Cretaceous period

If the asteroid-impact theory is correct, the extinctions should be repetitive and the Ir anomaly should be observed in other geological stratigraphic levels corresponding to known extinctions. About five other massive extinctions (besides the one at the end of the Cretaceous Period) have been noted [25]. These come at the end of the Cambrian ( 500 MY ago), the Ordovician (M35 MY age), the Devonian ( 345 MY ago), the Permian ( 230 MY ago) and the Triassic ( 195 MY ago) Periods. 

The unique chemical composition of cosmogenous debris has provided some insight into why approximately 70% of the species of organisms on Earth were driven extinct over a relatively short time interval approximately 66 million years ago. Evidence for this mass extinction has been observed in marine sediments throughout all the ocean basins. In a contemporaneous layer deposited at the end of the Cretaceous period, the hard parts of many species of marine plankton abruptly vanished from the sedimentary record. This sedimentary layer is also characterized by a large enrichment in the rare element iridium. 

An interesting story as to how most of the iridium appeared on Earth was explained recently by scientists who discovered a thin layer of iridium in the sediments that were laid down in the Earth s crust at the end of the Cretaceous period. This was a period about 65 million years ago when meteors and asteroids crashed into the Earth. These extraterrestrial bodies contained a high percentage of iridium. Dust from the impact spread around the Earth and blocked the sun for months, resulting in the extinction of many plants and animals, including the dinosaurs. This extensive dust cloud also deposited a thin coat of the element iridium that was contained in the fiery bolides. 

Fossil glycoproteins of the soluble organic matrix are present in an 80-million year old mollusc shell from the Late Cretaceous Period. Discrete molecular weight components, as determined by gel electrophoresis, are preserved. A particular repeating amino acid sequence (—Asp—Y—) found in contemporary mollusc shell proteins was identified in fossil glycoproteins425. ... 

During the Cretaceous period which dates back approximately 100 million years. Deposits include the predominately bituminous and sub-bituminous coal beds in the Rocky Mountain Province, extending in large, separated regions from central Montana into northeastern Arizona and northwestern New Mexico. 

Like iridium, arsenic is enriched in Cretaceous-Tertiary boundary shales from New Zealand (Brooks et al., 1984 Strong et al., 1987). The iridium is believed to have originated from an asteroid impact that caused the massive extinction at the end of the Cretaceous period about 65 million years ago. In contrast, most of the arsenic in the boundary shales probably had a terrestrial origin (Strong et al., 1987). The extinction of marine organisms, especially plankton, from the impact may have been responsible for increased anoxic conditions in the oceans, which led to the precipitation of arsenic in the marine deposits (Brooks et al., 1984), 541. 

Cephalopods are the smartest and most complex of the mollusks. This group includes squid, octopus, cuttlefish, and chambered nautilus. The ammonites were an extremely diverse and successful group of cephalopods that roamed the world s oceans for millions of years until their extinction at the end of the Cretaceous period, about 65 million years ago. 

Mesozoic Era The period of geologic time beginning 245 million years ago and ending 65 million years ago the age of the dinosaurs and cycads, the Mesozoic falls between the Paleozoic and Cenozoic Eras and includes the Triassic, Jurassic, and Cretaceous Periods. 

With the exception of burmite all the ambers mentioned so far are from the Tertiary period. There are many finds of amber from the Cretaceous period, 140 to 65 million years ago, but the vast majority of these are of little use as decorative material as they are far too brittle to work. They are usually found in very small pieces and tend to be a dull, opaque brown. They are, however, of great scientific interest, especially as some contain plant and animal inclusions. 

If photosynthesis were to cease, all higher forms of life would be extinct in about 25 years. A milder version of such a catastrophe ended the Cretaceous period 65.1 million years ago when a large asteroid struck the Yucatan Peninsula of Mexico. Enough dust was sent into the atmosphere that photosynthetic capacity was greatly diminished, which apparently led to the disappearance of the dinosaurs and allowed the mammals to rise to prominence. 

The last globally catastrophic collision between Earth and an asteroid probably took place 65 million years ago at the end of the Cretaceous period. It now seems reasonably likely that the extinction of many species, including the great dinosaur extinction which occurred at the Cretaceous-Tertiary boundary, was caused by the impact of an asteroid approximately 6.2 mi (10 km) in diameter. The submerged remnants of the giant impact crater produced in this terminal Cretaceous collision were recently discovered on the coast of the Yucatan peninsula in Mexico. The crater, Chixulub (pronounced CHIKS-a-lub), is approximately 112 mi (180 km) in diameter and has long been buried under coastal sediments. 

A rare kind of mountain is the individual volcano with no known relationship to a volcanically active region. Solitary volcanos like these have erupted in tectonically quiet landscapes, such as east Texas of the Cretaceous period, and their cause remains a mystery. 

A range of fault block mountains rose far inland as the continent was squeezed from west to east. The Sevier mountains stood west of the Cretaceous period s interior seaway, in what is now Montana, Idaho, Nevada, and Utah. The dinosaurs of that time (80-130 million years ago) left their tracks and remains in the mud and sand worn off these mountains. 

The original application of oxygen isotope thermometry. The thermal history (top) of the surroundings in South Carolina, USA, during the Cretaceous period as recorded by oxygen isotope changes (bottom) as a function of shell radius in the PeeDee belemnite (Urey et a/., 1951). 

Erbacher J., Huber B.T., Norris R.D., Markey M. (2001) Increased thermohaline stratification as a possible cause for an ocean anoxic event in the Cretaceous period. Nature 409, 325-9. 

The most well-known functions of secondary metabolites among arthropods include the use of pheromones for intraspecific communication, the employment of antipredatorial defensive agents, and the offensive use of paralytic and/or toxic agents, such as in the form of venoms, for the acquisition of prey. In recent years, considerable insight has been gained in all three categories, as well as in the discovery of heretofore unknown interactions (see Section 2.04.4.1). Fossil evidence of chemical defense in the insects reaches as far back as the Early Cretaceous period. The chemical defense of insects has been reviewed as... 

Cover illustration dragonfly Gomphaeschnaoides obliquus, about 110 million years old, from the Lower Cretaceous period, and found in north east Brazil. POPULAR SCIENCE Image kindly supplied by Dr David Martill. 

Wilson, P. A. Norris, R. D. 2001. Warm tropical ocean surface and global anoxia during the mid-Cretaceous period. Nature, 412, 425-429. 

Finally, Figure 23.5 is a schematic reconstruction of global temperature over the last 100 million years, based on analysis of various marine and terrestrial deposits. The Earth s climate, prior to the last million years, was considerably warmer than present. The warmest temperatures occurred in the Cretaceous Period, about 100 million years ago, when the global mean surface temperature may have been as much as 6-8°C above that of the present. During most of the long period of time up to about 1 million years ago, there is little evidence for ice sheets of continental scale. Subtropical plants and animals existed up to 55°-60° latitude, as compared with their present extent of about 30°N, the latitude of... 

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