stress, albeit not an outright mass extinction. In some regions, however, major floral extinctions did occur, with some researchers challenging the hypothesis of there being no significant floral mass extinction on this basis. In the Newark Supergroup of the United States East Coast, about 60% of the diverse monosaccate and bisaccate pollen assemblages disappear at the Tr–J boundary, indicating a major extinction of plant genera. Early Jurassic pollen assemblages are dominated by Corollina, a new genus that took advantage of the empty niches left by the extinction. Possible causes Gradual processes Gradual climate change, sea-level fluctuations, or a pulse of oceanic acidification during the late Triassic may have reached a tipping point. However, the effect of such processes on Triassic animal and plant groups is not well understood. The extinctions at the end of the Triassic were initially attributed to gradually changing environments. Within his 1958 study recognizing biological turnover between the Triassic and Jurassic, Edwin H. Colbert's 1958 proposal was that this extinction was a result of geological processes decreasing the diversity of land biomes. He considered the Triassic period to be an era of the world experiencing a variety of environments, from towering highlands to arid deserts to tropical marshes. On the other hand, the Jurassic period was much more uniform both in climate and elevation due to excursions by shallow seas. Later studies noted a clear trend towards increased aridification towards the end of the Triassic. Although high-latitude areas like Greenland and Australia actually became wetter, most of the world experienced more drastic changes in climate as indicated by geological evidence. This evidence includes an increase in carbonate and evaporite deposits (which are most abundant in dry climates) and a decrease in coal deposits (which primarily form in humid environments such as coal forests). In addition, the climate may have become much more seasonal, with long droughts interrupted by severe monsoons. Geological formations in Europe seem to indicate a drop in sea levels in the late Triassic, and then a rise in the early Jurassic. Although falling sea levels have sometimes been considered a culprit for marine extinctions, evidence is inconclusive since many sea level drops in geological history are not correlated with increased extinctions. However, there is still some evidence that marine life was affected by secondary processes related to falling sea levels, such as decreased oxygenation (caused by sluggish circulation), or increased acidification. These processes