LATE MESOZOIC ERA

CRETACEOUS PERIOD: 146 to 65 million years ago

continents

CONTINENTS

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:

Marine life flourished ranging from the diverse, squidlike ammonoids to giant carnivorous reptiles.

ammonoidselasmosaurus

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.

pterosaurextinct birddiving bird

Diverse dinosaur groups evolved including new carnivores, armored ankylosaurs, duck-billed hadrosaurs, and horned dinosaurs.

tyrannosaurusnodosaur

hadrosaurhorned dinosaur

Other reptiles evolved including snakes (which retained their legs at first) and new groups of crocodiles and turtles.

snake crocodile

Mammals diversified and produced marsupial and placental lineages, although they were still small, generalized forms.

mammalmammal

 

EXTINCTIONS


The Cretaceous-Tertiary (K/T) extinction killed off a number of groups of organisms. Given the great diversity of organisms which died in this extinction, it is unlikely that the extinction of the dinosaurs was something that would only have effected them (such as a virus or increased predation on dinosaur eggs by mammals). About 15% of marine invertebrate families become extinct; the ammonoids, rudists (reef building clams), and inoceramid clams were especially effected. Marine reptiles (mosasaurs, plesiosaurs), foraminiferan groups, dinosaurs, pterosaurs, several types of marsupial animals, and some plants all become extinct. Dinosaurs had adapted to such a broad range of habitats, including polar environments, which a sudden extinction event is more likely than a gradual changing of climate. A number of species may have survived the K/T boundary only to become extinct shortly afterwards (Kaiho, 1999; Jablonski, 1997; Jablonski, 1995; Drake, 1983; Marshall, 1996).

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 earth’s 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 earth’s 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 wouldn’t 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.

comet

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, 2003).