100 million years – 100 metres
Flowering plants are spreading. Dinosaurs are diverse and many theropod predatory dinosaurs, including birds, possess feathers. Marsupials evolve. The mid-oceanic ridges are volcanically active, the carbondioxide content of the atmosphere is high and the climate is warm.
The Pleiades, an open cluster, is born. The gas and dust similar to the original cloud can still be seen in photographs. The Pleiades is a clearly visible open star cluster in the constellation of Taurus. Also known as Seven Sisters, the star cluster was formed about 100 million years ago. Open star clusters contain young stars formed from same cloud at the same time. A gas cloud surrounding the Pleiades is not the original cloud that gave birth to the stars, but another interstellar gas cloud that the star cluster now happens to pass by. In reality, the Pleiades contains more than thousand stars, not just ”the seven sisters” that can be easily seen in the constellation.
The continents were drifting towards their current positions. The mid-oceanic ridges were volcanically active and oxygen and carbondioxide content of the atmosphere were high. The climate was warm and moist and the sea levels were high.
Life flourished in the shallow warm waters. The oceans hosted rich fauna of different fish and reptiles, which was fed by the rich populations of algae and plankton. Chalk deposits were formed sediments of algae shells. Coral reefs, crocodiles and dinosaurs spread into higher latitudes than ever before. Fauna included also different insects, spiders, amphibians, lizards, snakes, tortoises, and mammals.
Dinosaurs were very diverse. Many theropod predatory dinosaurs, including birds, Yutyrannus and Deinonychus (’raptors’ in the Jurassic Park movie), possessed feathers. Birds were similar to their predecessors Archeapteryx from Jurassic period, but could probably fly better. The most famous dinosaurs from Cretaceous were Tyrannosaurus rex and Triceratops, from the late part of the period.
Mammals were mostly small and unnoticable. Typical group of mammals were multituberculates that looked like rodents, even were not related to them, and primitive marsupials. The earliest marsupial fossiles (Sinodelphys) are found from China.
Cretaceous period also witnesses many special crocodiles, the largest being the Sarcosuchus imperator, living in the area which now is Nigeria in western Africa. It growed up to 12 meters long and it was preying also dinosaurs, along with the smaller prey. Also smaller crocodile species existed, some of which were apparently herbivores.
The first flowering plants (Angiosperms) are from early Cretaceous period. Their rapid proliferaton is probably related to the evolution of the pollinating insects. Flowering plants had advanteage due to the efficient pollination, fast growth, good dispersal to harsh and dry environments. Angiosperms are so successful that by the end of the Cretaceous period they form 40-60% of the flora, which affected to the evolution of the herbivore dinosaurs.