Huge carnivores appeared. There was a tremendous diversity in dinosaur species. Mammals were flourishing, and flowering plants developed and radically changed the landscape. The breakup of the supercontinent Pangaea into separate continents was underway. The separation of Laurasia and Gondwana was complete. In the first half of the Cretaceous, temperatures were warm, seasonality was low, and global sea levels were high. At the end of the Cretaceous, there were severe climate changes, lowered sea levels, and high volcanic activity. The Cretaceous period ended 65 million years ago with the extinction of the dinosaurs. The first placental mammals appeared at the beginning of the Cretaceous. The Cretaceous saw the rise and extinction of the toothed birds, Hesperornis and Ichthyornis. The earliest fossils of birds resembling loons, grebes, cormorants, pelicans, flamingos, ibises, rails, and sandpipers were from the Cretaceous. During the Cretaceous, primitive flowering plants (anthophytes, also called angiosperms) continued to develop (they evolved about 140 million years ago, during the late Jurassic period). Flowering plants (like magnolia, ficus, credneria, sassafras, viburnum) quickly outnumbered the other plants (mostly ferns , horsetails, trees (like conifers and gingkos), and cycads ), changing the environment tremendously. The extinction of bennettitaleans (a major group of plants) also occurred at this time. There was no polar ice during the mild warm, subtropical Cretaceous. The land was covered with forests surrounded by shallow seas. Seasonality was increasing. Most of the land mass was at or around sea level until the mid-Cretaceous, a time of high tectonic activity (continental plate movement) and accompanying volcanic activity. Many mountain ranges were formed, including California's Sierra Nevadas, the Rocky Mountains in the western USA and the European Alps. The sea levels rose during the mid-Cretaceous, covering about one-third of the land area. Toward the end of the Cretaceous, there was a drop in sea level, causing land exposure on all continents, more seasonality, and greater extremes between equatorial and polar temperatures. Source