LATE MESOZOIC ERA
CRETACEOUS PERIOD: 146 to 65 million years ago
In the Late Cretaceous, much of the interior of North America was covered by a vast inland sea that stretched from the Gulf of Mexico to the Arctic Ocean (Dunbar, 1969). By the end of the Cretaceous, Greenland had begun to separate from Eurasia. Enormous amounts of volcanic activity in the Deccan traps of India (perhaps associated with the separation of India from the Seychelles Islands) produced 550,000 square km (200,000 sq. mi) of basaltic rock that can be as thick as 3 km (Seyfert, 1979).
Marine life flourished ranging from the diverse, squidlike ammonoids to giant carnivorous reptiles.
Flowering plants became the dominant group of plants on earth. Small pterosaurs became extinct but were succeeded by enormous species. A diversity of bird groups evolved including the dominant Mesozoic birds, the enantiornithine birds (now extinct) and the ancestral members of the lineage which would produce modern bird groups in the Cenozoic.
Diverse dinosaur groups evolved including new carnivores, armored ankylosaurs, duck-billed hadrosaurs, and horned dinosaurs.
Other reptiles evolved including snakes (which retained their legs at first) and new groups of crocodiles and turtles.
Mammals diversified and produced marsupial and placental lineages, although they were still small, generalized forms.
The end of the Cretaceous witnessed increased volcanic activity due to the movement of continents. Volcanoes were active in the rising Rocky Mountains and in the southwest of India, volcanic eruptions produced a bout 480,000 cubic miles of basaltic lava covering an area about the size of California (with accumulated lava flows more than 1.5 miles thick in some areas. These Deccan traps are thought to have released about 21 trillion tons of hydrogen sulfate and 300 billion tons of hydrochloric acid leading to acid rain and the acidification of the oceans. In addition, 33 trillion tons of carbon dioxide were released which, after an initial cooling of 3-5 0C because of the volcanic debris blocking sunlight, may have raised global temperature by 5 0C. These eruptions, which are the largest known set of volcanic eruptions in earths history, occurred at the end of the Cretaceous over a period of 500,000 years between 66 and 65 million years ago (Dingus, 1998; Feduccia, 2003).
The earths continents were approaching their modern forms. The Rockies, Alps, and Andes were all rising. The oceans were regressing and the continents were more exposed than they had been in the previous 60 million years. There are some volcanoes in India that erupted in this time (60-65 mya) and could have released the iridium discussed below. All of this geologic activity could have affected climate and the volcanic activity could have released toxic gases into the atmosphere.
By the end of the Cretaceous, flowering plants had replaced the once dominant Mesozoic gymnosperms and ferns. Although such a drastic change in the vegetation seems a likely candidate for the extinction of some organisms, it certainly wouldnt have affected marine life and flying pterosaurs. If anything, the diversity of herbivorous dinosaurs (such as hadrosaurs, ceratopsians, and pachycephalosaurs) was increasing in the Late Cretaceous.
Did a meteorite/comet impact cause the end-Cretaceous extinction? There
are a number of craters that could be the right age and there may have been
multiple impacts. The Chicxulub crater off the Yucatan peninsula (Mexico)
has the appearance of an impact crater and is 65 million years old. This
crater is about 100 km in diameter, but is surprisingly deep. Its rocks
are full of iridium and shocked mineral grains. Ejecta and evidence of tsunamis
around the Caribbean, Central America, and the southern United States support
the conclusion that the meteor struck the Yucatan Penninsula (Chatterjee,
1997; Swisher, 1992; Melosh, 1997). The Shiva Crater located off the coast
of India has also been dated at 65 million years of age (Chatterjee, 1997).
The 10 km asteroid that impacted the earth struck at a speed of 90,000
km/h and a force of 100 million megatons. What would have been the effects
of such an impact? Effects of the impact probably included temperatures
at the impact site of several thousand degrees, global wildfires, an immediate
spike in global temperature, followed by a freezing period of several
months to a year because of the debris cloud which would have blocked
sunlight, a longer term global warming period, and intense acid rain which
would have acidified the ocean. It is estimated that sunlight would have
been blocked for 3 months because of the amount of debris ejected. Tsunamis,
global fires, a short-term temperature spike, and debris clouds which
blocked the sun and caused a longer-term temperature drop. In the Western
U.S., the layers of Cretaceous pollen lie under an iridium layer, which
lies under a layer with plant and fungal types which are consistent with
a cooling of an impact winter. Tsunamis are thought to be
responsible for an unusual layer of sandstone which underlies the iridium
rich layer in sites in the Southeast U.S. The tsunami is thought to have
reached a height of up to 300 feet and flooded regions which were 62 miles
inland. (Dingus, 1998; Wolfe, 1986; Tschudy, 1984; Vajda, 2004; Feduccia,