Who is responsible for the extinction of dinosaurs

Nevertheless, there has not been unanimous agreement in interpretations amongst these expert investigators. Because, from the vantage point of the present, the extinction of such a vast range of species seemed to have suddenly occurred, the concept has been proposed that some catastrophic environmental change is a unifying hypothesis to explain their disappearance.

Some investigators, on the other hand, suggest that the time frame of the process of extinction from a biological viewpoint was very long and that the various species had declined in number over many generations. The conflict between catastrophism and gradualism to describe the process of extinction has been debated extensively during the second half of the 20th century. The case for a catastrophic cause of mass extinction was strengthened by the discovery of a worldwide enrichment of the rare element iridium in a narrow band of geological strata formed 66 Mya , 1.

The source of that iridium was proposed to be from a huge asteroid or comet that collided with the Earth, coinciding with the time of the mass extinctions , 2 — 4. The discovery of the Chicxulub impact crater in the Yucatan Peninsula on the Gulf of Mexico confirmed, from its size and structure, that a large celestial object had collided with the Earth at the time that the mass extinctions had occurred , 5.

The consequences of that impact would indeed have been catastrophic, not just in the geographic region around the impact site, but worldwide because of environmental changes from an atmospheric dust and aerosol cloud that would have remained for years in the stratosphere , 2. Those who favour a gradual decline towards extinction of all species of the dinosaur family, except the ancestors of modern birds, have some plausible arguments.

The dating of fossils of all known dinosaur species is incomplete and is only well established for a range of dinosaurs in North America. It has been argued that long-term environmental changes had diminished the number of species over millions of years before the asteroid or comet impact, particularly the large-bodied Saurischian and Ornithischian dinosaurs , 6.

Careful review of fossil records of both classes of dinosaurs has indicated that the number of species was in decline over millions of years. Thus the family Dinosauria would have been susceptible to extinction by a range of environmental changes to which they could not adapt , 7. Nevertheless, others have concluded that there is little evidence for a gradual decline worldwide in the diversity of dinosaur species, in contrast to that in North America, over the long term before the fossil record indicated that all non-avian dinosaurs had become extinct , 8.

Although the debate between the gradualists and the catastrophists appears to be still unresolved, both groups of palaeontologists do agree that the dating of dinosaur fossils ended in the era of the Chicxulub impact event. Even if the diversity of dinosaur species had been in decline, the gradualists do acknowledge that the worldwide environmental changes caused by that impact could have led to their final extinction.

Clearly the effect of the asteroid or comet collision would have been devastating to all life forms in a wide geographical region around the impact site. The proposed environmental changes that would have occurred worldwide include a period of dim sunlight because of solar radiation absorption by ejected particles and aerosols in the atmosphere and stratosphere. Consequently, there would have been a short-term temperature drop , 9 , which, together with diminished sunlight, would have inhibited or killed photosynthetic plants.

However, perhaps a more significant effect, as far as the dinosaurs were concerned, was a consequence of the geological site of impact. A result of the asteroid impact would have been the ejection into the stratosphere of particulate carbon soot, CO 2 and sulfate aerosols , This would have had a marked cooling effect on the climate from the absorption of much solar radiation.

Because such geological sites that are rich in sulfur and carbon deposits are not plentiful, it has been suggested that if the asteroid impact had occurred in most other regions of the world, the probability of mass extinctions would have been much less , Conclusions about the biological effects of the Chicxulub impact have focused on the undoubted environmental changes of temperature, light intensity, sea temperatures, sea levels and climate disruption in the months and probably years following the impact.

The implications of these changes are that a consequent restricted food supply for many life forms that disappeared would have contributed to their extinction. One result of the asteroid collision with Earth, that has not been considered from a biological perspective, is a particular effect from the ejection into the stratosphere of the sulfur aerosols.

There are various calculations of the amount of sulfur, as sulfate or SO 2 , based on estimates of the size of the impacting asteroid and the density of sulfur deposits at the impact site. One estimate is that — gigatonnes of SO 2 were distributed globally in the stratosphere , Another, in broad agreement, put the quantity of stratospheric SO 2 in the range of — gigatonnes , The effect of such quantities of SO 2 on global climate can be deduced from observations on the changes from sulfur ejected into the upper atmosphere from volcanic eruptions , Yet in the following 12 months, this relatively small amount, compared with that from the Chicxulub impact, had a noticeable cooling effect on the world climate , An alternative environmental catastrophe postulated as the cause of dinosaur extinctions is a series of vast volcanic eruptions in India which created the Deccan basalt larval floods, known as the Deccan Traps.

