (Populus tremula x P. tremuloides) improves growth rate and biomass. In addition, these transgenic trees have more numerous and longer xylem fibers than unmodified wild-type (wt) plants. Long fibers are desirable in the production of strong paper, but it has not as yet proved possible to influence this trait by traditional breeding techniques. We also show that GA has an antagonistic effect on root initiation, as the transgenic lines showed poorer rooting than the control plants when potted in soil. However, the negative effect on rooting efficiencies in the initial establishment of young plantlets in the growth chamber did not significantly affect root growth at later stages.