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It has been hypothesized that increased water column stratification has been an abiotic ''universal driver'' affecting average cell size in Cenozoic marine plankton. Gradually decreasing Cenozoic radiolarian shell weight, by contrast, suggests that competition for dissolved silica, a shared nutrient, resulted in biologic coevolution between radiolaria and marine diatoms, which expanded dramatically in the Cenozoic. We present data on the 2 components of shell weight change-size and silicification-of Cenozoic radiolarians. In low latitudes, increasing Cenozoic export of silica to deep waters by diatoms and decreasing nutrient upwelling from increased water column stratification have created modern silicapoor surface waters. Here, radiolarian silicification decreases significantly (r ‫؍‬ 0.91, P < 0.001), from Ϸ0.18 (shell volume fraction) in the basal Cenozoic to modern values of Ϸ0.06. A third of the total change occurred rapidly at 35 Ma, in correlation to major increases in water column stratification and abundance of diatoms. In high southern latitudes, Southern Ocean circulation, present since the late Eocene, maintains significant surface water silica availability. Here, radiolarian silicification decreased insignificantly (r ‫؍‬ 0.58, P ‫؍‬ 0.1), from Ϸ0.13 at 35 Ma to 0.11 today. Trends in shell size in both time series are statistically insignificant and are not correlated with each other. We conclude that there is no universal driver changing cell size in Cenozoic marine plankton. Furthermore, biologic and physical factors have, in concert, by reducing silica availability in surface waters, forced macroevolutionary changes in Cenozoic low-latitude radiolarians.evolution ͉ microfossils ͉ micropaleontology ͉ morphometrics ͉ Ocean Drilling Program T he evolution of ocean plankton has played an important role in the development of the earth's climate system, and changes in ocean plankton may affect future changes in climate (1). The deep-sea microfossil record of protist plankton provides an unusual opportunity to understand how plankton evolution and environmental change mutually affect each other. Recently, it has been proposed that Cenozoic changes in upper ocean water column stratification have influenced the evolution of cell size in a variety of marine protist plankton groups, including planktonic foraminifera (2), diatoms (3), and dinoflagellates (4), and it has been suggested that these patterns are indicative of a ''universal driver'' of size change in Cenozoic plankton (4). The polycystine radiolarians are an important marine protist zooplankton group abundant as fossils in Cenozoic and Mesozoic deep-sea sediments. Their general size, feeding ecology, and distribution patterns are similar to those of the better-known planktonic foraminifera (5, 6), although radiolarians are more diverse and, at least since the Oligocene (7), possess distinct, diverse endemic high-latitude faunas. Radiolarians are most diverse in low latitudes and most abundant in near-surface waters, although some species inhabit ...

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The ejecta blankets of impact craters in volatile‐rich environments often possess characteristic layered ejecta morphologies. The so‐called double‐layered ejecta (DLE) craters are characterized by two ejecta layers with distinct morphologies. The analysis of high‐resolution image data, especially HiRISE and CTX, provides new insights into the formation of DLE craters. A new phenomenological excavation and ejecta emplacement model for DLE craters is proposed based on a detailed case study of the Martian crater Steinheim—a well‐preserved DLE crater—and studies of other DLE craters. The observations show that the outer ejecta layer is emplaced as medial and distal ejecta that propagate outwards in a debris avalanche or (if saturated with water) a debris flow mode after landing, overrunning previously formed secondary craters. In contrast, the inner ejecta layer is formed by a translational slide of the proximal ejecta deposits during the emplacement stage that overrun and superimpose parts of the outer ejecta layer. Based on our model, DLE craters on Mars are the result of an impact event into a rock/ice mixture that produces large amounts of shock‐induced vaporization and melting of ground ice, leading to high ejection angles, proximal landing positions, and an ejecta curtain with relatively wet (in terms of water in liquid form) composition in the distal part versus dryer composition in the proximal part. As a consequence, basal melting of ice components in the ejecta at the transient crater rim, which is induced by frictional heating and the enhanced pressure at depth, initiates an outwards directed collapse of crater rim material in a translational slide mode. Our results indicate that similar processes may also be applicable for other planetary bodies with volatile‐rich environments, such as Ganymede, Europa, and the Earth.

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