It has been argued that the timing of this volcanism is more likely to have been associated with the demise of the dinosaurs than the Chicxulub impact event , Nevertheless, geological evidence puts the timing of the Deccan volcanism and the asteroid collision so close together that it has been suggested that the Chicxulub impact actually triggered the largest of the volcanic eruptions which gave rise to the Deccan Traps , Although volcanic activity in general has not been considered to produce stratospheric sulfate aerosols in the quantities anywhere near that of the Chicxulub impact, the Deccan volcanism may have been an exception.

Calculations from paleomagnetic measurements in a Deccan basalt escarpment identified single eruptive events, the largest of which could have released quantities of SO 2 over years or decades, comparable with that released by the impact at Chicxulub , Together, from volcanism and the asteroid collision, the SO 2 emitted into the stratosphere could have had a prolonged climatic effect that would have lasted for years.

One biologically significant property of stratospheric SO 2 is that it strongly absorbs solar UV light in the UVB wavelength range of to nm. In the steady atmospheric state of the present-day era, the intensity of UV light reaching the Earth's surface is attenuated by ozone , The traces of SO 2 from human activity such as that emitted by cities has a limited effect on UVB at ground level , However, the mass of stratospheric sulfur from the Chicxulub impact, perhaps combined with that from the Deccan volcanism, would have been sufficient to completely block UVB reaching the Earth's surface for a decade or longer , One of the effects of UVB radiation is that it acts on 7-dehydrocholesterol in cells of the superficial integument of terrestrial animals to make, by a photochemical reaction, the pre-hormone, cholecalciferol.

A characteristic of animals that depend on solar UVB radiation to supply cholecalciferol is that its precursor, 7-dehydrocholesterol, the penultimate metabolite in the synthesis of cholesterol, accumulates in superficial skin cells.

In other cells, its concentration is very low because of rapid conversion to cholesterol. Cholecalciferol is more popularly known as vitamin D 3 but under natural conditions it is not a nutrient. The food of most animals contains, at most, only traces of cholecalciferol.

An adequate supply of cholecalciferol can therefore only be obtained by exposure of skin to UVB radiation from the sun.

A deficiency of cholecalciferol causes a range of pathological changes, the most prominent of which is the bone disease of rickets or osteomalacia. Much of the knowledge about the metabolism and function of cholecalciferol was obtained in the midth century from studies of the domestic chicken Gallus domesticus , a member of the avian descendants of the dinosaur family.

Create a List. List Name Save. Rename this List. Rename this list. List Name Delete from selected List. Save to. Save to:. Save Create a List. Create a list. Save Back. Grades 3—5 , 6—8. Iridium is relatively rare in Earth's crust but is more abundant in stony meteorites, which led the Alvarezs to conclude that the mass extinction was caused by an extraterrestrial object. At about 93 miles wide, the Chicxulub crater seems to be the right size and age to account for the dino die-off.

In , scientists drilled a rock core inside the underwater part of Chicxulub , pulling up a sample stretching deep beneath the seabed. This rare peek inside the guts of the crater showed that the impact would have been powerful enough to send deadly amounts of vaporized rock and gases into the atmosphere, and that the effects would have persisted for years.

And in , paleontologists digging in North Dakota found a treasure trove of fossils extremely close to the K-Pg boundary , essentially capturing the remains of an entire ecosystem that existed shortly before the mass extinction. Tellingly, the fossil-bearing layers contain loads of tiny glass bits called tektites—likely blobs of melted rock kicked up by the impact that solidified in the atmosphere and then rained down over Earth. However, other scientists maintain that the evidence for a massive meteor impact event is inconclusive, and that the more likely culprit may be Earth itself.

Ancient lava flows in India known as the Deccan Traps also seem to match nicely in time with the end of the Cretaceous, with massive outpourings of lava spewing forth between 60 and 65 million years ago. Today, the resulting volcanic rock covers nearly , square miles in layers that are in places more than 6, feet thick.

Proponents of this theory point to multiple clues that suggest volcanism is a better fit. Other research has found evidence for mass die-offs much earlier than 66 million years ago, with some signs that dinosaurs in particular were already in a slow decline in the late Cretaceous. This all makes sense, supporters say, if ongoing volcanic eruptions were the root cause of the world-wide K-Pg extinctions.

Increasingly, scientists trying to unravel this prehistoric mystery are seeing room for a combination of these ideas. This nearly whole, deep-black skull belongs to the most complete specimen of Tyrannosaurus rex on display in Europe, an individual nicknamed Tristan Otto.

But that notion depends a lot on more precise dating of the Deccan Traps and the Chicxulub crater. This debate may rage for years, as scientists dig up new clues and develop new techniques for understanding the past.

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